Selective Laser Melting of Ni-based Superalloys: High Speed Imaging and

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Selective Laser Melting of Ni-based Superalloys: High Speed Imaging and

Process Optimisation

Himani Siva Prasad

Materials Engineering, masters level 2016

Luleå University of Technology

Department of Engineering Sciences and Mathematics

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Abstract

Additive manufacturing is the process of joining or adding material to build an object from 3D model data. Selective Laser Melting (SLM) is an additive manufacturing technology that generates components layer by layer. Though it is already being used in the industry, some aspects are not very well understood. In this thesis, high speed imaging is used to gain new insights about the interaction of laser light with material. A number of parameter sets for high efficiency and good surface finish were found for a nickel based superalloy, Hastelloy X^TM. Three setups are discussed:

single laser pulse interaction with powder, low speed SLM and high speed SLM. It was found, that in order to observe powder behaviour, a narrow bandwidth illumination source is necessary. The low speed SLM process was imaged clearly and revealed three stages of the process, i.e. powder redistribution, melting and drop incorporation. In contrast, the high speed process included vertical powder displacement. Influence of various process parameters is also discussed.

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Acknowledgements

I would like to express my gratitude to Torbj¨orn Ilar, my supervisor, for his support and encouragement through my endeavours while writing this thesis, and Alexander Kaplan for the opportunity to work in the field of laser manufacturing.

I would also like to thank Frank Br¨uckner for the opportunity to carry out some of my experiments at Fraunhofer IWS, and Thomas Finaske who spent so much time helping me through my days there.

A special word of thanks to everyone at the manufacturing systems group for the innumerable times they helped me and for being such amazing company through the months. To my family and friends, who have always been there for me.

Himani

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List of Figures

2.1 Additive manufacturing processes . . . 4

2.2 Objects manufactured using SLM . . . 5

2.3 Schematic of SLM process . . . 6

2.4 SLM process parameters and layerwise structure . . . 6

4.1 HSI setup for single laser pulses . . . 17

4.2 Setups for building tracks . . . 18

4.3 Scraper . . . 19

4.4 Powder paving arrangement . . . 21

4.5 HSI setup for high speed laser scanner . . . 22

5.1 SEM image of Hastelloy X^TM powder . . . 24

5.2 Samples from the single laser pulse trials . . . 25

5.3 Images from a high speed video from single pulse trials . . 27

5.4 Samples built with powder layer thickness 1 mm . . . 28

5.5 Layer built with layer thickness 1 mm . . . 29

5.6 Cross section of multilayered structure . . . 29

5.7 Top view of tracks built . . . 30

5.8 Cross sections of optimal tracks . . . 31

5.9 Cross section showing crack formed . . . 32

5.10 Cross section of 5 layered structure . . . 32

5.11 Microstructure of multilayered structure . . . 33

5.12 Frame from high speed video . . . 34

5.13 Frames from high speed videos . . . 36

5.14 Frame from HSI of thin track . . . 37

5.15 High speed scanning: samples . . . 38

5.16 Powder redistribution around tracks . . . 38

5.17 Description of high speed video . . . 39

5.18 Comparison of various high speed videos . . . 40

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Abbreviations

Term Abbreviation

Additive Manufacturing AM

Selective Laser Melting SLM

Selective Laser Sintering SLS

Direct Laser Deposition DLD

High Speed Imaging HSI

Ytterbium Aluminium Garnet YAG Computer Aided Design/Drafting CAD Scanning Electron Microscope SEM

Frames per second fps

Computerised Numerical Control CNC

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1. Introduction

Selective Laser Melting (SLM) is an additive manufacturing (AM) process where complex structures can be generated directly out of metal powder using a CAD model. It allows almost infinite design freedom and is suitable for rapid manufacturing. The tool-less system enables every component manufactured to be unique, making the process very attractive in a world with short product life cycles.

AM technologies have been around since the 1980s, but their details are not very well understood. Over the last few years, they are gaining popularity for the manufacture of tooling, implants and other complex structures.

However, they are not conventionally used due to low efficiency and long process times. Understanding and improving the process could lead to increased efficiency, enabling us to use them in serial production.

High speed imaging (HSI) is a process of capturing videos at a very high frame rate. It can lead to gaining new insights about the process imaged.

There are very few documented attempts to carry out HSI of the SLM process.

The aims of this thesis are:

– To find SLM parameters for given powder Hastelloy X^TM that result in high efficiency and good surface finish.

– To better understand the SLM process through HSI - by visualising what exactly takes place on interaction of the laser beam with the powder bed. And to better the high speed imaging process itself.

– To compare how processes vary based on parameters selected.

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2. Theory

2.1 Laser Processing

The first laser was constructed in 1960, and the interaction of laser light with materials was studied as early as in 1963. Its properties, i.e. monochro- maticity, focussability, coherence, low divergence etc. make it special. [1]

The term laser is the acronym for Light Amplification by Stimulated Emis- sion of Radiation. The prerequisites for laser action are:

– Optical resonator

– Active gain medium (with a host medium) – Population inversion in the active medium – Means of excitation

The properties of the above, along with the beam transmission system decide the characteristics of the resulting laser light. The light can then be used in various ways to obtain di↵erent e↵ects in materials. The power, laser travel speed, pulse length, focal position etc. can be adjusted. The behaviour of materials on interaction with laser light varies based on both, the material and laser light properties. A shielding gas is often used to optimise the atmosphere.

During the experimental part of this thesis, a Nd:YAG, and Ytterbium Fiber lasers were used. They are both solid state lasers, i.e. the active medium is solid.

Laser light is used as a heat source in manufacturing. Today, lasers are widely used in the industry for welding, cutting, marking, various additive manufacturing processes etc. From table top systems to large industrial setups, laser systems are available with a wide range of specifications with

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varying wavelength, power, beam guidance systems etc. The most significant advantages of using lasers in manufacturing are the speed, efficiency, precision and ease of automation of the processes. Hybrid systems combin- ing conventional manufacturing methods with lasers are also used. Though lasers have been used in industries for decades, a lot of research is done in order to understand the interaction of laser light and matter.

2.2 Additive Manufacturing

Additive manufacturing is defined as the process of joining or adding material to make an object from 3D model data as opposed to conventional ma- chining, where material is removed to produce an object, i.e. subtractive manufacturing. It enables rapid manufacturing, repair and prototyping of parts that cannot be done through conventional processes, increasing the freedom of design ideas. AM is carried out with a wide range of materials including metals, ceramics, composites, polymers and biological systems.

[2, 3]

AM processes can classified broadly based on the energy source, material and the feed system.

Commonly used energy sources are:

– Laser

– Electron beam – Arc

Examples of material feed systems are:

– Powder bed systems: A thin layer of powder is layed down and the energy source melts or sinters areas as needed.

– Powder feed systems: Powders are fed via nozzles and a laser sourse is used to melt layers to create the shape desired.

– Wire feed systems: Wire is fed and melted using an energy source.

Material is deposited pass by pass to create the object required.

Although the above list is not exhaustive, they are used most often.

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AM has a huge impact not only on the way products are designed and manufactured, but also on material wastage, costs, logistics, post processing and the environment. Components can be custom made for the needs of customers without the need for changeovers and procedures that limit the variability of designs on conventional systems. Increasing the efficiency of AM systems can make costs comparable to conventional manufacturing.

Objects generated through AM go through a large number of thermal cycles with rapid solidification and directional cooling. This results in anisotropy, complex microstructures and, therefore, complex physical, me- chanical properties and surface characteristics. AM systems are used by the industry, but a lot of work is still done to understand the process. [2]

Selective Laser Sintering (SLS), Selective Laser Melting (SLM), Direct Laser Deposition (DLD) etc. are examples of AM processes. The basic di↵erences can be seen in figure 2.1 The SLM process is described in section 2.3.

Figure 2.1: Additive manufacturing processes[4]

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2.3 Selective Laser Melting

Selective Laser Melting is an AM process, that enables generation of near net shape objects out of metal powder. SLM is used widely in aerospace, biomedicine, and other engineering fields.

Figure 2.2: Objects manufactured using SLM

L: Turbine seal, C: 3D network, R: Dental implants [5, 6, 7]

The SLM process enables the manufacture of parts with almost infinite geometrical freedom, including undercuts and hollow objects. The resulting object is homogenous with a density of approximately 100%, since the powder is completely melted. This gives it properties identical to those achieved by conventional manufacturing methods. [8, 9] Some examples of objects manufactured by SLM can be seen in figure 2.2.

Using a software, the CAD model of the object to be created is sliced into layers. Metal powder is paved to create a powder bed (also known as pre- placed powder). These powders generally have a grain size of 10 µm to 45 µm and the layer thickness is variable up to 100 µm. A laser beam scans certain areas of this powder bed in an inert atmosphere, based on the CAD data, melting the powder in these areas. The platform is lowered and a new layer of powder is paved, and melted selectively. Partial remelting of the previous layer enables metallurgical binding. This process is repeated layer after layer. When the object is complete, it is removed from the work surface. Post processing such as hot isostatic pressing of parts can be done, if need be. The process is summarised in figure 2.3. [9]

The parameters that describe a particular SLM process are as follows:

– Laser power

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Figure 2.3: Schematic of SLM process [1]

– Laser travel speed – Layer thickness

– Spot diameter/ Spot size – Hatch distance

– Shielding gas/ Atmosphere

The laser scans with a di↵erent direction every layer, so that the product is homogenous through its entire volume. The above parameters can be seen in figure 2.4

Figure 2.4: SLM process parameters and layerwise structure[1]

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Since the model is reduced to a series of two dimensional images, the costs only depend on the volume of material melted, not the geometrical complexity. Hollow structures can be manufactured, reducing time and costs. SLM also allows customisation and individualisation of every single object manufactured. This is because the manufacturing process is ‘tool- less’, i.e. a common system is sufficient no matter what the design of the object is. Prototypes can therefore be made using SLM with the shortest lead time. [6]

The energy supplied to the material per unit volume is called the specific energy. It can be calculated using the following formula:

Specific energy = Power

Speed ⇥ Spot diameter ⇥ Layer thickness

The SLM process also has some drawbacks. The high temperature gradi- ents in each track gives rise to internal stresses. In contrast to the SLS process, full melting may cause dross formation and balling, resulting in bad surface finish. [10]

Objects generated through SLM undergo little or no post processing.

Therefore, the surface characteristics of the object are significant. Sur- face properties and efficiency of SLM processs are discussed in sections 2.3.1 and 2.3.2 respectively.

2.3.1 Surface Characteristics

The characteristics of the surface resulting from the SLM process is significant for two main reasons. First, as mentioned in section 2.3, good surface finish reduces the need for post processing. Second, the surface characteristics of each layer, a↵ects the properties of the next layer.

Some factors decide the behaviour of the molten material. Laser travel speed and how the powder melts, and joins the melt pool defines the splashing of the melt pool. This a↵ects the surface roughness. The next factor is the surface energy of the molten metal.

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Molten tracks during the SLM process tend to decrease their surface energy through surface tension. It leads to each track having a curved surface, increasing the unevenness of the layers built. This is known as the balling phenomenon, detrimental to the quality of the object generated. Balling causes high surface roughness, pores and could hinder the paving of the next powder layer. Small sized balling (spheres), however, can be ignored and are universally present in SLM processes.

The aggravation of balling e↵ect can be explained in the following process:

1. Paving of powder layer.

2. Laser scanning and melting of selected areas.

3. Surface tension- resulting in some balling 4. Solidification of tracks.

5. Paving of next powder layer, with uneven thinkness due to balling in previous layer.

6. Laser scanning of layer.

7. Aggravated balling e↵ect.

Paving of new layers could be obstructed when the height of the solidified track exceeds the layer thickness.

Li et al. studied the balling phenomena for nickel powder and listed factors that a↵ect it. Most of these factors influence the wetting ability and, as a result, the balling. They are listed below:

– Oxygen content: Oxygen content of less than 0.1% in the SLM atmosphere results in flat and smooth surfaces with harmless small sized balling. At 1% a di↵erence is seen and at 10% there is serious balling.

– Laser power: Higher laser powers increase the input energy, and therefore the wetting and spreading of the melt pool. This decreases the balling e↵ect.

– Laser travel speed: With increasing speeds, the width of tracks reduce implying worse wettability, flowing and spreading e↵ects, even- tually causing balling.

– Hatch distance: Increasing the hatch distance produces more irregu- larities. But no significant balling phenomenon is seen based on the

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hatch distance.

– Layer thickness: Thicker layers imply less energy absorbed per unit volume of the powder, resulting in a small wet area, and balling.

– Surface remelting: Remelting results in melting of most small spheres on the edges of the tracks, producing a smooth surface. [11]

From the above list, one can see that most factors influencing balling are connected to the quantity of energy supplied. Changes in the viscosity between solid and liquid states also influences the shape and smoothness of the tracks. [10]

2.3.2 Efficiency of the SLM process

Efficiency is calculated as the energy used for melting of the necessary volume divided by the total energy supplied. The efficiency of the SLM process is quite low, i.e. only a few percent of energy is used for the actual process.

Some of the losses encountered during SLM are listed below:

– Reflection of laser light

– Absorbtion of energy by the surroundings of the melt pool, powder particles

– Scattering due to particles – Overheating of the melt pool – Horizontal remelting

– Vertical remelting

Losses have to be minimised to make the process economically viable for large scale production. This thesis concentrates on reducing losses due to horizontal and vertical remelting.

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2.4 Superalloys

Superalloys are nickel-, iron-nickel-, and cobalt- based alloys used at high temperatures (above 540 C). Ordinary materials deform at considerably less loads at high temperatures (above 0.5⇥ Melting Temperature) than at room temperature. This also applies to creep and fatigue properties. Iron- , nickel- and cobalt- based superalloys are generally have a face centered cubic (fcc) crystal structure. This means, they have no ductile to brittle transition temperature. They derive their strength not only from their crystal structure, but also due to precipitation hardening, solid solution hardening and work hardening.

Superalloys are a very cost e↵ective way to achieve performance in high temperatures and stress conditions. They are, therefore, commonly used in industrial and aircraft turbines. Many superalloys are now available in a powder form with varying grain sizes for additive manufacturing. [12]

2.4.1 Hastelloy X

^TM

Hastelloy X^TM is a nickel-chromium-iron-molybdenum superalloy, invented in 1952 by Haynes International. It is known for its fabricability, corrosion resistance, high strength at elevated temperatures and resistant to hot petrochemical reactions. As a result of these properties, it is used in the combustion zone of gas turbines, industrial furnaces, flash dryer components etc. The nominal composition can be seen in Table 2.1. Due to its good ductility, it can be cold worked. It also is easily formed and can be welded using various methods. [13, 14]

Table 2.1: Composition of Hastelloy X^TM[13]

Element Ni Cr Fe Mo Co W C Mn Si B

Weight % 47** 22 18 9 1.5 0.6 0.1 1* 1* 0.008

*maximum, ** balance

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2.4.2 MAR M 247

^TM

MAR M 247^TM is a nickel based superalloy, initially developed by Martin Metals. It can be used at temperatures upto 1010 C in aeroengines. It has good corrosion and oxidation resistance. It often work hardens, because of its high strength and low thermal conductivity. This along with its high toughness makes it difficult to machine in comparison to other superalloys.

Its composition can be found in table 2.2. [15]

Table 2.2: Composition of MAR M 247^TM[16]

Element Ni W Co Cr Al Si Mn C

Weight % 66.16 - 9.3 - 9.7 9 - 9.5 8 - 8.5 5.4 -5.7 0.25 0.1 0.06-

68.24 0.09

2.5 High speed imaging

High speed imaging (HSI) is the process of taking pictures very fast. There are two kinds of HSI: First, to take a photograph so as to freeze the motion and obtain a clear image. And second, to take a large number of photographs in a short period of time, resulting in a video. The videos are later played at a low frame rate, to view the recording in slow motion.

The first application of HSI was by Eadwaerd Muybridge in 1878. Tech- nology has since then developed, enabling us to have extremely high frame rates. The types of HSI cameras are: intermittent motion cameras which use films, rotating prism cameras and rotating mirror cameras. With the development of electronics, frame rates of over 100,000,000 frames per second could be reached by the 1960s.

Today’s cameras have a large internal memory, enabling them to capture and save long videos. The size of the video depends on the number of pixels captured. For a given frame rate, longer videos can be captured, when the region of interest is small.

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Correct illumination is crucial for good quality HSI, especially while filming processes that produce a high amount of light. A popular method is to use a monochromatic light source to illuminate the region of interest. A narrow band pass optical filter is used, so the camera only captures light with a narrow bandwidth. Diode lasers are a common source of illumination, since the wavelength of light produced is di↵erent from that produced by commonly used fiber laser systems. The light source is generally pulsed with the same frequency as the camera frame rate, and synced with the camera software to provide illumination at exactly the same points of time when the shutter of the camera is open.

The aperture, exposure time, frame rate and focus of the camera have to be adjusted to record an optimal video. A common problem while filming laser processing is that the melt pool is extremely reflective. Therefore, the right angle for the illumination and camera have to be chosen.

The aperture size is described by the f-number (ratio of the focal length to the e↵ective aperture diameter). Smaller the f-number, larger is the diameter of the lens opening, allowing more light to reach the sensor.

Larger the opening, smaller is the depth of field.

HSI can provide photographic evidence of phenomena that cannot be observed under ordinary circumstances. Examples are melting or boiling of material, plume behaviour etc. However, it can be challenging to interpret the videos. [17, 18]

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3. Equipment and Materials

The experimental work for this thesis was carried out at the labs at Lule˚a University of Technology, Lule˚a, Sweden and Fraunhofer IWS, Dresden, Germany.

The equipment used during the course of the thesis are listed below.

– Powder characterisation

– Scanning Electron Microscope – Building of Tracks

– Nd:YAG laser, workstation and optics

– Ytterbium fibre laser, isel workstation and optics – High speed ytterbium fiber laser scanner

– Plexiglass environment container

– Scrapers (to spread powders at pre-determined thicknesses) – 0.1 mm thick frame

– Ruler

– High Speed Imaging

– Redlake Integrated Design Tools NR4-S2 high speed camera – Photron Fastcam SX series high speed camera

– Nikon Micro-Nikkor 200 mm lens – Nikon Micro-Nikkor 105 mm lens

– DHG Macro Achromat 3dpt 67 mm Magnifying lens

– Cavirar Cavilux HF diode laser light source (808 nm wavelength, 500 W)

– Narrow band pass filter (808 nm)

– Lamps (wide band illumination source) – Cables

– Magic arms and clamps – Tripods

– Software: Motion studio, PVF Photron and Cavilux

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– Sample preparation

– Cutting machine: Struers Discotom-100

– Automatic Mountic Press: Buehler SimpliMet-1000 – Circular grinding machine: Buehler MetaServ-250 – Elliptical polishing machine: Struers LaboPol-5 – Electrolytic etching equipment: Struers LectroPol-5 – Sample imaging

– Nikon stereo microscope and software – Optical microscope

– Digital cameras – Vernier calipers – Safety

– Safety goggles – Nitrile gloves

– Half face particulate mask and filters

The consumable materials used during the thesis are as follows:

– Powder characterisation

– Double coated carbon conductive tabs – Building of Tracks

– Hastelloy X^TM powder

– MAR M 247^TM powder: Coarse, fine – 4mm carbon steel plate (substrate) – Argon (Shielding gas)

– Ethanol

– Sample preparation

– Cutting wheel: Struers 300 mm 40A30 – Phenolic resin powder

– SiC grinding paper, grit: 60 µm to 6.5 µm – Polishing disks

– Buehler diamond slurry, grit: 3 µm to 0.25 µm, MasterMet – Ethanol

– 5% Sulphuric acid solution in distilled water – 10% Oxalic acid solution in distilled water

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4. Method

This section describes the procedures used to carry out all experimental work, sample preparation and analysis.

4.1 Powder Characterisation

Hastelloy X^TM

The Hastelloy X^TM powder was imaged using a Scanning Electron Micro- scope (SEM) to get an idea of the size and surface properties. The powder was applied on a double coated carbon conduction tape and attached to the SEM mount. The SEM was used in the secondary electron mode with high vacuum and 15 V acceleration voltage. Focus, brightness, contrast and astigmatism were adjusted to obtain clear images.

MAR M 247^TM

The MAR M 247^TM powders were not characterised using an SEM. Instead, the data provided by the suppliers were considered. Two powders with di↵erent grain sizes were used.

4.2 SLM and HSI

SLM and HSI was carried out with three experimental setups. They are listed below and later discussed in detail.

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– Setup 1: Interaction of single laser pulses with powder – Setup 2: Low speed SLM

– Setup 3: High speed SLM

Nitrile gloves and a particulate mask were used at all times while handling powder. Safety goggles were used when the laser was active.

4.2.1 Setup 1: Interaction of Single Laser Pulses With Powder

Hastelloy X^TM powder was melted using 1 ms long laser pulses. A ytterbium fiber laser was used for these experiments. The main goal of these experimental trials were to observe the behaviour of powder on interaction with the laser beam, and to attempt HSI with a wide bandwidth illumination source.

A setup, using a Plexiglas environment container, held the substrate with the powder during the procedure. Argon was supplied via a tube fitted to the inside of the container. Since argon is heavier than air, it fills the container bottom-up, providing an inert atmosphere.

Since a 5 mm thick powder bed was used, no interaction with the substrate was expected. The amount of melt, its basic appearance and spatter were observed. To protect the sensitive optics from spatter, a pressurised air wall aka crossjet was used. The substrate was raised such that the crossjet was outside the container, so that the argon atmosphere was left undis- turbed. Trials were carried out with di↵erent laser powers (between 1.5 kW and 3 kW) and spot diameters (between 0.6 mm and 1 mm).

HSI of Setup 1

A Redlake IDT NR4-S2 high speed camera was used with a Nikon AF Micro-Nikkor 200 mm lens and a narrow band filter (for wavelength 808 nm). The telephoto macro lens allowed macro imaging with a large working distance, i.e. 200 mm. The camera was positioned to have an unob-

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structed view of the substrate.

The most significant feature of the HSI of this setup was that wide bandwidth illumination lamps were used. The setup can be seen in figure 4.1.

Figure 4.1: HSI setup for single laser pulses

Since the lamp can stay on for long periods of time, no syncing was required with the camera software. Combinations of di↵erent apertures, exposures, frame rates and number of lamps were attempted. Up to 3 lamps were used simultaneously.

Apart from illumination and focus, aperture and exposure are the two main settings that can be varied for imaging. Aperture sizes between f/11 and f/22 were chosen on the lens. Exposure times were varied between 10 µs and 200 µs.

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4.2.2 Setup 2: Low Speed SLM

Tracks and layers were built using the continuous mode of the Nd:YAG laser. The material was Hastelloy X^TM with 4 mm thick carbon steel plates as the substrate. The Plexiglas environment container described in section 4.2.1 was used in this setup. The substrate was raised such that the lens was outside the container, to avoid collisions. The basic setup can be seen in figure 4.2a.

(a) Basic setup for building tracks (b) HSI setup for tracks

Figure 4.2: Setups for building tracks

To pave a layer, some powder was scooped onto the substrate and spread evenly using specially made scrapers for various layer thicknesses (0.1 mm to 1 mm). The scraper was held perpendicularly such that both ends were in contact with the substrate, and moved along the length necessary. A schematic diagram of a scraper can be seen in figure 4.3.

To pick a certain starting focal position, tracks were built with constantly increasing focal distance. The track begins with the focal point below the surface and ends with the focal point a few cm above the surface.

This allows for one to pick a suitable portion of the track, and find a focal position that is likely work. A defocussed beam with spot diameter between 1.6 mm to 3.3 mm was used to build the tracks.

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Figure 4.3: A scraper - to pave x mm thick powder layer

For each track, the power (between 600 W and 2 kW), scanning speed (between 17 mm/s and 117 mm/s), track length and argon flow rate (up to 20 L/min) were chosen and entered into the CNC system. The focal position was adjusted by altering the position of the lens (z-axis of the CNC system). A guide laser helped in adjusting the start position of the laser beam. The position of the optics remained constant, and the table was moved to build the tracks. Around 80 di↵erent sets of parameters were attempted, along with HSI of many.

To build layers, multiple tracks were built next to one another. A track was built, the table was moved by a predetermined distance (hatch distance) and the next track was built.

HSI of Setup 2

For HSI of low speed SLM, the same camera and lens assembly mentioned in section 4.2.1 was used. A high frequency diode laser with wavelength 808 nm was used for illumination. The laser was synced with the camera such that a the area of interest was illuminated when the camera shutter was open. Identical frame rates and exposure times were selected on both software (Cavilux for the diode laser and Motion Studio for the camera).

For some recordings, an additional magnifying lens was used to magnify the image. The setup can be seen in figure 4.2b.

The initial camera settings, e.g. choosing the area of interest and focus adjustments were carried out using low light settings. Low light settings

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include a low frame rate/ frequency of illumination laser (around 50 Hz) and the maximum possible exposure time. The aperture was chosen as necessary.

The maximum frame rate possible (⇠6000 fps) was chosen. Exposure time of 3 µs and aperture size f/16 was used in most cases. The maximum recording time for the chosen settings was ⇠1.9 s. The syncing was done such that the camera software was controlled by the illumination laser software. When completed, the necessary frames were chosen and saved.

The videos were analysed later using Motion Studio. The brightness, contrast, gamma correction and playback speed could be varied to optimise the video.

4.2.3 Setup 3: High Speed SLM

A high speed fiber laser scanner was used to build tracks using powder MAR M 247^TM. The aim of these trials was to observe the powder and melt behaviour while using powder with di↵erent grain sizes, powers and laser travel speeds through HSI. The speeds achieveable in this setup were much higher than those possible in setup 2.

The beam had an elliptical shape and at focus the length of the major axis was 0.25 mm and that of the minor axis was 8 µm. The beam was oriented such that the major axis was along the direction of scanning. This means that very thin tracks were built.

The powder layers were built using a 0.1 mm thick aluminium frame and flat scrapers. Some powder was placed on the substrate, a 4 mm thick carbon steel plate, and spread using the scraper. This arrangement can be seen in figure 4.4. For each powder, a combination of powers (between 200 W and 2 kW) and speeds (between 10 m/s and 300 m/s) were attempted with a focussed beam. No protective environment container was used during these experiments, since it was an open setup as seen in figure 4.5. Each track was scanned four times in the same direction by the laser.

In this case, the position of the substrate was constant, while the beam moved. Melting of powders and their attachment to the base material were

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Figure 4.4: Powder paving arrangement checked for.

HSI of Setup 3

The HSI was carried out using the Photron Fastcam SX with a Nikon 105 mm macro lens. The illumination source was a diode laser, like the one described in section 4.2.2. A frame rate of 100,000 fps was used. This was possible due to the higher internal memory of the camera and a smaller area of interest in comparison to setup 2. Since the frame rate is so high, the exposure time was chosen to be 0.2 µs.

The camera and the illumination laser were synced and arranged appro- priately. The process could be viewed through the side of a machine, so the imaging was done from the side, unlike setup 2. The area of interest was around 7 cm wide and the path where the track was built was in focus.

The HSI setup for these experiments can be seen in figure 4.5.

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Figure 4.5: HSI setup for high speed laser scanner

4.3 Sample Preparation

All samples from Setup 2 were prepared at the sample preparation lab at the material science department, LTU.

The steel plates with the SLM tracks were cut at the appropriate locations.

The pieces were mounted in phenolic resin using a mounting press. The samples were ground using a circular wet grinding machine using grinding papers from grit size 60 µm to 6.5 µm in 4 to 5 steps. Before each step, the sample was washed under running water and turned 90 degrees. Each sample was ground for ⇠30 seconds per sheet. Further, the samples were polished on an elliptical polishing machine with a lubricant and diamond slurry with grit size 3 µm, 1 µm and 0.25 µm. The polishing was completed using a final polishing suspension called MasterMet. Electrolytic etching was done with either a 5% solution of H2SO4 in distilled water or a solution of 10 g of Oxalic acid in 100 ml of distilled water at 6 V for 2 s.

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4.4 Analysis

The videos captured in setup 1 were played back at a very low frame rate (3 fps to 5 fps). Optimal imaging settings were identified through analysis of the videos.

The samples from Setup 2 were analysed by visual inspection, where the surface finish was evaluated subjectively. A cross section of every sample was analysed by stereo microscopy. The track height and width were measured using tools in the microscope software. Microstructure of selected samples were photographed using an optical microscope.

Specific energy was calculated for every trial from setup 2 and an attempt was made to find a range of values that produced the best results, i.e.

sufficient melting while avoiding deep vertical remelting.

Samples from setup 3 were only analysed by visual inspection and checked for attachment of powder to the substrate.

Videos captured in setup 2 and setup 3 were played at a low frame rate (5 fps to 25 fps). Details about each process were noted and comparisons were made for di↵erent process parameters. Videos from setup 3 were compared to find the influence of the chosen parameters and powders.

Overall di↵erences between videos captured in setup 2 and setup 3 were also observed.

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5. Results and Discussion

The observations and results from all the experiments conducted are discussed in this section. Each setup is discussed separately followed by a comparison of videos captured in setup 2 and setup 3.

5.1 Powder Characterisation

One of the obtained SEM images of the HastelloyX^TM powder can be seen in figure 5.1. The grains were mostly spherical and smooth with diameters between 10 µm and 40 µm.

Figure 5.1: SEM image of Hastelloy X^TM powder

The characteristics of the MAR M 247^TM powder used were provided by the supplier. The fine powder grains had diameters between 25 µm and

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45 µm, while the coarse powder grains had diameters between 47 µm and 80 µm. Both powders had spherical grains.

5.2 Setup 1: Interaction of Single Laser Pulses with Powder

Powder is melted in all cases of laser power between 1.5 kW and 3 kW (with spot diameter 0.6 mm to 1 mm). When increasing laser power, a drilling e↵ect was seen, i.e. the melted material was found in deeper pits on the powder bed. Spatter also increased with increasing power. Some oxidation was observed. An image of the samples obtained can be seen in figure 5.2.

Figure 5.2: Samples from the single laser pulse trials

With longer exposure times and larger apertures, the images are too bright due to the process light though a narrow band filter was used. Short exposure times and smaller apertures resulted in darker videos. In none of the videos, the powder bed or its behaviour on interaction with the laser beam could be seen. However, splashing of melt was observed in videos captured with low exposure times (between 10 µs and 40 µs) and moderate

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aperture size (f number between 11 and 16).

Images from a video captured can be seen in figure 5.3. The time between consequent frames is 0.14 ms. The bright areas seen on the images in figure are molten metal. A plume can be noticed in frames (iii) to (xii).

The laser radiation also contributes to the brightness in these frames. In frames (xiii) to (xv), a drop of molten metal is seen moving away from the melt pool. Drops like these form the spatter. As the melt solidifies, it gets darker on the video and nothing is visible when it has solidified.

Videos captured for di↵erent powers have minute di↵erences. The quantity of melt splashing increases with power, though not much can be seen on the videos due to the drilling e↵ect. This can be noticed in videos captured at rates higher than 7000 fps and played back at lower than 3 fps.

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Figure 5.3: Images from a high speed video captured for 3 kW, 1 mm defocussed beam captured with frame rate 7000 fps, aperture 16 and

exposure time 40 µs

5.3 Setup 2: Low Speed SLM

At various layer thicknesses, parameters that resulted in melting of powder and attachment with the substrate were found. Further, for 0.1 mm thick layers, parameters for high efficiency and low surface tension were found.

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5.3.1 Layer Thickness: 1 mm

For 1 mm thick powder layers, sufficient melting and attachment was seen at power 1.5 kW, speed 25 mm/s and spot diameter 2.5 mm. Layers were built using these parameters with hatch distance is 2 mm. Further decreasing the speed to 17 mm/s and defocussing to a spot diameter of 3.3 mm improved surface tension characteristics. Increasing the power to 2 kW resulted in a large plume, and was not used to avoid equipment damage. No suitable parameters resulting in flat tracks with low surface tension could be found within the limits of the equipment.

(a) Melted tracks (b) Result of low power density

Figure 5.4: Top view of samples built using 1 mm powder layer thickness Some tracks built can be seen in figure 5.4. In figure 5.4a, most tracks were melted and attached to the substrate. Figure 5.4b shows the result of insufficient power where there is no melting of the substrate material.

While building some layers, refilling of powder was necessary owing the reduction of volume on melting, which empties some space on both sides of the track. Attempts made to build layers without a powder refill resulted in uneven structures. An example of a layer built and its cross section can be seen in figures 5.5a and figure 5.5b respectively. In figure 5.5b pores can be seen between each track. The possible reason for this is spatter and small balling combined with insufficient power for remelting while building the next track.

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(a) Top view of layer (b) Cross section of layer built

Figure 5.5: Layer built with parameters: layer thickness 1 mm, power 1.5 kW, speed 25 mm/s, spot diameter 2.5 mm, hatch distance 2 mm with

powder refill

Figure 5.6: Cross section of multilayered structure built with power 1.5 kW, speed 20 mm/s, spot size 3.3 mm and hatch distance 2 mm

5.3.2 Layer Thickness: 0.5 mm

For layer thickness 0.5 mm, flat and wide tracks could be built at power 1.5 kW, speed 20 mm/s and spot diameter 3.3 mm. A 3 track, 3 layered structure was built with these parameters, whose cross section can be seen in figure 5.6. Single layers built with these parameters had a resulting thickness between 0.65 mm to 0.8 mm, implying that the next layer of powder layed out was less than 0.5 mm thick, explaining why the structures built were uneven.

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5.3.3 Layer Thickness: 0.1 mm

Most of the tracks were built using layer thickness of thickness 0.1 mm. At power 600 W, spot diameter 2.5 mm and speeds as low as 25 mm/s balling and very little attachment can be seen. A number of optimal parameter sets were obtained with powers higher than 900 W. Images of some of the samples can be seen in figure 5.7.

Figure 5.7: Top view of some tracks built with powder thickness 0.1 mm Parameters were chosen based on track shape (from the cross sections), height and width. Track height was measured from the surface of the substrate to the top of the track. Wide, flat and smooth tracks with height closest to the layer thickness were preferred. This results in minimal horizontal and vertical remelting, and thus, high process efficiency. The smooth and wide tracks seen in the top-right corner of figure 5.7 had the best surface characteristics, but had 1.5 times the desired track height.

This means that the next layer built will not have the same properties.

The best parameters found are listed in table 5.1.

Cross sections of the tracks built using parameters from table 5.1 can be found in figure 5.8.

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Table 5.1: Optimal parameters for 0.1 mm thick powder layer Number Laser power Speed Spot Track height Track width

Diameter

(W) (mm/s) (mm) (µm) (mm)

1 900 25 3.3 107 1.9

2 1200 100 2.5 97 1.617

3 1200 67 3.3 104 1.9

(a) Track 1 (b) Track 2 (c) Track 3

Figure 5.8: Cross sections of samples built with optimal parameters for 0.1 mm thick layers

From the cross sections, it can be seen that there is always a clear sep- aration between the substrate and track materials. This is a feature of the powderbed process. More vertical remelting is observed in track 1 and track 3 compared to track 2. Some degree of vertical remelting is necessary for good attachment between layers and can only be selected once multilayered structures are built and tested.

Some cross sections of samples built with powers higher than 1.5 kW showed cracks in the body. One example is with parameters power 2 kW, scan speed 117 mm/s and spot diameter 3.3 mm. Though these parameters resulted in very flat and wide tracks, they cannot be used. The cross section can be seen in figure 5.9. More experiments would have to be carried out to find optimal parameters at powers higher than 1.5 kW.

A multilayered structure was built using the following parameters: power 900 W, speed 42 mm/s, spot size 2.5 mm and hatch distance 1.5 mm.

An image of the sample can be found on the top-left corner of figure 5.7 and its cross section can be seen in figure 5.10. The first layer was built

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Figure 5.9: Cross section showing crack formed

with 6 tracks, the tracks on each layer were placed betwen the tracks of the previous layer. Therefore each succeeding layer had one track less than the previous. Boundaries between the layers can be identified in the figure.

Also, no cracks or pores are seen in the cross section.

Figure 5.10: Cross section of 5 layered sturcture built from 0.1 mm thick powder layers

The microstructure of the multi layered structure can be seen in figure 5.11. In figure 5.11a, some boundaries between individual tracks can be seen. Figure 5.11b shows the microstructure under a higher magnification.

The presence of both, long needle like grains and small rounded grains can be seen. The microstructure continues through the structure, showing recrystallisation of a significant volume of the material everytime a track is added.

Remelting of individual tracks and remelting between tracks were attempted in order to improve the surface characteristics of the tracks and to reduce surface tension. No significant benefit was observed. A decrease in surface tension and plume was seen on increasing the argon flow time.

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(a) Boundaries between tracks (b) Microstructure

Figure 5.11: Microstructure of multilayered structure

However, no quantitative results can be stated, since the oxygen content in the container was not measured. In general, a defocussed beam produced better results in terms of surface tension than a focussed beam.

On comparison with typical SLM machines, the obtained results use larger layer thicknesses, spot diameters and powers. This is due to the limits of the equipment used. There is a possibility to use these parameters to build the insides of structures to reduce build time.

5.3.4 Specific Energy

The energy supplied per cubic millimeter, i.e. the specific energy was calculated for all tracks built with layer thickness 0.1 mm. The values of specific energy ranged from 38 J/mm³ to 171 J/mm³. However, the attempt to find a range of values that always resulted in sufficient melting was not successful.

The same specific energy produced di↵erent results when di↵erent parameters were used. A trend can be spotted when one of the parameters was varied, keeping the others constant. For example, increasing the power while keeping the other three parameters (speed, layer thickness and spot diameter) constant resulted in increased melting and deeper tracks. For a particularvalue of specific energy, high powers and high speeds produced

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better significantly better results than low powers and low speeds. Exam- ple: for specific energy 60 J/mm³, power 600 W, speed 42 mm/s and spot diameter 2.5 mm did not produce a continuous track, while power 2 kW, speed 100 mm/s and spot diameter 3.3 mm resulted in a flat and wide track.

5.3.5 HSI of Setup 2

The best results for the HSI was achieved on using a macro magnifying lens, which was the only case where details could be seen. For the chosen area of interest, the highest achieveable frame rate was around 6000 fps.

Figure 5.12 shows a single frame from one of the high speed videos captured. The parameters used were: power 1200 W, speed 42 mm/s and spot diameter 3.3 mm. A solidified track can be seen on the left side, fresh powder on the right and the melt pool in the centre. On the right side, rearrangement of powder can be noticed, as well as some spheres of molten metal. Drop incorporation into the melt pool can also be seen.

Figure 5.12: Frame from high speed video of SLM process

On interaction with the laser beam, the powder grains move to redistribute themselves on the substrate or the previous layer. They seem to move together and melt to form a droplet of molten metal. This can be accounted for by the reduction of volume on melting. The droplets are incorporated

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into the melt pool when the laser beam moves in relation to the substrate.

On either side of the track, small balling can be seen.

Di↵erences can be seen in videos when the powers and speeds are varied.

Frames from videos of two tracks being built can be seen in figure 5.13.

In figure 5.13a, a process with insufficient power can be seen, while figure 5.13b shows images from an optimal process. The frames in the figures were captured approximately 0.83 ms apart.

In both figures, rearrangement of powder on the powder bed can be seen, followed by melting into small droplets. Often these droplets move and join others forming larger droplets.

In the case with insufficient power, i.e. figure 5.13a, there is no real melt pool. The parameters used were: power 600 W, speed 42 mm/s and spot diameter 2.5 mm. The droplets join together to grow, and when the surface tension can be low enough, they solidify at their position. Sometimes they attach to the substrate, and might also solidify without any attachment.

When sufficient power is supplied, a melt pool can be clearly seen, as in figure 5.13b. The parameters used were: power 2000 W, speed 7000 mm/s and spot diameter 3.3 mm. The laser beam melts the substrate and powder in its area of influence. On the edge, we see redistribution of powder etc.

as discussed above. Some droplets may combine to form larger droplets before joining the melt pool. Some droplets join the melt pool from the sides occasionally. Ripples can be seen when powder or droplets join the melt pool.

In many cases, enough power is supplied to create a melt pool but insufficient to melt a wide track. This happens because the laser beam used has a gaussian profile, and the melting takes place in the centre of the beam. The results are thin and uneven tracks, with small balling on the edges. These can be observed in the high speed videos, a frame can be seen in figure 5.14. Parameters used in this case are: power 1.5 kW, speed 100 mm/s and spot diameter 3.3 mm. These are not optimal for both efficiency and surface characteristics.

On comparision of videos where the substrate was moved with di↵erent speeds, some observations about the melt pool can be made. At higher

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(a) Process with insufficient power (b) Process with sufficient power

Figure 5.13: Frames from high speed videos

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Figure 5.14: Frame from HSI of a thin track

speeds, more disturbances and ripples are seen in the melt pool. This influences the surface characteristics of the solidified track. At low speeds, tracks are smoother.

5.4 Setup 3: High Speed SLM

Among the seven sets of parameters attempted for both the fine and coarse powder, only a few resulted in melting. For the fine powder, melting was only achieved at speeds lower than 10 m/s, and powers higher than 1.5 kW. Melting could be seen starting at power 800 W and speed 10 m/s for the coarse powder.

A possible explanation is heat is conducted better by the coarse powder.

The average grain diameter is approximately double that of the fine powder. Energy is lost during conduction between grains, which are fewer per unit volume of the coarse powder. Thus, more energy is used for melting in case of the coarse powder, resulting in melting at lower powers.

Since the material MAR M 247^TM has a lower thermal conductivity than HastelloyX^TM, the parameters cannot be compared.

Since the maximum power was 2 kW and minimum speed was 10 m/s and

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the experiments were carried out with a focussed beam, no further trials were possible. All the tracks can be seen in figure 5.15.

Figure 5.15: Tracks built using the high speed scanner

Parameter sets with higher speeds (above 100 m/s) or lower powers (below 800 W) resulted in some sintering of the powder. There was no attachment with the substrate, but some changes in the appearance of the powder can be seen along the path scanned. This can be seen in figure 5.16. In the figure, small and dark metal bits can be seen. These were formed by the powder material which melted but did not attach to the substrate.

Figure 5.16: Powder redistribution around the tracks. Wide area cleared at power 1.5 kW and thin line at power 800 W for fine powder at speed

10 m/s

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5.4.1 HSI of Setup 3

HSI of the process revealed details about the process of a laser beam with a small spot size scanning a powderbed with high speeds. The minimum speed used was almost 100 times the maximum speed used in setup 2, while the range of powers remained the same. Even with a frame rate of 100000 fps, it was difficult to see details.

Melt behaviour could not be observed in any of the cases, but the behaviour of the powder on interaction with the laser beam was visible. This was especially the case wih the lowest speed attempted, i.e. 10 m/s. At higher speeds (100 m/s and 300 m/s), no details could be observed.

The basic process was similar in all cases. When the laser beam scans the powder, a small quantity powder is melted onto the substrate and the rest is displaced. The displaced powder is kicked up due to the energy supplied and some of it forms clusters or agglomerates. When the powder has settled down, the path can be observed. These can be seen in figure 5.17. The time taken, quantity of powder displaced and width of the track vary based on the parameters used, and are discussed in this section.

(a) Laser beam path (b) Agglomerates of sintered powder

(c) Result

Figure 5.17: Description of clips from the high speed video captured for the fine powder at power 1.5 kW and speed 10 m/s

Figure 5.17a shows the path followed by the laser beam. It is a frame where the beam has passed 3/4^th the width of the area of interest. Figure 5.17b shows some clusters of powder grains formed and displaced during the process and figure 5.17c shows the end result. As discussed above, the substrate was una↵ected in many experiments.

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(a) 2 kW, 10 m/s, Coarse powder

(b) 2 kW, 10 m/s, Fine powder

(c) 800 W, 10 m/s Coarse powder

Figure 5.18: Comparison of videos using di↵erent powders and powers during similar stages

Figure 5.18 compares videos captured at 100000 fps at speed 10 m/s using

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the coarse and fine powder at 2000 W and 800 W. Clips in the figure were captured at 0 ms, 1.9 ms, 10 ms, 60 ms and 80 ms.

Comparison of figures 5.18a and 5.18b reveals that the same parameters produce di↵erent results in the coarse and fine powders. The fine powder is displaced more than the coarse powder, and forms larger clusters of sintered material. Figure 5.18c shows a process with power 800 W, much lower than 2 kW used in the other two figures. Clearly, the results are not as striking. The same process can be seen, but on a smaller scale. This particular trial resulted in a thin track being melted and can be observed in the last clip shown.

The videos revealed that the quantity of powder displaced and the size of clusters were always higher for the fine powder. The width of the track cleared increased with increasing laser power and decreasing speed. At speeds above 100 m/s, redistribution of powder was negligible and any clusters seen in the fine powder trials were no larger than 3 to 5 grains.

5.5 Low Speed vs High Speed SLM

Di↵erences between processes in setups 2 and 3 lies in the laser travel speed and spot size of the beam. Setup 2 used low speeds and larger spot sizes, while setup 3 used high speeds and a small spot size.

Experiments in setup 2 resulted in melting of powder and formation of a melt pool. Powder displacement was close to the surface and in small steps.

Little sintering was noticed close to the edges of tracks. Displacement of powder in the vertical direction was not noticed, however molten droplets seemed to ‘jump up’ in early stages of interaction with the laser beam.

Setup 3 displayed a di↵erent dynamic altogether. In the videos captured no melt pool was seen. Powers possible were also insufficient for the given speeds and layer thickness, albeit the small spot size. A large percentage of the powder grains were displaced, clearing a track along the path of the laser beam. When the power was high enough, melting and sintering was noticed. For successful melting with this setup, thinner powder layers or

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higher powers are recommended. Better conclusions could be made from high speed videos captured at frame rates higher than 100000 fps.

5.6 Sources of Error

Identified sources of errors include:

– Paving powder layers: In setup 2 scrapers were designed to pave even layers of powder, however a deviation as small as 5 while using the scraper could a↵ect the thickness of the layers. Similar inci- dents could e↵ect the powder layer thickness in setup 3. The coarse powders were more difficult to pave evenly.

– Argon atmosphere: The Plexiglas environment container used was an open setup. The argon content could di↵er through the course of the day, causing discrepancies in the melt behaviour. To avoid this, argon was allowed to flood the container for a a minimum of 5 minutes before the first experiment each day.

– Cross sections: Only one cross section was made for most samples.

Therefore, no generalised values of track height and width can be stated.

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6. Conclusions

– HSI of the SLM process using wide bandwidth illumination sources does not produce good videos. To see details of the powder behaviour and the melt pool, a wide bandwidth source would have to be extremely bright, leading to an uncomfortable working environment.

– Multiple parameter sets were found for each layer thickness.

Number Layer thickness Laser Power Speed Spot diameter

(mm) (W) (mm/s) (mm)

1 0.1 900 25 3.3

2 0.1 1200 100 2.5

3 0.1 1200 67 3.3

4 0.5 1500 20 3.3

5 1 1500 17 3.3

– A single value of specific energy that results in flat and wide tracks does not exist. High powers and high speeds result in higher degrees of melting than low powers and low speeds for the same specific energy.

– Details of the low speed SLM process were revealed by HSI. Three main steps were identified: redistribution of powder, melting and drop incorporation into the melt pool. Influences of process parameters were seen in the high speed videos.

– HSI of high speed SLM proceses shows di↵erent powder behaviour compared to the low speed processes. The powder was displaced in the vertical axis and sintering was seen. Frame rates greater than 100000 fps are necessary to gain more knowledge about the process.

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7. Future Work

Further attempts at HSI of SLM processes with frame rates around 200000 fps are suggested. This would give insights into the melt behavior, and therefore, better understanding of the process.

Building structures with HastelloyX^TM in conventional SLM machines using parameters determined in this thesis would help determine their valid- ness.

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