ADDITIVE MANUFACTURING OF PURE COPPER USING ELECTRON BEAM MELTING (EBM)

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DEGREE PROJECT IN MECHANICAL ENGINEERING SECOND CYCLE, 30 CREDITS

ADDITIVE MANUFACTURING OF PURE COPPER USING ELECTRON BEAM MELTING (EBM)

PRITHIV KUMAR CHINNAPPAN VISHAL SHANMUGAM

Stockholm, Sweden 2022

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Abstract

Pure copper (Cu) has the properties of high optical reflectivity and surface tarnishing as well as excellent thermal and electrical conductivity. Accordingly, laser-based additive manufacturing (AM) techniques confront various difficulties to produce this material. In contrast, the electron beam melting (EBM) process is paving to become an excellent method to manufacture AM parts from such materials. This is since the electron beam is not influenced by the optical reflectivity of the material. Furthermore, EBM works under vacuum that can protect the powder material from oxidization. In addition, the high working temperature and preheating process for each layer can ensure a uniform heat input and a much lower cooling rate. Hence, the EBM process can significantly prevent the parts from delamination failure caused by residual stress.

Accordingly, this research work is intended to investigate the EBM processability and geometrical freedom/accuracy of EBM made copper components.

The 99.95% pure Cu powder with a particle size range of 45-100μm are used to produce samples. All the samples are built with a certain layer thickness of 50μm with altering parameters, including the processing temperature, line offset, focus offset, beam speed, and beam current. It is found that the processing temperature of 500°C leads to low density and severe lateral melting/sintering. Accordingly, the temperature is lowered to 450°C, 400°C, 350°C, and 310°C to control the excessive lateral melting.

Since dense parts could only be produced above 400°C, this work focuses on developing 400°C processing temperature with different line offset, focus offset, beam speed, and beam current. However, it is observed that the processing window of the EBM process is rather narrow, too high or too low energy input could both result in a porous part with severe distortion. After many experimental optimizations runs, the combination of the optimum parameters is reached which can deliver parts with over 99% density and a good geometrical stability. After optimization, the benchmark parts are designed and manufactured according to electrical and thermal applications (using the optimum parameters). Afterwards, the corresponding geometrical freedom and accuracy of the copper components made by EBM is assessed and discussed.

Keywords: Additive Manufacturing, Electron Beam Melting, Pure Copper, Process Optimisation.

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Sammanfattning

Ren koppar (Cu) har egenskaper som hög optisk reflektivitet och ytans anlöning samt utmärkt termisk och elektrisk ledningsförmåga. Följaktligen möter laserbaserad additiv tillverkning (additive manufacturing, AM) olika svårigheter när det gäller att producera detta material.

Däremot är elektronstrålesmältning ("electron beam melting", EBM) på väg att bli en utmärkt metod för att tillverka AM-delar av sådana material. Detta beror på att elektronstrålen inte påverkas av materialets optiska reflektivitet. Dessutom arbetar EBM under vakuum som kan skydda pulvermaterialet från oxidering. Dessutom kan den höga arbetstemperaturen och förvärmningsprocessen för varje lager säkerställa en jämn värmetillförsel och en mycket lägre kylningshastighet. EBM-processen kan därför i hög grad förhindra att delamineringsfel orsakade av restspänningar uppstår. Syftet med detta forskningsarbete är därför att undersöka EBM-processbarheten och den geometriska friheten/precisionen hos EBM- tillverkade kopparkomponenter.

Det 99,95 % rena Cu-pulvret med ett partikelstorleksområde på 45-100 μm används för att producera prover. Alla prover är byggda med en viss tjocklek på 50 μm med ändrade parametrar, inklusive bearbetningstemperatur, linjeförskjutning, fokusförskjutning, strålhastighet och strålström. Det har visat sig att bearbetningstemperaturen på 500°C leder till låg densitet och allvarlig lateral smältning/sintring. Följaktligen sänks temperaturen till 450°C, 400°C, 350°C och 310°C för att kontrollera den överdrivna laterala smältningen.

Eftersom täta delar endast kunde produceras över 400°C, fokuserar detta arbete på att utveckla 400°C bearbetningstemperatur med olika linjeförskjutning, fokusförskjutning, strålhastighet och strålström. Det observeras dock att bearbetningsfönstret för EBM- processen är ganska smalt, för hög eller för låg energitillförsel kan båda resultera i en porös del med allvarlig förvrängning. Efter många experimentella optimeringskörningar uppnås kombinationen av de optimala parametrarna som kan leverera delar med över 99% densitet och en god geometrisk stabilitet. Efter optimering designas och tillverkas benchmarkdelarna i enlighet med elektriska och termiska applikationer (med optimala parametrar). Därefter bedöms och diskuteras motsvarande geometriska frihet och noggrannhet hos kopparkomponenterna tillverkade av EBM.

Nyckelord: Additiv tillverkning, Elektronstrålesmältning, Ren koppar, Processoptimering.

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Acknowledgement

We want to convey our heartfelt gratitude and appreciation to our thesis supervisor, Dr. Sasan Dadbakhsh and our guide, Xiaoyu Zhao, for the continuous support, insightful comments, and encouragement throughout our thesis. We also want to acknowledge the help received from Zeyu Lin in the Department of Production Engineering , Christopher Hulme in the Department of Material Science and Engineering and SWERIM for assisting us in carrying out the necessary experiments required for our thesis.

Also, we would like to thank our families and friends who gave great support during this thesis work.

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In the world of Manufacturing technology, Additive Manufacturing (AM) is the latest fast- growing technology, which is also commonly known as 3D-printing. In other terms Additive Manufacturing is also known as solid free form manufacturing and rapid prototyping. AM builds the part in a layer-by-layer process based on a sliced CAD model. The success of the AM technology lies in its ability to build complex shaped parts. The complexity of the part does not add any cost in the AM^[1], which is not a case in the conventional machining processes. In AM the part is manufactured primarily by adding material only where it is required in a layer unlike the conventional machining processes where the material is removed from the base resulting in wastage of the raw material. This gives AM an edge over the conventional machining processes as it can produce parts with high complexity, lighter structures and allowing customization of the parts.

Copper has been one of the most important metals discovered by mankind from the bronze age. Copper has been one of the most significant metals in industrial applications due to its distinctive properties such as high thermal and electrical conductivity^[2] and antibacterial properties. Nowadays, the recent advancement in the modern electronic industry requires the Copper parts to be designed with complex shapes^[3]. The complex shapes that are required by the industry are very hard and challenging to be produced by the conventional subtractive machining processes. This is where the latest Additive Manufacturing technology comes into picture as the fabrication of such complex shaped components can be realized by this method.

The most predominantly used methods in AM for producing metal components are Selective Laser Melting(SLM) and Selective Electron Beam Melting(SEBM) or commonly known as Electron Beam Melting(EBM). Both the SLM and the EBM technique is a powder bed-based AM production method. In these methods the powder is laid layer by layer and the laser beam or the electron beam is focused on the powder and the selective region is melted with the required energy based on the sliced CAD model. SLM has been the most used metal fabricating method as the powder bed is heated around 90˚C and approximately sustained at this temperature. Whereas in the EBM the powder bed is heated to approximately 0.8 times of the melting temperature of the metal to be melted^[4]. Another thing to be noted in these two processes is that the SEBM is carried out in a vacuum chamber whereas the SLM is carried out in a pure nitrogen or argon filled atmosphere, this eliminates the possibility of the component being oxidized during the build. Since the SLM uses the inert gas atmosphere the rate of cooling after the build is faster compared to the vacuum chamber used in the SEBM.

Even though there is an excellent possibility for SLM being the upcoming manufacturing technology, its working principle makes it primarily suitable only for materials with low reflectivity and thermal conductivity^[5]. The lasers that are very generally available have a wavelength of 1000 – 1100 nm. Copper has an energy absorption of about 2% at these wavelengths due to its high reflectivity, this leads to unstable SLM processes and less dense samples. On the other hand, Copper has high electrical conductivity which makes it more suitable for SEBM, as SEBM is not affected by the reflectivity of the material. Also, an energy absorbance of about 80% can be achieved by using SEBM^[6]. Therefore, in this research work we are using SEBM as the AM method to print copper and optimize its process parameter to produce parts that are about 99.5% dense.

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2. Literature Review 2.1 Powder Impact

In comparison to SLM, the EBM method uses coarser powder grains, which reduces the resolution of built objects slightly. SLM was found to achieve Ra = 11 µm, while EBM was reported to achieve Ra = 25-35 µm ^[7]. This is influenced by the angle of the surface being built ^[8], and the sample thickness has been found to influence the roughness ^[7]. The higher roughness is due in part to the coarser powder used in the EBM process, as well as the absence of a sintering stage in the SLM process.

The use of coarser powder in EBM is deliberate, as the EBM uses electrons with significant mass to transfer heat to the powder bed, as opposed to SLM that uses photons with no mass. This has a knock-back effect because electrons collide with powder particles, transferring kinetic energy and causing the powder to be ejected from the layer. The angle of the beam versus the base plate can aggravate this. Unless the powder is properly grounded, the electrons will cause a charge to build up in it. If the charge is allowed to rise, repulsive forces will eventually outweigh the forces holding the particles to the powder bed, causing the powder to disperse and clear areas. Smoking is the electrostatic ejection of charged powder particles that, if severe, can cause problems with powder density as well as obstructing the electron beam, reducing the efficiency of energy transfer ^[9].

The sintering stage prior to melting prevents these two factors, charge build-up and powder being physically ejected, by allowing for grounding through the main build as well as keeping powder particles fixed, resulting in proper conversion of kinetic energy into heat. Reflection and deflection, in which the incoming energy beam is partially diverted away from the build, are factors that affect both SLM and EBM. The impact of electron deflection in EBM has not been thoroughly investigated and estimates of how much of the incoming energy is deflected are only educated guesses. Several factors contribute to this energy deflection, including electron collisions with gas particles, ejected powder or welding gas, and insufficiently sintered powder, all of which result in less-than-optimal energy conversion. The angle of impact between the electron beam and the powder bed can also affect the latter.

Powder is recycled and reused between each build to keep AM as environmentally and financially sustainable as possible. Even when a sieve technique is used to remove agglomerated powder, it is difficult to keep the quality of recycled powder like that of virgin powder. Studies on the influence of flaws in powder particles after recycling and their effect on subsequent builds have found that EBM causes a modest shift in grain coarseness and a loss in flowability, but that no substantial effect on build quality should be detected ^[10].

2.2 Defects

Temperature during the process, as it relates to unmelted powder, remelting, in-situ heat treatment, and many other factors, has a significant impact on the presence of defects. Defects in the EBM process can be compared to similar effects in powder welding, and the latter defects are visible in the former.

2.2.1Porosity

Pores in a material are hollow sections that can cause increased stress and crack propagation.

In EBM, there are two types of pores: spherical gas induced pores and elongated process induced pores. The first can happen because gas is trapped in the powder particles during atomization, then released during melting and trapped by rapid solidification, or because of the powder's surface chemistry. The latter appear elongated and are caused by suboptimal parameters where unfused powder stretches cause cavities (Lack of Fusion). Solidification shrinkage could also result in cavities between grains if internal stress becomes too high to

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maintain cohesion. These cavities also contribute to lower thermal conductivity, allowing heat to stay in the top layers for longer than intended, potentially leading to balling, or swelling defects ^[11].

2.2.2 Balling

Spherical droplets can form because of wettability concerns, or the ability of one fluid to spread out over a solid surface, with preceding layers because of melt pool properties or surface tension. This can cause hardened segments to rise above the powder layer and obstruct the powder rake. In the worst-case scenario, if sensors identify physical barriers, the machine will enter failsafe mode and abort the construction to avoid machine damage, or the powder will not be distributed on the start plate evenly which results in the powder sensor commanding the rake to move frequently.

Particles may detach from the layer below and follow the rake, either being pushed off the printing area or remaining in areas where they appear as a large inclusion. The resulting pits where the particles can be refilled by subsequent layers of powder, but there is a risk that the powder layer there will become too thick to fully melt, resulting in pores with unfused powder, or that smaller pits will become trapped under subsequent layers ^[12].

2.2.3 Delamination

Residual stress, which is a common defect in AM, can be caused by temperature gradients and varying thermal expansion during construction. On a macroscopic level, residual stress can cause serious defects in the build, such as warping. When combined with insufficient support, warping can occur, in which the material bends to relieve stress. This bend, also known as warping, can put additional strain on the adhesion between layers, leading to delamination. If the warping occurs during the process, it can result in uneven layers of powder where in areas with less powder, the beam power will be too high, resulting in destroyed material, and it will lead to porosity and lack of re-melting if the power is too low. Depending on the geometry being constructed, the stress can cause the base plate to warp as well, which is more likely to occur when the object is built directly onto the plate without any support ^[11]. As layers are stacked on top of each other, it is crucial that they adhere to each other properly by promoting remelting between the layers and minimizing unfused powder and porosity. As a result, residual stresses exceed the bonding abilities between layers, as well as excessive energy is added which prolongs the life of the melt pool before it solidifies ^[13].

2.2.4 Shrinkage

Shrinkage in additive manufacturing of metals is inevitable and it must be accounted for while designing the part for manufacturing. There are many factors that account for shrinkage which includes solidification shrinkage and uneven distribution of powder over the surface which leads to irregular solidification. Further the dimensions and volume of the part greatly affects the solidification and cooling rate which directly influences the shrinkage effect.

2.2.5 Powder Spreading or Smoke

Powder spreading or pushing is one of the most common defects in the EBM process where the powder layer is blown from the start plate due to three reasons: Water content or residues in the powder which leads to immediate vaporization when the beam hits the powder and causes a local explosion, the higher momentum of electrons than the cohesive force of powder

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particles and the electrostatic charge of the powder which is generated by the electron beam when instead of passing through the layer, it surrounds the particle and creates a negative charge and gets a repulsive force between themselves.

To eliminate this problem, preheating has been identified as a viable solution. It increases the conductivity of the powder layer and helps in reducing the electrostatic charge between the particles. Further use of very fine particles is also avoided. Thickness of the build plate and grounding of the plate also has significant results in reducing the smoking effect. Moreover, Helium gas is used to avoid charge in EBM machines ^[14].

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3. Methodology

3.1 Electron Beam Melting

In Selective Electron Beam melting, a constantly accelerating 60KV electron beam is used to selectively melt the powder in a layer-by-layer pattern that produces a highly denser near net shaped parts. The main components of a Selective Electron Beam Melting Machine are shown in the following figure 1.

The beam column(a) consists of Filament, Focus Coil and a Deflecting Coil. The filament emits the electron beam that passes through the focus coil which controls the spot size of the electron beam, and the direction of the electron beam is controlled by the deflecting coil. Then the focused electron beam selectively melts the layered powder on the powder bed(b). The height of the powder bed is controlled by the build platform(c). After a layer of powder on the powder bed is melted, the build platform is moved downwards according to the layer thickness defined in the system. The powder rake(d) then applies the fresh layer of powder from the stationary hoppers(e) on to the powder bed. The excess powder during raking is removed to the overflow bins(f) which can be reused later. These steps are repeated until the build cycle is completed.

After the build is completed, that part is removed from the powder bed. Any excess powder sticking on to the part is removed using the Powder Recovery System(PRS) and reused.

Figure 1. Main components of an Electron Beam Melting system: (a) beam column, (b) powder bed, (c) build platform, (d) powder rake, (e) powder hoppers, and (f) overflow bins.

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3.2 Build Process

The Arcam A2X, SEBM machine is used to conduct this research work. The characteristics of the Arcam A2X machine is given in the following table.

Beam Type Electron Beam

Max. build size 200 x 200 x 380 mm (W x D x H)

Max. beam power 3kW

Cathode type Tungsten filament

Min. beam diameter 250 µm

Max. EB translation speed 8,000 m/s

Minimum chamber pressure 5 x 10-4 mbar

Typical build atmosphere 2 x 10-3 mbar

Typical process temperature range 600⁰ - 1,100°C

Table 1. Arcam A2X machine parameters

This research work is carried out using gas atomized 99.95% pure Copper that has a particle of 45 – 100 µm. A stainless-steel start plate measuring approximately 75 x 75 x 10 mm has been used. The start plate is levelled on a 15mm thick loose copper powder. The levelled start plate is then pre-heated to a temperature of 500°C. The pre-heating of the clean start plate sinters the powder around the start plate and solidifies the loose powder, this is because the thermal conductivity of the solidified powder is more than that of the loose powder. After preheating of the start plate, the build platform is moved in the down direction of 50µm since the layer thickness of the build is 50µm. The rake then distributes the powder on to the start plate evenly. The distributed powder is pre-heated, this pre-heating of the powder layer helps in sintering the powder layer that binds the powder together. When the powder is exposed to the electron beam, it imparts a negative energy in the powder. This negative energy could be very high so that the powder particles to smoke as they are repelled from the powder bed^[15]. This preheating not only helps in avoiding smoking during the process, but it also helps in reducing the residual stresses due to faster cooling rate due to elevated temperature of the powder bed during melting and comparatively lower temperature around the melt region^[16]. Since there is no build up of stresses during build in EBM, no heat treatment would be required to relieve the stresses post-build^[17]. The pre-heating of the powder bed takes less than or about a sec.

ARCAM A2X EBM machine comes with 2 themes of pre-heating, pre-heating I and pre-heating II. Pre-heat I is used to heat up the entire powder bed whereas pre-heat II is used to heat-up only the melt region. In this work we have used only the pre-heat I. After the pre-heat the electron beam melts the region according to the sliced CAD model and the build height. Then the build platform is moved downwards equivalent to the build layer thickness i.e., 50 µm and then the sequence is repeated until the build is completed. After the build is completed, the part is vacuum cooled until it reaches the desired temperature. Once the desired temperature is reached the machine is opened and the start plate along with the part is removed. Any excess powder around the part is removed using the PRS, the PRS uses a high-pressure air system to remove the powder from the part. The removed powder is recycled and reused. The fabricated part is then cut and removed from the start plate with the help of Wire Electrical Discharge Machining also known as Wire EDM.

3.3 Density Measurement

The relative density of the fabricated part is measured based upon the Archimedes principle^[18]. The principle asserts that the buoyant force experienced by the object submerged in a liquid act upwards and is equivalent to the weight of the liquid displaced by the object. In other words, this principle can be simplified as the density of the object can be determined irrespective of the change in volume of the liquid. The density of the object can be determined

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using the following equation (1) ,

𝜌_𝑜𝑏𝑗= 𝜌_𝑙𝑖𝑞 ∗ _(𝑚 ^𝑚^𝑎𝑖𝑟

𝑎𝑖𝑟− 𝑚_𝑙𝑖𝑞) (1)

Where, 𝜌_𝑜𝑏𝑗 = 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑜𝑏𝑗𝑒𝑐𝑡 𝜌_𝑙𝑖𝑞 = 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 𝑜𝑓 𝑡ℎ𝑒 𝑙𝑖𝑞𝑢𝑖𝑑 𝑚𝑎𝑖𝑟 = 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 𝑜𝑏𝑗𝑒𝑐𝑡 𝑖𝑛 𝑎𝑖𝑟 𝑚𝑙𝑖𝑞 = 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 𝑜𝑏𝑗𝑒𝑐𝑡 𝑖𝑛 𝑙𝑖𝑞𝑢𝑖𝑑

An analytical balance was used to measure the mass of the object in air and in the liquid as shown in fig. 2. The liquid medium that is used in this method is the isopropyl alcohol or commonly known as isopropanol. The density of the isopropanol at different temperatures is given in the Appendix-1. The balance was zeroed each time before measuring the mass of the object. Any air bubble adhering to the object when submerged in the liquid is carefully removed with a thin wire to ensure that no liquid is removed from the beaker. The weight of the object in air and liquid is measured three times for each of the objects and the mean value is obtained. The obtained mean value is then used in the equation (1) to determine the density of the fabricated object.

3.4 Porosity Measurement

For measuring the porosity, the fabricated part is compression mount using the KonductoMet™ from Buehler, KonductoMet is a conductive filled phenolic mounting compound that used for mounting objects that are to be analyzed using SEM, when carbon is not the object to be analyzed. The mounted samples are grinded in the planar grinder using Silicon Carbide paper of Grit 220, 280, 320, 400 and 600^[19]. The grinded sample is then polished using the 3µm diamond liquid. The polished sample is then scanned using the Phenom ProX Desktop Scanning Electron Microscope (SEM).

The scanned image is then analyzed for porosity using the ImageJ, which is an open-source

Figure 2. Illustration showing the setup of density measurement

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image analysis software. The scanned image is opened in the ImageJ software, image analysis can only be performed on the 8bit image. In case if the scanned image is not an 8bit image then it can be converted to 8bit by choosing Image option, Type and 8bit as shown in the below fig. 3.

The 8bit image still cannot be used for image analysis as the scanned image has the mounting compound around the edges as the edges are uneven. Hence, only the center region of the scanned image needs to be analyzed. This can be done by selecting the center region leaving out the edges and duplicating it by pressing Shift+Control+D. Then the porosity for the duplicated image is analyzed by adjusting the threshold of the image until all the pores are covered as shown in fig. 4. The resulting threshold value is the porosity of the analyzed sample, which in this case is 0.05%.

3.5 Powder Characterization Particle Size Distribution:

The particle size of the virgin powder is said to be 45-100µm by the manufacturer. To check how the particle size has been affected after a few runs, the ImageJ software is used for analysis. The recycled powder after a few runs is taken and applied on to the carbon tape before scanning it in the SEM. After scanning the image is loaded to the ImageJ software, for the particle size analysis the image requires to be in 8bit format. If not, then the image needs to be converted as said before. And the image needs to be in black and white otherwise known as binary image, before converting the image to binary using threshold, we must trim the edges and duplicate as said above. A threshold range is used to differentiate the object of significance from its background. All pixels in the image whose values lie under the threshold are converted to black and all pixels with values above the threshold are converted to white, or vice-versa. Then click on analyse, analyze particles in Imagej as shown below. And then on the analyze particles window select the desired options as shown and click on ok. Now the results and the summary is displayed. The results tab provides the area of the individual particles from the image from which we can calculate diameter or size of the particle.

Figure 3. Illustration showing image type change in Image J

Figure 4. Illustration showing threshold in Image J

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Flowability:

“Powder flow, also known as flowability, is defined as the relative movement of a bulk of particles among neighboring particles or along the container wall surface. In other words, powder flowability refers to the ability of a powder to flow in a desired manner in a specific piece of equipment. Powder flow is thus a multidimensional problem where powder characteristics are studied in conjunction with the geometry of an application to predict flowability. Two kinds of flow tests done in this research work, 1. free flow test and 2. dynamic flow test using Rheometer. The free flow test is performed only under the influence of gravity.

50g of the desired powder whose flowability needs to be measured is filled in the top while the opening in the bottom through which the powder flows is closed using a finger, which is like a funnel as shown in the figure of the free flow apparatus. The finger closing the hole is removed and the time for the powder to flow completely is measured using a stopwatch and is noted.

The same test is repeated for two more times and the average time is taken to minimise any errors during measurement. Dynamic flow test using a Rheometer is done to know how much of the specific energy is required to move the powder particles with respect to different moving speed of the rake or blade that spreads or distributes the powder. The rheometer used in the analysis shown in the figure below. Comparing the results for the Virgin and Used powders shows how processing the powder in AM has significant effect on the flow energy of the powder.

Figure 6. Image Free flow test apparatus(Left) and Rheometer(Right) Figure 5. Illustration showing particle size analysis in Image J

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3.6 Surface Roughness:

The surface roughness of the copper part built using electron beam melting is measured using the white light interferometer, Zygo NewView™ 7300 and the obtained data is processed using the Mountains®9 software from Digital Surf.

Figure 7. White Light Interferometer, Nygo Newview 7300

The measured surface profile of the printed copper part is shown in the below figure 8. The measured profile contains the outliers and non-measured (NM) points from the white light interferometer. The outliers are removed, and the non-measured points are filled using the in- built algorithm of the Mountains9 software.

Figure 8. Surface profile measured using white light interferometer

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Figure 9. Removing Outliers using Mountains9

Figure 10. Extracted Surface profile without outliers and non-measured points

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Figure 11. Surface Roughness value of the extracted profile

3.7 Tensile Test

The tensile test was carried out at Swerim, Stockholm. The tensile test specimen was built as per the drawing in figure 12. The tensile test specimen was built to a total height of 14mm and sliced using wire EDM to produce a tensile specimen each of 2mm thick. The specimen mounted to the tensile test machine shown in figure 14 and the tensile stress is applied until the specimen ruptures. The machine contains the extensometer which measures the elongation of the specimen until failure. This data is fed to the computer which automatically calculates the necessary tensile properties based on the thickness and width of the specimen.

Figure 12. Drawing of the tensile specimen built

Figure 13. . Printed tensile specimen on the start plate(left), Sliced tensile specimen(Right)

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Figure 14. Tensile Test Machine, Instron 4505

3.8 Electrical Conductivity

Copper is an excellent conductor of electricity. Thus, it is important to check the conductivity of the parts produced by additive manufacturing to have an insight on how the conductivity differs with respect to different processing parameters. The conductivity of the parts is compared with the conductivity of pure copper examined by IACS. The samples were printed with the dimensions 50mmx6mmx1mm with the optimal parameters obtained through the experimentations. The samples were printed in Vertical and Horizontal orientations and printed as a whole block of five samples together which is later sliced using Wire EDM.

Sample Type Beam Current Beam Speed Focus offset Line offset

Vertical block 10mA 500mm/s 10mA 200µm

Vertical block 10mA 500mm/s 10mA 100µm

Horizontal block 10mA 500mm/s 10mA 200µm

Horizontal Block 10mA 500mm/s 10mA 100µm

Table 2. Electrical Conductivity samples process parameters

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Later the produced samples are prepared for the testing by sanding the sides and make the side surface as smooth as possible. A four-point resistivity measurement is performed where the sample is fed with 1.49A of constant current and another set of wires held near the ends to check the voltage across the sample as shown in the image.

Figure 15. Four-point electrical resistivity testing equipment

3.9 Microstructure Preparation

Three samples were taken for further study of microstructure in their respective horizontal and vertical direction of the print. The reason behind the selection of three samples is that they possess the highest possible density during the entire study. given below the processing parameters of these samples.

Sample No.

Beam Current

Beam Speed

Focus offset

Line offset

Contouring Strategy

A 10mA 500mm/s 10mA 100µm Only Hatching

B 10mA 500mm/s 10mA 200µm Hatching with contour start MECC 75%

C 10mA 500mm/s 10mA 200µm Hatching with contour end

MECC 75%

Table 3. Process Parameters of the samples A, B and C

The microstructure analysis is done to learn more about the possible subsurface defects and how the parts are formed. The samples that are subjected to analysis follows the same procedure as for the SEM analysis with an additional step of etching. Moreover, during the process of polishing, an additional step of 1µm diamond paste polishing is also performed. As the standards set for etching are high, the need for perfect micro polishing is highly recommended.

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The etchants are carefully selected and prepared with high caution. As the quality of the etchant reflects the reveal of the grain boundaries, the preparations are done under the safety shields with proper protective gears. The products for creating the etchant are listed below.

Serial No. Product Quantity 1 Ferric chloride 5g 2 Hydrochloric Acid 10ml

3 Glycerol 50ml

4 Water 30ml

Table 4. Chemicals for preparing the etchant

The order of preparing the etchant is chosen carefully as the tendency of the reaction of hydrochloric acid is very high. The salt is measured and crushed into fine powder as the ferric chloride is hydrated at room temperature. Later the water after measuring is poured and stirred until the salt is completely dissolved. Then the glycerol is measured accurately using a beaker and poured until the last drop as the glycerol is highly viscous. The products are then mixed thoroughly using the stirrer and finally, the acid is poured. The acid is poured very slowly as the reaction is exothermic. Once the etchant is created, a separate flask of pure water is placed adjacent to the etchant as the reaction time required to expose the grain boundaries are less than a second and the part is immediately dipped inside the water to stop the etching process.

Then the parts are dried using a hot dryer and cleansed to make them available for the subsequent steps.

The parts are then viewed in an optical microscope as an initial step to make sure that the etchant has done its work as the etching time is very short. Later for further examination, the parts are then viewed in an advanced optical microscope with a magnification of 50x to view the actual grain boundaries. Once it is clearly visible, the samples are then moved to the next stage of examining under the Scanning electron microscope. The previous steps are done to make sure the microstructure is clear and visible as the Scanning in SEM takes time. With a magnification of 100x and 200x the samples are investigated.

3.10 Benchmark

After acquiring good results from synthesizing and printing copper in the earlier methods, it is always necessary to test the machine with the parameters such that it can be able to print all kinds of shapes and structures. One such method to testify our results is the state of art benchmarking. Since there is no readily available source of methods to create parts made from copper, carefully evaluated models have been created using the evaluation methods defined by Moylan et al.

• Have a significant number of small, medium, and large features.

• Not consume large quantities of material.

• Have many features of the real-world parts.

• Have simple geometric shapes that can have perfect definition and easy control of the geometry.

• Allow repeatability measurements.

• Requires no post-treatment and manual intervention (support structures)

• Not take long to build.

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It is noted that the last point is entirely dependent on the layer thickness and machine speed, so it is not considered as an issue for this benchmarking. Printing a benchmark considering all the features and shapes is easy, but the most important task is to make sure that the benchmark can be evaluated with proper testing equipment. Thus, making the benchmark considering the fifth point is important for the test to be successful.

The proposed benchmark is designed to fit into the current testing setup with its overall dimension of 70 x 70 x 12 mm with the provision to stick to the baseplate. Further, the part is designed in such a way that it doesn’t need support structures, but the powder layer provides enough support compared to other printing technologies. As the major concern is dimensional evaluation, inheriting simple geometry with varying sizes would be able to synthesize the results easily as they are pre-defined with tolerances and form errors. Simple classic geometries such as cuboids, thin walls, holes, cylinders, etc., are represented with varying sizes in the model.

The size change is addressed for the measurements which fit into the ISO ranges for the preliminary sizes such as 1 to 3mm, 3 to 6mm, 6 to 10mm, 10 to 18mm, 18 to 30mm, 30 to 50mm, 50 to 80mm and 80 to 120mm. Further, the parts are printed above the baseplate, there is no need for a separate copper base as the plate is not affecting the parts and left along with the parts for measurement. The parts that are considered for printing are explained with a model and listed below.

Figure 16. Isometric view of Benchmark CAD drawing

• A set of six cuboids (R): the blocks have a 15mm length and 11mm height as common which vary with different thicknesses such as 0.5mm, 1mm, 1.5mm, 2mm, 2.5mm and 3mm. Further, the blocks are separated by 2mm between them.

• A set of seven tilted cuboids (TC): The cuboids be inclined to the base plate with an increasing angle of 15 degrees per step. The length and breadth of 15mm x 5mm remain common for all the inclined parts starting from 0 degrees and ending with 90 degrees.

• A set of five slotted rectangular cuboids (SR): the parts are designed with the purpose of checking the capability of producing internal slots as well as the thin wall feature.

Each block is a unique thickness and has dimensions but with a constant length and height of 20mm x 11mm. The change in thickness is as follows:

o 1^st cuboid with 8mm total breadth and has a slot of 14mm x 2mm o 2^nd cuboid with 6mm total breadth and has a slot of 16mm x 2mm o 3^rd cuboid with 4mm total breadth and has a slot of 19mm x 3mm o 4^th cuboid with 4mm total breadth and has a slot of 17mm x 1mm o 5^th cuboid with 4mm total breadth and has a slot of 18mm x 2mm

• An overhanging bridge with 5 different gaps (OB): 6 square blocks of dimensions 10mm x 2mm x 10mm separated by distances of 1mm, 2mm, 3mm, 5mm and 10mm and the blocks have a common horizontally placed rectangular block of the dimension 33mm x 10mm x 2mm which is placed on top of the blocks.

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• A set of 6 semi cylinders with different radii (SC): An array of closely packed semi cylinders of radii 6mm, 5mm, 4mm, 3mm, 2mm and 1mm. All the semi cylinders have the same length of 4mm.

• A set of 8 horizontal holes (HH): A hole of different radii are placed in a direction parallel to the base plate. The holes are arranged in a wall of thickness 5mm. The holes have a diameter ranging from 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 4mm, 5mm and 6mm.

• A set of 8 vertical holes (VH): A hole of different radii are placed in a direction perpendicular to the base plate. The holes are arranged in a cuboid of thickness 5mm.

The holes have a diameter ranging from 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 4mm, 5mm and 6mm.

• A set of 5 Straight cylinders (CY1): Five cylinders of the varying diameters of 5mm, 4mm, 3mm, 2mm and 1mm placed perpendicular to the base plate.

• A set of 5 inclined cylinders (CY2): Five cylinders of the varying diameters of 5mm, 4mm, 3mm, 2mm and 1mm placed at an inclination of 60degrees to the base plate.

• A set of 5 inclined cylinders (CY3): Five cylinders of the varying diameters of 5mm, 4mm, 3mm, 2mm and 1mm placed at an inclination of 45degrees to the base plate.

The parts are organized and aligned rationally to be representative for the evaluation of geometrical tolerances during the inspection. The common additive manufacturing defects such as the staircase effect which is typical in processes that involve layer by layer manufacturing can also be evaluated for straightness and curvatures in the model.

In short, more than sixty geometrical features have been found on the references part. The parts are printed with carefully selected parameters from the experiments carried out for the optimal processing parameters for printing copper. The processing parameters for printing this benchmark are listed below. The outcomes of the print are discussed in the results section in detail.

Model No. Beam Current

Beam Speed

Focus offset

Line offset

Contouring Strategy

1 10mA 500mm/s 10mA 200µm Only Hatching

2 10mA 500mm/s 10mA 200µm Hatching with contour

start MECC 75%

3 10mA 500mm/s 10mA 100µm Only Hatching

4 10mA 500mm/s 10mA 100µm Hatching with contour

start MECC 75%

MECC Parameters

8.5mA 170mm/s 10mA 200µm

Table 5. Process Parameters for the Benchmark Models

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4. Process Parameter Optimization

The process parameter optimization of pure copper was done by building 10x10x6mm cubes by varying the combination of different process parameters. The built cube is separated from the start plate with the help of the wire EDM. These cubes are then measured for density by using Archimedes principle as discussed above in the methodology. Based on the density the process parameter is optimized until a density of 99.5% is reached.

4.1 Important Process Parameters

● Beam Current (BC) sets the amount of the beam generated. BC is measured in mA.

● Scanning velocity, A measure of how fast a beam travels across a powder bed along a scan vector. It is also referred to as Beam Speed(BS) and measured in mm/s.

● Hatching Offset, the spacing between adjacent scan line. It ensures total melting of the region to be melted by allowing remelting to a certain extent of the previous weld track. It is also referred as Line offset(LO)

Figure 17. Image showing hatching offset

● Focus Offset (FO) is regulated by the focus lens and controls the beam spot size of the electron beam on the start plate. In the figure 18, (i) indicates the negative FO value where the beam spot size is above the start plate. (ii) indicates FO value of zero where the focal point is on the start plate. (iii) indicates the positive FO value where the beam spot size is below the start plate

Figure 18. Image showing varying focus offset

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4.2 Beam Energy Calculation

When it comes to the Powder Bed Fusion (PBF) process, energy input is the most important parameter. Energy per unit length of track is calculated by dividing beam power by scanning speed, which is the ratio between beam power and scanning speed for a single track.

During the scanning of a single layer, the energy input (J/m³) is measured as the average applied energy per volume of material.

4.3 Contour Strategy

It is well known that the additive manufacturing techniques do not produce parts with good surface roughness. To improve the surface roughness of the additively manufactured product various post processing technique is used. Contour, which is a melting approach is used to improve the surface roughness and mechanical properties of the additively manufactured parts. Figure 19 shows the top view of a simple cube to illustrate the scanning strategy. A hatch defines the inner region of the part and is enveloped by contours on the outside. There are two contours on this cubic sample shown: one inner and one outer. Individual user can specify how many contours they want.

Figure 19. Top view of cubic sample with showing different melt strategy

Arcam provides the options, Start with contour or End with contour. In start with contour, the contour melting is carried out before the hatch melting. In end with contour, the hatch melting is carried out before the contour melting. The hatching pattern can be rotated at an angle for each layer or every few layers to control the microstructure of the part. In this work we have employed outer contour with hatching and hatching pattern is rotated to a 90⁰ angle for each layer.

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5. Challenges Faced in 3D Printing of Pure Copper

One of the most important takeaway of this thesis work is the challenges faced in realizing the 3d-printing of pure copper in EBM. The challenges that we faced during the process can be classified as start plate, processing temperature, repeatability and defects formed during the EBM process.

5.1 Start Plate

The start plate or the build plate is the base upon which the powder is spread and melted by the electron beam. Hence it is of utmost important to choose the right start plate as it heavily influences the process. The two kinds of start plate that we have used for the additive manufacturing of pure copper powder are, pure copper and stainless-steel start plate. Each start plate had a thickness of 10mm.

Due to the high thermal conductivity of copper, the preheat of the copper start plate was relatively faster. The desired preheat temperature for the start plate was 400⁰C which could be achieved in ~8minutes. As the process temperature was reached faster there was not enough time for the sintering of the powder under the start plate. This resulted in a weak and soft powder cake under the start plate. Also, while printing it was noticed that the printed parts were weakly attached to the start plate. This posed the threat of removal of the melted layers from the start plate during the recoating of the powder layer by the rake. Once the build was completed and the start plate was removed from the powder bed, it could be observed that the start plate was bent significantly due to the local thermal gradient during the process. Due to the extent of bent, the copper start plate could not be reused even after machining as it reduced the thickness of the start plate drastically.

Figure 20. Bent copper start plate placed on a flat surface

The thermal conductivity of stainless steel is 20 times lesser than that of the copper. Due to the low thermal conductivity of stainless steel, the desired pre-heat temperature of 400⁰C was reached in ~20mins. This provided sufficient time for the powder under the start plate to be sintered which resulted the powder cake to be comparatively harder which could be crushed with bare hands. Post build it could be noted that the parts were well attached to the start plate which nullified the threat of layer removal by the rake. The bent in the stainless-steel start plate was very negligible that could be removed by face milling and reused.

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Figure 21. Stainless-steel start plate post preheating(left), Status of powder cake post preheating and start plate removal(right)

5.2 Processing Temperature

The desired process temperature was 400⁰C to print the copper parts. Due to the high thermal conductivity of copper the heat transfer from the stainless-steel build plate to the surrounding powder bed was rapid. This results in temperature being rapidly dropped during raking of the powder and increase in temperature during the melting of the powder layer causing an unstable process temperature. Arcam has provided the option of preheating the powder layers which aims to maintain the process temperature among other uses. During the preheating the electron beam scans the entire build area to maintain the process temperature this can either be controlled manually or automatically. Controlling the preheat temperature manually during the process requires the process to be stopped to change the preheat parameters. By doing so the process temperature drops further as restarting the process takes few seconds for the machine to assess the situation. Using the automation mode, the machine itself calculates the necessary process parameters and the time required for the preheat. But the process parameter window for the automation mode depends on the material type used in the process is generally provided by the manufacturer, in our case which is General Electric. And we were not provided with the automation process parameter window. Hence, we have not used the preheat of the powder layers. Only when the process temperature was too high than the desired, we have used the false preheat to bring down the temperature to the desired level.

Also, it is to be noted that when the process temperature dropped below 300⁰C the powder started to smoke due to lack of sintering of the powder particles.

Figure 22. Graph from the log showing the variation in temperature with respect to time

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5.3 Repeatability

We expect the same process parameters to produce same kind of result each time it is used.

But in our work, it was noted that the same process parameter produced different kind of result each time used. Initially it was hard for us to understand the phenomenon behind this and as far as we researched there was no other study on electron beam melting that had report this kind of repeatability issue. Initially we were able to produce 10x10x6mm cube with good top surface using the parameters of Beam Current 10mA, Beam Speed 500mm/s, Line Offset 200µm and Focus Offset 0mA. But when the same process parameter used in the subsequent times, we found that the top surface of the cube was porous as shown in the figure 23 and we were not able to produce the cube with good top surface for the same parameters. Our initial thought on this was that the powder was relatively new when we were able to produce cube with good top surface and as the powder was reused the quality of the powder would have gone down which resulted in the porous top surface. But when compared the powder quality of the virgin and the recycled powder using the element analysis in Scanning Electron Microscope(SEM) there was not any noticeable change between the quality of the powders.

Even though the SEM was not the reliable method to check for the powder quality, the results did not show any noticeable deviation. Hence, the powder quality was considered to be unchanged.

When the same process parameters were used with a change in the Focus Offset from 0mA to 10mA, the cube that was produced had the good top surface. The subsequent build with Focus Offset of 10mA produced the same kind of result. Hence there was a shift in the focus offset of the machine from 0mA to 10mA and the reason behind this is unknown. Also, it was noted that the materialization on the brass grid cups on the top column had a large influence on the quality of the parts produced. Hence, these brass grid cups need to be polished to restore it to its original condition after every few runs.

Figure 23. (a) & (b) Showing the top surface of the cube produced with FO 0mA, (c) top surface of cube produced with FO 10mA

5.4 Defects

Copper parts produced by EBM poses a challenge every time as many factors for printing copper kept on changing due to certain circumstances, which lead to creation of unwanted defects that are harmful for the product. Some defects which arise during optimization of the processing parameters were listed below.

Keyhole Defect:

The Keyholes appear on the top surface as shown in the Figure 1 of the part as it relates to the melt pool. When the electron beam passes through the line, a melt pool is generated which

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is made by melting the powder on top of the previous layer and penetrates some parts of the previous layer for effective bonding. If the beam energy is too high, the melt pool curves and creates a swirl during the process. Once it starts to cool down, the surrounding walls of the melt pool starts to converge and leaves a vacuum space trapped in between the layer which eventually termed as pores.

Figure 24. Keyholes on the top surface of the part

Figure 25. Schematic explanation of keyhole formation

Once the final layer is reached, the process happens but now there isn’t a layer of powder on top to get the pores closed and forms a keyhole. In some cases, the top of the melt pool just contracts and gets solidified immediately once it starts to cool down, leaving the vaccum space trapped in a shape of a key as shown in the Figure 25.

Figure 26. Keyhole on the vertical section, X-Z plane of the cube produced

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Lack of fusion:

A characteristic internal defect which serves as a root cause for many other defects which may arise either internally or externally. Lack of fusion generally occurs if the melt pool is not sufficient or if the beam length is not sufficient to penetrate till the previous layer or there is an uneven or lots of powder distributed on top of the building part as explained in the Figure 27.

This sometimes happen at fewer specific spots or can occur to the entire print zone depends upon the beam energy and spot size. Lack of fusion is generally characterized by a clump of unfused powder as shown in the Figure 28 trapped inside the part that has been built. This typically reduces the density of the part and might also pose to internal pores. As mentioned in the literature, this defect can be much avoided as it is mostly dependent on the processing parameter and thus, the parameters that causes such defect can be avoided to use in the future.

Figure 27. Schematic representation of lack of fusion formation

Figure 28. Showing Lack of fusion in the horizontal cross section of the cube

Delamination Defect:

Delamination occurs mostly at the edge of the parts when there is an excess beam energy input at the part. This defect has many correlations where the start point of the defect is hard

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to calculate. Generally, when the process parameters have more energy input than what the material needs, the residual thermal stresses will try to release to the rest of the built area through the edges which causes a warp when cooling. The higher operating temperature, incomplete or insufficient remelting of the previous layer is also a reason which might lead delamination. When the rake spreads the next layer of powder the warped zones might get some extra powder underneath the warped layer or in the valley of the zone which makes the beam current to develop this defect even worse and finally the built part tends to come off the build area. This leads to much worse after effect as the continuation of the built is seen near impossible. Figure 29 clearly shows the warped zones which was developed during the print of benchmark. This defect might arise irrespective of the processing parameters as it totally depends upon the amount of heat getting transferred. But over melting of the layer with higher input energy will lead to more delamination defect.

Figure 29. Delamination of layers in the printed Benchmark

Balling Defect:

Balling is a structural and aesthetic defect which tends to affect the top surface of the printed parts. Even if there is a highly dense part, this defect may arise depends upon few factors.

This is identified in parts if there is a ball like round structures attached on top of the parts as shown in the Figure 30 which was taken using the secondary SED mode in SEM. As mentioned in the literature, this defect is because of wettability concerns of the molten fluid or the ability of the molten fluid to spread evenly on top of the previous layer which is due to melt pool or the surface tension. Eventually, it will leave some large inclusions which obstructs the rake resulting in uneven spreading of powder for the next layer. Sometimes, the rake tends to scrap out the inclusions which might either be pushed out of the build area or will get stuck in some places of the build zone causing subsurface defects such as pores and lack of fusion as it is hard for the beam to melt those inclusions. In some cases, the rake will tend to move the entire build zone, resulting in aborting the build process.

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Figure 30. Balling formation on the top surface of the cube

Smoking Defect:

Smoking is a very serious defect as it results in aborting the build process or serious structural deformity in the parts that undergoes this defect. This defect is identified during the process when the preheating of the powder layer is off, and the built height is too high, or the size of the parts is too low. It is generally occurred due to the electrostatic forces that act upon the powder layer when the beam strikes, which in turn creates a repelling force that pushes most of the powder away from the build zone. It is deteriorating effect which causes some serious problems to the machine. As mentioned before, copper is an excellent conductor, thus having the least probability of smoking. But this can happen if the build zone temperature drops too low or possible contamination of powder having moisture in it. This defect can be eliminated to a great extent by incorporating preheating of the powder layer before every layer is printed and proper storage of copper powder to minimize the risk of possible contamination of moisture. As shown in the Figure 31 large sums of powder being pushed outside the built zone and sometimes there won’t be any powder left for the rake to spread across the build plate resulting in automatic abortion of the process.

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Figure 31. Showing scattered powder due to smoking during the process

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6. Results and Discussion 6.1 Powder Characterization Particle Size Distribution:

The powder particle size distribution is shown in the below table 6. The powder particle size is compared between new powder and the recycled powder after the 7th run. The distribution indicates that there is a reduction in size of the powder as it is recycled. The SEM image shows that the particles have become more edge after the 7th run compared to the new powder which more spherical. This is because the copper is soft in nature, which means that the more the particles shear with each other during the process or during recycling the more edgy it becomes.

Table 6. Powder particle size distribution for new and recycled powder after 7th run

Figure 32. SEM image showing new powder particles(left) and powder particles after 7th run(right)

Flowability:

The free flow test result is given in the table 7 below. The free flow test results indicate that there is an increase in the flowability of the powder with respect to the number of times the powder has been processed in the EBM machine. The increase in flowability could be the result of the reduction in small satellite particles as mentioned in the above particle size distribution. This means that the powder flowability from the powder hopper to the build table

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is getting better with each run as the powder flows from the hopper only under the influence of gravity. The rheometer test results shown in the below graph 1 indicates that the specific energy required to move the powder is increased from the new powder to the old powder. This could be explained by a change in particle shape as mentioned in the above particle size distribution. As we have an increase in edginess of the particles due to its softness which affects the flowability when compared to the spherical particle shape. Hence the specific energy to spread or distribute the powder particle over the build platform increases with the number of times the powder is recycled.

Table 7. Free flow test result between different powder sample

Graph 1. Graph indicating energy required to spread new and recycled powder after 7th run

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6.2 Density vs Different processing Parameters Density vs Energy:

Graph 2. Graph showing the variation in density with respect to energy at various processing temperature

The above graph 2 indicates how the density is varying with respect to energy at various processing temperatures. For this we have tested 3 different temperatures. From the results it can be seen that the energy of 1.2J/mm at 400°C produces parts that are ~99% dense.

Figure 33. Parts produced at various processing temperature, 310°C(left), 400°C(center) and 500°C(right)

The above figure 33 shows the image of the parts produced at varying processing temperatures. All the three parts have been produced using the same energy of 1.2J/mm. That part that is produced at 300°C looks more porous and less dense whereas the geometrical accuracy of the part looks good, and the density of the part is ~88%. That part that is produced at 400°C has good surface quality and the geometrical accuracy of the part looks good, and the density of the part is ~99%. That part that is produced at 500°C has poor surface quality and the geometrical accuracy of the part isn’t good due to the over melting of the powder particles at high temperature. Even though the part is over melt the density of the part is ~99%.

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Density vs Beam Speed:

Graph 3. Variation in density with respect to beam speed at various processing temperature

The above graph 3 indicates how the density is varying with respect to beam speed at various processing temperatures. For this we have tested 3 different temperatures. From the results it can be seen that the beam speed of 500mm/s at 400°C produces parts that are ~92% dense.

It can be seen that the Beam Speed of 1000mm/s at 450°C produces more than 92% dense parts. Since the variation in density is less and the processing temperature is more, we consider that beam speed of 500mm/sec at a processing temperature of 400°C is the best suited.

Density vs Line Offset

Graph 4. Variation in density with respect to line offset at various processing temperature

The graph 4 indicates how the density is varying with respect to line offset at various processing temperatures. For this we have tested 3 different temperatures. From the results it can be seen that the line offset 200μm at 400°C produces parts that are ~88% dense. It can be seen that the various line offset at 450°C produces more than 92% dense parts. Since the

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variation in density is less and the processing temperature is more, we consider that beam speed of 500mm/sec at a processing temperature of 400°C is the best suited. And also the lesser the line offset the more the energy consumption and the time to print the as more scanning would be needed to produce the part.

Density vs Line Offset at varying Focus Offset:

Graph 5. Variation in density with respect to line offset at various focus offset

The graph 5 indicates how the density is varying with respect to line offset at various focus offset. For this we have tested 3 different focus offsets. From the results it can be seen that the line offset 100μm at 400°C with a focus offset of 10mA produces parts that are ~99.5%

dense. It can be seen that the focus offset of 20mA at 100μm produces a part that is close to 99.5% dense. But as shown in the figure 11 the surface quality of the part produced using LO of 100μm with a focus offset of 20mA is not as good as part produced using LO of 100μm with a focus offset of 10mA.

Figure 34. Parts produced at various focus offset, 0mA(left), 10mA(center) and 20mA(right)

ADDITIVE MANUFACTURING OF PURE COPPER USING ELECTRON BEAM MELTING (EBM)

ADDITIVE MANUFACTURING OF PURE COPPER USING ELECTRON BEAM MELTING (EBM)

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