Chapter 3 The Processing Science of AM and the Influence on Material Properties
3.2 Process Parameters
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3.2 Process Parameters
(i) Surface roughness and feature size of products
In additional manufacturing industry, surface roughness, geometrical accuracy and feature size of products are typical concerns for equipment operators. But they shouldn’t be over-emphasized since post processing and machining usually takes place for the products. The minimum feature size is determined by the minimum size of the heat source and the size of feedstock. Table 1 summarized the data for PBF and DED processes.
Table 1 Typical layer thickness and minimum feature size or beam size for PBF and DED processes
Process Typical layer thickness/µm Minimum feature size or beam size
PBF-SLM39 10-50 75-100
PBF-EBM 50 100-200
DED-Powder fed 250 380
DED-Wire fed40 3000 16000
As can be seen from table 1, the resolution is high to low in the following sequence: SLM, EBM, powder fed-DED, wire-fed DED. The resolution of SLM, EBM also depend on the operating parameters. Due to the finer feedstock of powder fed than wire fed, powder fed DED has better resolution. Compared with PBF, DED is usually used to fabricate components with larger dimension and simpler geometry due to the relatively large feature size. There is a tradeoff between feature size and deposition rate. 17,25,41
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Surface roughness dependent on the orientation in the building process. The vertical surfaces of parts are rougher than the horizontal ones since 3D printing build components layer by layer.
(ii) Build chamber atmosphere
The atmosphere under which the parts were fabricated could influence the chemistry and heat transfer. As discussed in chapter 2, the SLM processes usually use inert gas like Argon or nitrogen filling the environment. The inert atmosphere could provide protection to the melt metal powders. The EBM processes happening in vacuum because of the accelerated electrons has high energy. To reduce the building up of charges in powder particles, a small quantity of helium is injected to the environment. The near vacuum environment result in the increase of melt vapourisation. DED processes are operated under the shield of gas flowing over the metal surface or under an inert gas atmosphere.
(iii) Feedstock quality
The quality of feedstock in the AM processes has an important impact on the final products.
The quality of the powders is determined by the shape, size, morphology, composition and internal porosity. And it affects physical properties such as followability, apparent density.
The techniques used to produce powders impacts the qualities directly. The techniques include gas atomization, rotary atomization, plasma atomization, plasma rotating electrode process.
Among them, plasma atomization and plasma rotating electrode process could produce powers in highly spherical shape and with smooth surfaces. Rotary atomization yields powders with irregular shape. Gas atomization tend to produce powers with a large amount of satellite. In addition, during gas atomization, the inert gas could be trapped into the powders and cause porosity. 17,42
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The recycled powders need to take chemical composition measurement due to the evaporation loss, reaction with oxygen, nitrogen or other gases and contamination during the production process. Tang et al. have researched on the effect of powder reuse times on the AM of Ti-6Al-4V by EBM and found that oxygen content increased from 0.08 to 0.19% by weight, while aluminum content decreased from 6.47 to 6.37% and vanadium content decreased from 4·08 to 4·03% over 0 to 21 reuse cycles.43
3.3 Defects in Metal AM parts
The common defects that could be found in metal AM parts are porosity, balling, cracking, delamination, swelling and residual stress. These defects can be caused due to improper way of manufacture, improper processing parameters or the defects existing in powders prior to fabrication.
Porosity can be powder induced, process induced or happens during solidification. The pores formed are different and are differentiated in Fig. 3.643 Based on the studies, porosity formation is mostly caused by processing, so the process parameters need to be properly set up to avoid creating pores.
Process-induced porosity refers to the pores formed by processing technique, formed when applied energy is not enough for sufficient melting metal powders or the applied energy is too high and result in spatter rejection. When not enough applied energy focused on the powders, lack of fusion occurs. When the applied energy is too high, spatter ejection may occur, which is known as keyhole effect.
Balling forms due to insufficient wetting of the preceding layer and surface tension where molten metal forms spheroidal beads.44 It obstructs the continuous melt lines, forming rough and
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bead-shaped surfaces. More intense extent of balling can aggravate in subsequent layers and the layers are overcrowded with large metallic beads that extend above the powder bed.32 Different extent of balling is shown in Fig. 3.7.
Fig. 3.6 Light optical microscopy showing the comparison of process-induced porosity to gas induced porosity43
Li et al. ‘s research on balling process has shown that balling could be greatly reduced by keeping the oxygen level at 0.1%, applying a combination of high laser power and low scanning speed or applying laser re-scanning.45 Das et al. illustrated that the oxide film on the preceding layers will impede the interlayer bonding and wetting of powders. 44Poor layer bonding could easily lead to the delamination of parts. Therefore, lowering the oxygen layers to prevent of oxide film formation and use laser scanning repeatedly to break to oxide films and wetting the powders are key to reducing balling phenomena.
Cracking may occur during solidification or heating process. Porosity may lead the microstructure cracking. Delamination is the separation of adjacent layers within parts due to incomplete melting between layers. Fig. 3.8 shows the delamination lead to the cracking between
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layers. When the processing temperature is too high, the surface tension together with the melt pool size may lead to swelling.17
(a) (b)
(c) (d)
Fig. 3.7 Illustrations of single-layer formed due to selective laser melting of powder under adjustable balling processes: (a) ordinary sintering before the balling processes start; (b) beginning of the balling process; (c) strongly limited balling process; (d) light limited balling process.45
Regarding the crack formation in AM parts, there are different material-dependent mechanisms.46 Here two types of mechanism are discussed. For some materials, solidification cracking can occur if the applied energy is too high and stresses are induced between the solidified areas and not yet solidified areas in the melting pool. So it depends on the solidify nature of materials and caused by the strain on the melting pool. Upon the higher energy that energy source provided, the thermal strain is larger. So for solidification cracking, adjust the applied energy properly could reduce the cracking. Grain boundary cracking occurs along the grain boundaries.
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The cracking is dependent on the materials in grain boundary morphology and the formation or dissolution of precipitates.46
Fig. 3.8 M2 parts produced by SLM47
Solidification cracking and grain boundary cracking are both phenomena that occur within the microstructure. However, generally cracking is sometimes used to describe macroscopic cracks in the material. These cracks may nucleate due to other macroscopic defects such as delamination that are not related to excessive energy input.47
Residual stress is a commonly seen defect in AM products and it has a negative effect on the mechanical properties of AM parts. Residual stress can occur through a variety of mechanisms including plastic deformation, temperature gradients and phase transformations. In AM processes, the main concern is the large thermal gradients. Residual stress is defined as a stress remains in a material after the original cause of the stresses has been removed. If residual stress exceeds the local yield stress of material, warping or plastic deformation may occur. If residual stress exceeds the local ultimate tensile strength of the material, cracking or other defects may occur. Macroscopic residual stresses have larger effect on the whole
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component compared with the microscopic ones. While microscopic residual stresses resulting from precipitates or atomic dislocations are more localized. Macroscopic residual stress can be thermally introduced in metal AM by rapid and uneven cooling or heating during and after solidification.17,48
The residual stress can be measured using the techniques such as micro-hardness, x-ray diffraction, neutron diffraction.48,49 Rangaswamy et al. has demonstrated that residual stress is closely related to temperature. It tends to be higher for substrates operating near room temperature as in DED or SLM than those operating at higher temperature such as EBM.50 Heating of the substrate could reduce the residual stress.48 Therefore, the fabricated parts by EBM process have a lower residual stress than those by SLM and DED processed due to the higher operating temperature.
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Chapter 4 Metals and Application
Currently the metal materials used in additive manufacturing include steels and iron-based alloys, pure titanium and titanium alloys, nickel-based alloys, cobalt-chromium alloys, gold and silver. As the new techniques and applications emerge, the range of metals available for use in AM continues to grow.
AM materials have high potential for a wide range of applications in the areas of aerospace, energy, biomedical, semiconductor manufacturing and customer products. To utilize the full potential and market of AM, the materials used will need further research and development.
Despite the progress and improvement on the new AM materials and processing techniques, there are remaining challenges, which will be discussed in this chapter. The commonly used metal systems, their properties and applications are explored.
As previously mentioned, the publications for AM metals processing technique mainly focused on PBF and DED. In this chapter, the components are manufactured through SLM processes.
4.1 Steel and Iron-based Alloys
Based on the publications, M2 tool steel and 316 stainless steel are intensively studied.
Jandin et al. reported the fabrication of 316L stainless steel powder by SLM for the first time. In their study, the 316L stainless steel powders were processed using yetterbium fiber laser with a wavelength of 1065 nm.51 In 2010, Tolosa et al. achieved 99.9% relative density with of 316L stainless steel powder.52 Yap et al. presented an excellent review of SLM materials, with the comprehensive data and substantive research reviews on the properties of different metal systems.
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Different types and steels and iron-based alloys are investigated, including H20 tool steel, 17-4 precipitation hardening steel, Fe-Ni, Fe-Al, Fe-Cr-Al alloys, are also been tested despite there is not as much publications as that on steel. So far, the research works mainly investigated the optimization of processing parameters to achieve full density parts.18
(i) Property
A. Relative density. Relative density is one important quality indicator. Relative density is the ration of density of the AM product to the theoretical density of the bulk materials. The theoretical density could be calculated based on the ideal crystal structure and atomic weight. ASM International also provides the material data sheet for reference. Most SLM steel and iron-based alloys could achieve the relative density above 90%. Table 2 summarizes the reported maximum relative density of various SLM materials.
Table 2 The reported highest relative density of SLM steel and iron-based alloys18
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B. Strength. Steel has superior strength so it is important for its application. SLM parts usually have better strength than cast since the chemical composition is more uniformly distributed.
Table 3 shows the highest yield strength and ultimate tensile strength of SLM steel and iron-based alloys.
Table 3 Highest reported tensile strength of SLM steel and iron-based alloys18
C. Surface roughness. For SLM process, post processing is need to reduce the surface roughness of component. And in general, for AM processed, machining is needed. It also depends on the resolution requirement of the products. As shown in Table 4, the surface roughness could be as low as 3.78 µm. The surface roughness commonly seen in the SLM products are about 20 µm.
Table 4 Lowest reported surface roughness of SLM steel and iron-based alloys18
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D. Micro-hardness. There are many techniques and units set for the micro-hardness. The highest reported micro-hardness of SLM steel and iron-based alloys is shown in Table 5.
Table 5 The highest reported micro-hardness of SLM steel and iron-based alloys18
(ii) Application
The parts manufactured through SLM methods are still more expensive than that of conventional manufacturing, so they are used in high-end areas such as medical applications. The advantage of produce metal parts with complex geometries allow the fabrication of parts with inner channels, which meet the requirement of heat exchangers and light weight structures. And they could be applied in aerospace and automotive industries. The applications depend on their mechanical properties, biocompatibility.
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The benefit of SLM parts in medical applications lies in easily customized. The ability to create complex structures make the fabrication of light-weight structures. There are published research in this area. Kruth et al. published the biocompatible metal frames could be used as dental prostheses.53 Wang et al. optimized SLM parameters and lead to high shape precision of customized brackets with the application in orthodontic products.54 316L stainless steel was mostly often used in the research of light structure materials. Shen et al. studied the mechanical properties and the failure mechanism of the sandwich structure.55 Due to the high hardness of the SLM of steel or iron based alloys, the products can be used in tooling.
4.2 Titanium and Titanium alloys
The published research of titanium and its alloys is second to that of steel and iron-based alloys. Pure titanium and Ti-6Al-4V are intensively studied. Due to the active chemical nature of titanium in the liquid state, producing in AM methods which under inert gas or near-vacuum environment could protect the components. Besides, titanium and its alloys has excellent strength to weight ratio, making them suitable materials for aerospace products. The biocompatibility that titanium possess enables their application in medical applications.
(i) Properties
A. Relative density. Compared with steel and iron-based alloys, the relative density of titanium and its alloys is higher. And the highest reported data on Table 6 has shown the parts almost achieve the theoretical density. That means only a limited small portion of porosity exist in the parts and the mechanical properties are enhanced.
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B. Strength. Titanium parts through SLM can achieve better strength than that of casting due to the more uniform chemical composition and microstructure. The highest reported strengths of titanium parts are shown in Table 7.
C. Surface roughness. There is few reports on the surface roughness of Ti parts. Zhang et al.
reported the surface roughness of pure Ti is 5 µm.56
Table 6 Highest reported relative density of Ti and Ti alloys
Table 7 Highest reported strength of Ti parts
D. Micro-hardness. The highest reported micro-hardness of Ti parts is shown in Table 8.
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Table 8 The highest reported micro-hardness of Ti parts
(ii) Applications
Titanium has great potential on biomedical applications due to the biocompatibility and its high specific strength and elastic moduli close to that of human bones. Thus, the titanium body implants are a focus of the research. Santos et al. built the bones and dental crowns using pure tiannium.57 Warnke et al. researched on tissue engineering and porous titanium alloys scaffolds and has shown SLM Ti64 scaffold allows the growth of bone cells.
The fabrication of titanium scaffolds could also be applied in aerospace area, enabling the fabrication of light weight structure parts. There has been multiple research and publications on porous titanium parts. Wang et al. researched on the porous TiH2-Ti scaffolds with a high porosity of about 70%.
4.3 Nickel-based Alloys
Nickel-based alloys are one of the most intensively studied by researchers followed iron-based alloys and titanium alloys. Inconel, the Cr-Ni-Fe alloy are the main focus due to the resistance to high temperature. NiTi, is a type of temperature-sensitive shape memory alloy.
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(i) Properties
A. Relative density. The highest reported relative densities of Nickel-based alloys are shown in Table 9. As it shown that Inconel 625, Inconel 718, Niomonic 263 achieved near 100%
relative density.
Table 9 Highest reported relative densities of Nickel-based alloys
B. Strength. The strength of SLM Nickel-based alloys has a higher strength that the casting counterparts. The reason is same to that of titanium parts and iron parts. The highest reported strength of Nickel-based alloys is shown in Table 10.
Table 10 Highest reported strength of Nickel-based alloys
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C. Surface roughness. There is hardly any report on the surface roughness of Nickel-based alloys.
As the AM technology develops and more applications of AM Nickel alloys, there should be more research on the surface roughness of Nickel alloys.
D. Micro-hardness. There are not many tests on the micro-hardness. But research found that aging treatment could improve the micro-hardness of Nickel-based alloys. It could be seen from Table 11.18
Table 11 Highest reported micro-hardness of Inconel and Nickel-based alloys
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Chapter 5 Summary and Future Research Trends
In this thesis, an overview of the basics of the additive manufacturing is covered. A fundamental understanding of the advantages of 3D printing, the historical development, how the AM products are produced has been shown. Among a variety of category of materials, metals stood out due to their potential application in aerospace, medical, energy and the benefits of able to customization, reduce materials waste and cost, and fabricate complex geometry.
A diverse set of processes have been used to fabricate 3D objects. The AM manufacturing systems can be divided into powder bed, powder fed, wire fed depending on the feedstock. The way how the feedstock works, the strength and weakness of the system corresponding upon the requirement of the components are explored. Based on the standards on process terminology set by ASTM, each AM technique that could process metals are categorized. The working principals, process parameters, examples of machines and machine manufactures, their advantages and weakness of those methods are compared and discussed.
The defects of AM components are essential to determine the quality of AM manufacturing.
Process parameters plays a key role in decreasing or even eliminating the AM defects. Therefore, the interactive relationship between materials, processing, property and applications are explored.
Some physical phenomena and some commonly adjusted parameters for optimization are explained. The defects that commonly exist in AM products and how to avoid or alleviate through parameters adjustment and materials preparation are illustrated.
The most studies AM metals and alloys are steel and iron-based alloys, titanium and titanium-based alloys, Inconel and nickel-based alloys. The relevant noted research, their properties and applications are reviewed.
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However, to utilize the full potential of AM, every part of AM process including design, process control, supply chain, powder properties and standardization need to be further developed.
Based on the roadmap for additive manufacturing and measurement science roadmap for metal-based additive manufacturing, the desired capabilities and recommendations for future research is summarized below:6,11
Design
Design