The purpose of this paper is to investigate the density, surface quality, microstructure and mechanical properties of the components of the selective laser melting (SLM) parts made at different building orientations. SLM is an additive manufacturing technique for three-dimensional parts. The process parameters are known to affect the properties of the eventual part. In this study, process parameters were investigated in the building of 316L structures at a variety of building orientations and for which the fracture toughness was measured.
Hardness and tensile tests were carried out to evaluate the effect of consolidation on the mechanical performance of specimens. Optical and electron microscopy were used to characterise the microstructure of the SLM specimens and their effects on properties relating to fracture and the mechanics. It was found that the density of built samples is 96 per cent, and the hardness is similar in comparison to conventional material.
The highest fracture toughness value was found to be 176 MPa m^(1/2) in the oz. building direction, and the lowest value was 145 MPa m^(1/2) in the z building direction. This was due to pores and some cracks at the edge, which are slightly lower in comparison to a conventional product. The build direction does have an effect on the microstructure of parts, which subsequently has an effect upon their mechanical properties and surface quality. Dendritic grain structures were found in oz. samples due to the high temperature gradient, fast cooling rate and reduced porosity. The tensile properties of such parts were found to be better than those made from conventional material.
The relationship between the process parameters, microstructure, surface quality and toughness has not previously been reported.
Alsalla, H., Smith, C. and Hao, L. (2018), "Effect of build orientation on the surface quality, microstructure and mechanical properties of selective laser melting 316L stainless steel", Rapid Prototyping Journal, Vol. 24 No. 1, pp. 9-17. https://doi.org/10.1108/RPJ-04-2016-0068Download as .RIS
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Stainless steels are widely used as feed powder materials within selective laser melting (SLM) due to their comparatively low cost, their safety and their ease of use. 316L stainless steel is a common alloy that is used in aerospace applications due to its resistance to corrosion and good specific strength (Yan et al., 2012). The SLM of 316L stainless steel was previously investigated to illustrate the effect of the process parameters on the density, the mechanical properties (Childs and Hauser, 2005; Childs et al., 2005) and to look for defects, such as porosity, cracks and balling, which were found to occur in SLM produced parts. Different SLM processing parameters, including laser power, scan speed, layer thickness and hatch distance, have been examined in previous studies; however, the understanding of the inter-relationship between these parameters is still not clear, in particular in relation to the effect of balling (Tolochko et al., 2004), the interaction mechanism between material and laser beam (Fischer et al., 2003) and the powder solidification on the substrate (Schoinochoritis et al., 2015). Earlier studies have yet to apply SLM to make 316L stainless steel parts with mechanical properties that are sufficient for use in applications (Zhang et al., 2011). There is also a limited characterisation of the critical mechanical properties for aerospace applications, particularly for fracture toughness, which has not been tested in different build directions (Kruth et al., 2010). So there is a need for an in-depth and systematic study on the SLM processing of stainless steel for aerospace applications. This is essential both to further improve the capability of SLM to process 316L, and for the resulting properties of parts for aerospace applications. A systematic study is required to provide a clear understanding of the fundamental process mechanisms governing the resulting microstructure; and thus the properties of 316L parts.
The research presented in this study has focussed on analysis of the mechanical properties and microstructures of SLM fabricated parts made with 316L steel, and in particular, on the effect of build orientation on these properties. First, the effect of build orientation on the toughness properties, strength and ductility of the SLM fabricated stainless steels was investigated. Second, the microstructure of the SLM parts was characterised, and their effects on mechanical properties, in particular on fracture toughness, were evaluated. These measured properties were compared with previous research, providing an understanding of SLM processing for different metal alloys. The findings of this study provide new knowledge about the use of the SLM process for the fabrication of stainless steel aerospace components.
2. Experimental procedures
Using SLM (M1 CUSING, Concept Laser Ltd), 15 sets of 316L stainless steel flat samples for tensile testing, and 15 sets of single edge notched bend (SENB) samples for fracture toughness testing were produced. The samples were produced in different build orientations, which were named the ox, oz. and six orientations, as shown in Figure 1. The SLM machine has a base plate with x- and y-axes, and a build direction in the z-axis. The naming convention uses two letters. The first letter is the axis of the machine in which the longest axis of the sample lies. The second letter is the machine axis in which the second longest sample axis lies. These sets were designed to investigate the effects of different building layouts and orientations on the properties of the SLM stainless steel parts. In Figure 1, the samples, made in three different build directions were produced with a laser power of 180 W, a scan speed of 1,600 mm/s, a layer thickness of 30 am and a scanning laser spot with a 75 am diameter.
After removal from the plate, all the SLM-fabricated samples were machined to remove the remaining support structure and were then tested in an as-built condition. For each direction, five specimens were built for the tensile testing and five for fracture toughness testing. The densities of these specimens were evaluated by two methods, direct measurements, by determining their weight and dimensions (mass/volume), and the Archimedes principle. All of the specimens were pulled until they fractured for the tensile testing, and the toughness notched specimens were subjected to a three points test through compression to failure. All of the samples produced were manufactured and tested according to ASTM E8 and ASTM E399-9, for tensile and fracture toughness tests, respectively.
The metallographic investigations were carried out for the samples that were etched in a solution containing 122 ml alcohol, 122 ml hydrochloric acid and 6 ml nitric acid. The scanning electron microscope (SEM) and optical microscopy (OM) were used to characterise the surface morphology and microstructural features of the specimens. The samples were analysed for the actual surface at different building strategies by surface profilometer (Talyscan-150, Taylor Hobson Ltd); the data were collected through two measurements, measured on the parallel and perpendicular to the building direction. The average values of Ra were obtained from both parallel and perpendicular measurements.
3. Result and discussion
3.1 Density analysis
The first and the most important concern about the use of the SLM process is the density of the fabricated parts, as this has a direct influence on the product’s performance and mechanical properties (Morgan et al., 2004). The result exhibited the same density for ox, oz. and six parts at 7.7 g/cm3, with a standard deviation of ±0.01. The samples that were built in the six directions represented a larger building height, and, consequently, there are a larger number of layers. Each layer of the z built group has a relatively smaller cross section than that of the ox and oz. groups, its external surface boundary is more likely to be bonded with the semi-melted particles after its exposure to, and interaction with, the laser beam scanning. Overall, the effect of the building directions on the density of the SLM parts is very minimal. In general, post polishing or machining would be applied to the SLM parts for aerospace application. The average density of 7.7 g/cm3 for the SLM stainless group accounts for 96 per cent of the density of 8 g/cm3 for AISI 316L stainless steel which is made by conventional production means. This experimental density result is slightly lower in comparison to a previous study, which achieved 99 per cent of the density of 8 g/cm3 (Yasa and Kruth, 2011). This previous study showed that the preheating temperature plays an important role in the SLM process, and high-density parts of 99 per cent are made when a high preheated temperature of the powder bed of 150 and 200°C is used. The laser re-melting may be another approach to enhancing the density of SLM parts as it allows porosity of about 0.77 per cent for the parts with no laser re-melting to 0.036 per cent for the parts with laser re-melting when the parameters selected properly (Boisselier and Sankaré, 2012).
In general, the SLM process is required to produce fully dense parts to meet strict mechanical property requirement for aerospace application. However, it might be difficult to achieve complete metal powder densification during the SLM process, as there is no mechanical pressure applied to metal powder, as in moulding process. The metal powder densification during SLM is predominantly influenced by temperature, capillary force and gravity (Kruth et al., 2010). Gas bubbles can be entrapped in the powder material through the SLM’s solidification. The dissolved element’s solubility was decreased through solidification in the melt pool (Rombouts, 2006). Hence, further research and development on SLM is still required to deliver high-density stainless steel components for aerospace applications. Future research should provide an in-depth understanding of the laser material interaction mechanism to manipulate the powder consolidation and to achieve enhanced densification.
3.2 Surface and cross-section microstructure
Figure 2 shows different views (both parallel and perpendicular to the build direction) of the microstructure of ox, oz. and six samples. Densification is clearly a little more complete in the oz. and six samples, as they exhibit fewer voids and pores, due to the short scanning length when compared to samples built in the ox direction. We define “pores” as being empty spaces in the sample following manufacture, and “voids” as being empty spaces brought into existence by fracture of un-melted particles left over from manufacture. At the overlap joint zone (this phenomena occur when a short distance between the scan tracks), the additional heat was transferred from the melted track to the previous solidified track, leading to better consolidation and densification. The microscope shows that the scan tracks are clearly distinguishable in Figure 2(b) and (c) through a few large pores that are caused by the balling phenomena in Figure 2(c); the dark band areas are present due to etchant. The cross-section images show that the stainless steel powder particles also melt well.
Figure 3 presents the region between the scanning tracks of the material where the melting shows good diffusion. Fine dendritic structures are presented in the perpendicular view in the very dense xz samples, whereas some pores and voids are shown in the yx and zx samples. The microstructure of parts produced in this study varies in different directions. Gäumann et al. (1999) have shown that the quality of metal parts formed by laser melting depends on the thermal gradient and growth speed. Rombouts (2006) also found similar microstructures had formed as the result of rapid solidification due to a high cooling rate. The small pores found in xy and z build samples can be explained through three different resources such as incomplete melting, shrinkage and the composition of gas voids (balling) (Hao et al., 2009). These small pores have an average size of 8 μm and are probably produced as a result of shrinkage and balling due to the solidification phenomena. In Figure 3, it was observed that the pores in zx samples have an irregular shape and appear more frequently than in samples built in the yx direction, while samples built in the xz direction reveal no pores, implying good consolidation which can be observed through the dendritic microstructure. This porosity may concentrate stress through mechanical loading and can increase during applied stress. Soboyejo (2002) reported that the growth of three-dimensional defects may lead finally to catastrophic failure in the structure and in engineering components. Consequently, it is axiomatic that these pores affect the mechanical properties, especially tensile strength, elongation and fracture toughness, and should show fewer properties than those found in conventional material (this will be further clarified in Section 3.4).
Figure 4 presents OM and SEM images of the xz build direction sample, where the dendrite arms were normal to the surface. So, this structure showed that the solidification is dendritic or cellular, with a size of about 3 μm, and also in Figure 6(b), higher magnification reveals that the intercellular spacing is less than 1 μm, which contributes to the excellent strength that can be achieved, both in processed and aged conditions (Cherry et al., 2015; Gu and Shen, 2009; Kruth et al., 2010). Though previous research in SLM processing has found that these microstructures are common, this microstructure is formed as result of high thermal gradients and rapid solidification, due to a very high cooling rate, and this helps to reduce crack nucleation at the pores between scan tracks (Takalo et al., 1979). The tensile strength significantly increases as a result of the amount of primary austenite dendrites in the multi alloy. This microstructure has been reported in previous studies and was explained by the relation between dendrites and mechanical properties (Kaiser et al., 2013).
There are correlations between dendrite arm spacing and mechanical properties, such as yield strength, hardness and ultimate tensile strength (UTS) (Chirita et al., 2010). Generally, a refinement of the microstructure or grain leads to improved mechanical properties. The dendrite arm spacing was found to be significantly affected by the different cooling conditions (Ejiofor and Reddy, 1997). So, it is important to understand the microstructure of material from different views because the overlapping of the laser’s scanning track can produce defects, such as cracks and voids. Such defects, when located at the interface between the rows and scanning tracks, are known to produce a point that is vulnerable to crack growth. As the temperature gradient and local heat transfer conditions determine the grain growth in the parts that are produced by SLM (Kruth et al., 2010), it is expected that changing the process parameter, as well as the build direction, which may affect the microstructure of the parts and the mechanical properties of hardness, UTS and yield strength, could all be increased with an increase in the cooling rate, while the ductility decreased gradually (Kaiser et al., 2013; Mallapur et al., 2010; Osorio et al., 2006).
3.3 Mechanical properties
3.3.1 Tensile test
Figure 5 reveals the tensile stress-strain curves that were calculated for the yx, xz and zx build direction. The elastic slope in this figure is usually named the Apparent Young’s Modulus. The total elongation to rupture, the ultimate and yield strength were determined after examination of the plastic and elastic parts of the curves (Table I and Figure 6). The total elongations were found to be between 35 and 41 per cent, and these results corroborate with the results presented in previous studies that were carried out by John (2001) and Yang et al. (2012) and the UTS recorded a large difference from 564 MPa for the zx samples to 695 MPa for the xz samples (standard deviation 3 MPa). The lowest value of UTS was recorded in the z samples, because of the number of irregular pores (Figure 3) and the presence of a weak region between the scanning tracks and the melting layers where all the fractures had occurred. On the other hand, the yield strength varied from 387 in the zx samples to 423 MPa in the xz samples and was situated above the previous research results of 261 MPa (de Lima and Sankaré, 2014; John, 2001).
This enhancement in the tensile properties was due to the proper selection of the process parameters. For example, the width of hatch spacing such as 70 μm was reasonable to ensure good consolidation, as opposed to a 120 μm hatch spacing, which had been applied previously and was to leave some gaps un-melted or partially melted between the tracks. From Table I, it was noted that the samples built in the xz direction show the highest performance. This is owing to fewer pores and voids being present and finer grains (dendrite microstructures), as shown in Figure 4(a) and (b). Generally, the tensile properties obtained in this experiment are high in comparison to previous studies, and they seem to be usable for aerospace applications. Consequently, the mechanical properties of the SLM of 316L stainless steel depend not only on the material composition but also on build direction. In the meantime, it must be borne in mind that the defects, such as cracks and pores, which occur following the SLM process, can be reduced by post process treatment and residual stress relief to further improve the tensile strength and the fracture toughness.
3.3.2 Fracture toughness test
Fracture toughness testing results, presented in Table II, were calculated according to the maximum flexural load by concentrating the stress in the area of minimum cross section of as built samples made in different build directions (see Section 2 for test conditions). The fracture toughness varied between 145.5 in the zx direction (standard deviation ±1.1) and 176 MPa m1/2 in the xz direction (standard deviation ± 0.9) and a situated low value when compared to the conventional method of austenitic stainless steel grade 316L, from 112 up to 278 MPa m1/2, and 316L annealed stainless steel at 210 MPa m1/2 (Maloy et al., 2001). These low values should be caused by the cracks, pores and voids in the parts produced, as well as the residual stress that remains in the 316L parts after the SLM process, are other reasons that affect the mechanical properties of materials, especially fracture toughness.
The differences in the fracture toughness values obtained in this study with different build directions had been anticipated by the previous investigation that is mentioned in Section 3.2 of the microstructure analysis. The tensile test result also showed that the samples built in the xz direction recorded the highest values of UTS and elongation, meaning that there is a larger area under the stress strain curve, i.e. the material absorbs more energy before failure. These combined values enhance the fracture toughness of parts built in the xz direction because of the fast cooling rate and the resulting dendritic grain structure. On the other hand, parts built in the yx and zx directions presented slightly lower values of fracture toughness than those built in the xz direction because of the more prevalent and larger pores and defects, especially in the samples built in the zx direction, which were subjected to loading perpendicular to the build direction. The results in Table II reveal that the build orientation has a strong effect upon properties. For example fracture toughness was the smallest in the samples built in the z direction because of the pores, voids and cracks that are present at the edge of the parts. This is shown in Figure 6(c), which reveals the fracture surface of the samples.
In summary, the toughness is the ability of a metal to deform plastically and to absorb energy before rupture. The key to toughness is also a good combination of ductility and strength in these properties, and according to the results obtained in this study, they are considered good. Further treatments, such as Hot Isostatic Press (HIP), could reduce the amount of residual stress and also the re-melting of parts is necessary to reduce the defects that are mentioned above.
3.3.3 Vickers hardness test
The Vickers hardness test for as build samples built in different build orientations was found to demonstrate no significant difference between 226 HV in the xy samples and 221 HV in the xz and zx samples, with a standard deviation of 2 HV. The xz build records the lowest value of hardness, which means that the samples that were built in the xz direction have more ductility than those built in other directions (as can be noticed in Figure 5), while the zx direction build also has a low hardness due to the porosity that is revealed in this build direction (Table III). The result presented in this study is similar to those found in conventional products that are made of this alloy, and in a previous study of SLM 316L (Cherry et al., 2015).
Hardness is the term used to describe the amount of energy required to permanently deform (stretch, bend, compress, etc.) a material. Toughness describes the energy required for fracture. It is often the case that hard materials are not tough, and vice versa (Osakada and Shiomi, 2006; Leskovšek et al., 2002). In this study, the xz samples showed the least hardness, as shown in Figure 7, but they also demonstrated the highest fracture toughness, as well as the highest strength and ductility values at 695 MPa and 41 per cent, respectively.
3.4 Surface quality
Figure 8 shows that the surface roughness and the quality of the components produced by SLM of 316L stainless steel do not vary with build direction, but vary between surfaces that are perpendicular and parallel to the build direction. Perpendicular surfaces presented the highest values of roughness, with an average of Ra 4.25 μm (standard deviation of 2.5 μm), while surfaces parallel to the build direction had an average Ra of 3.1 μm (standard deviation of 1.2 μm). These differences are related to the occurrence of an elevated solidification ridge at the edge of the sample [Figure 9(a)]. These ridges may affect the dimensional accuracy of a part. However, the parallel surfaces were affected by partially melted powder particles clinging to the surface. Mumtaz and Hopkinson (2010) found that surface roughness was generated by the rippling effect that can occur during SLM, when the laser, moving the temperature gradient between the solidifying zone, produces a shear force on the liquid surface, which is contrasted by the surface tension force. This shear force results in the formation of residual rippling on the surface as the relaxation process could not be fully realised on occasions due to the extremely short solidification times of the melt pool (Kruth et al., 2010; Strano et al., 2013). These experimental results agree well with previous studies (Badrossamay and Childs, 2006; Chen et al., 2004; Rombouts et al., 2005) which showed that SLM has not yet achieved high surface quality of components. This is still one of the major drawbacks of the process, particularly in the fabrication of high performance aerospace components. This issue has to be further addressed by applying new process parameters, such as re-melting, which has been applied previously, and thus to reduce the surface roughness from 12 to 1.5 μm, as shown in Figure 9(b) (Kruth et al., 2010), or a number of surface modifications, such as machining and etching oxidation. The etching solution could be applied as a post-process to enhance the surface quality. In this experiment, all the investigations were done for as build samples and without any re-melting to reduce the energy consumed and the production costs.
3.5 The effect of microstructure and fabrication on the quality and mechanical properties
In terms of sample quality, the SLM process investigated in this study delivers a better surface roughness (Ra) on the surface that is parallel to the build direction. This means that the surfaces perpendicular to the laser are rougher, and this is due to the rippling effect, scan tracks and the elevated solidification ridges that arise at the edge of samples. Figure 9 summarises this by showing how the surfaces perpendicular to the laser have rough surfaces. However, here, these differences in surface quality are not related directly to mechanical properties. On the other hand, these results have proven that the build direction has an influence on the microstructure of the samples produced, as shown in Figures 2 and 3. This is because the samples built in the zx direction have more irregular pores, and they also have more partially melted particles stacked onto the parallel surface. In contrast, samples manufactured in the xz direction present fewer pores and dendritic grains due to good consolidation. These microstructures gave a very clear indication of how to predict the mechanical properties of the samples. So, under these conditions, it can be concluded that the influence of the build direction on mechanical properties is greater, due to the difference of the microstructure, especially in relation to the fracture toughness and the ultimate tensile strength.
The xz samples record the lowest value of hardness, which means the microstructure (the dendritic grains) have more ductility than the other samples in the zx and yx build directions, as well as high strength (Table II). This leads to a high fracture toughness value, due to the higher energy absorption of the material.
In the context of previous research, it can be concluded that the mechanical properties of the SLM of 316L stainless steel components are comparable to those made from conventional materials Cherry et al. 2015; Kalu, 2013 and Soboyejo, 2002). All of the results discussed in this experiment are only valid for the range of experimental processes that are considered in this study.
This paper has investigated the density, surface quality, microstructure and mechanical properties of the components of the SLM parts made at different building orientations. It illustrates the effects of different building orientations on the microstructures and the mechanical properties, especially the fracture toughness of the 316L stainless steel after it was fabricated by SLM. Below, we summarise the major findings:
Samples fabricated in the yx and zx orientations presented defects, such as pores and cracks, which affect the fracture toughness, strength and total elongation.
The density of the SLM parts is around 96 per cent in comparison to the conventional product in AISI 316L stainless steel.
The tensile properties for the parts produced are good in a particularly total elongation of between 35 and 41 per cent, and the tensile and yield strengths are seen to be quite high in comparison to previous research. Special mention should also be given to the yield strength and tensile strength, which are higher than those in conventional material while maintaining the high elongation values.
In the fracture toughness test (SENB), the results of anisotropy and the values are slightly lower than those in conventional products (average values from 145.5 to 176 MPa at room temperature).
Vickers hardness test results are similar to those found in conventional products made of this alloy, and the value of the hardness decreases with an increase in toughness.
The fracture surface and the microstructure show evidence of voids, cracks and pores in the samples produced in the yx and zx build orientation, and a few pores are present in samples with an xz building orientation with dendrite grains.
The orientation during the build affects the mechanical properties, particularly fracture toughness. The weakest build strategy recorded was in the zx direction, because of pores, voids and cracks that are present at the edges of the parts.
Dendritic grains appear in the xz parts due to a high temperature gradient and fast cooling, which seem to increase the toughness and ductility while the number of pores decreases.
Most pores had aspherical shapes, and sizes of between 10 and 150 μm in the yx built parts and 10 to 67 μm in the zx built parts, which also recorded the highest porosity.
Build orientation has a slight effect on surface roughness, with the xz built samples having 3.8 Ra roughness, while this is increased to 4.7 Ra in the yx and zx built parts. Further work such as HIP treatment or re-melting could improve the mechanical properties, surface roughness and reduce the internal stress.
With regards to the manufacture of stainless steel components for use in the aerospace industry, or indeed other industries where fracture toughness and strength are critical, it is clear that parts should be designed so they can be made in additive layer manufacturing (ALM) with the highest in services load carried in the build direction z direction. Furthermore designers and manufacturers should be aware that because of the inherent limitations of ALM in surface roughness parts will require subsequent heat treatments to solve stress concentration problems at sharp corners and fillets. Solidification ridges near sample edges mean that post-processing may be required if such features are critical.
Tensile properties obtained from the 316L stainless steel made by SLM in different directions
|As built samples||UTS (MPa)||Yield strength (MPa)||Elongation (%)|
|yx||668 ± 3||397 ± 3||37 ± 1|
|xz||695 ± 3||423 ± 3||41 ± 1.9|
|zx||564 ± 3||387 ± 3||35 ± 0.6|
The fracture toughness property of 316L stainless steel made by SLM in different build directions
|As built sample||Fracture toughness (MPa m1/2)|
|yx||152.6 ± 1.16|
|xz||176 ± 0.9|
|zx||145.5 ± 1.1|
Vickers hardness result of 316L stainless steel
|Build direction||Vickers hardness ± HV||±SD HV|
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