Mechanical Properties of SLM-Printed Aluminium Alloys: A Review

Selective laser melting (SLM) is a powder bed fusion type metal additive manufacturing process which is being applied to manufacture highly customised and value-added parts in biomedical, defence, aerospace, and automotive industries. Aluminium alloy is one of the widely used metals in manufacturing parts in SLM in these sectors due to its light weight, high strength, and corrosion resistance properties. Parts used in such applications can be subjected to severe dynamic loadings and high temperature conditions in service. It is important to understand the mechanical response of such products produced by SLM under different loading and operating conditions. This paper presents a comprehensive review of the latest research carried out in understanding the mechanical properties of aluminium alloys processed by SLM under static, dynamic, different build orientations, and heat treatment conditions with the aim of identifying research gaps and future research directions.


Introduction
Additive manufacturing (AM), more popularly known as 3D printing (3DP), has been extensively applied for various engineering applications. When compared to conventional manufacturing, highly complex parts such as lattice structures can be produced by AM because of the layer-by-layer fabrication process [1]. In recent times, AM technologies are being sought after for providing customized solutions to problems arising due to the COVID-19 virus [2]. Several technologies are in practice for the AM of metallic parts, which include selective laser melting (SLM), electron beam melting (EBM), laser engineered net shaping (LENS), direct metal deposition (DMD), and cold spray additive manufacturing (CSAM). Each of these processes offers its own merits and limitations in terms of quality of the print-part, mechanical property, performance of the component, and the range of materials that can be fabricated. Among these AM processes, SLM is being widely accepted by industry to manufacture customized, high value-added, and complex metal components for aerospace, automotive, defence, and biomedical applications [3]. SLM is a powder-bed fusion type process that produces metal parts by selectively fusing metal powders on a platform using a laser beam. Figure 1 presents the schematic illustration of the SLM process. According to Kempen et al. [4], the fusing occurs through the melting and rapid solidification of metal powders scanned by a laser beam along a 3D print-path created by a processing software. Aluminium alloys, titanium alloys, steels including stainless steels and tool steels, nickel superalloys, and cobalt-chromium alloys are the most common SLM-processed metals. There is a growing need for SLM to create fully dense parts, where mechanical, thermal, and other properties are comparable to those of the wrought and/or conventionally fabricated materials. However, SLM is a very complex process and a lot of research is targeted at understanding the effects of various process parameters on mechanical properties, performance, and quality of printed parts. Some of the main process parameters affecting part quality and properties in SLM are the laser power, hatch spacing, defocus distance, powder layer thickness, scan speed, and scan strategy. Maamoun et al. [6] presented a detailed study on the influence of these processing parameters on relative density, porosity, surface roughness, and dimensional accuracy. They identified an optimal processing window using process maps to achieve desired values for each performance characteristic. Likewise, Rashid et al. [7] have reported that varying a single SLM parameter, scan strategy, can result in significantly different microstructural and mechanical properties of the printed parts. Therefore, it is very important to understand the effects of the different SLM process parameters on the part quality and properties.
Aluminium (Al) and its alloys are characterized by their light weight, high strength, corrosion resistance, and good weldability, making them suitable for a range of applications in industries such as automotive, aerospace, machinery and tooling, defence, and construction. Of the different alloy combinations, aluminium-silicon-based alloys (Al-Si), specifically AlSi10Mg, AlSi12, A356 (AlSi7Mg0. 3), and A357 (AlSi7Mg0. 7), have been extensively used in the SLM process owing to their fabricability. This paper presents a comprehensive review on latest research conducted on mechanical properties of Al alloys processed by SLM. A large number of articles have been published on process optimization, anisotropy effects due to build orientation, and the improvement of SLM built part properties by heat treatment. This paper presents a classification and discussion of these published papers into various research categories to identify research directions and research gaps for different types of mechanical properties and performance under static and dynamic loading conditions. Aluminium alloys, titanium alloys, steels including stainless steels and tool steels, nickel superalloys, and cobalt-chromium alloys are the most common SLM-processed metals. There is a growing need for SLM to create fully dense parts, where mechanical, thermal, and other properties are comparable to those of the wrought and/or conventionally fabricated materials. However, SLM is a very complex process and a lot of research is targeted at understanding the effects of various process parameters on mechanical properties, performance, and quality of printed parts. Some of the main process parameters affecting part quality and properties in SLM are the laser power, hatch spacing, defocus distance, powder layer thickness, scan speed, and scan strategy. Maamoun et al. [6] presented a detailed study on the influence of these processing parameters on relative density, porosity, surface roughness, and dimensional accuracy. They identified an optimal processing window using process maps to achieve desired values for each performance characteristic. Likewise, Rashid et al. [7] have reported that varying a single SLM parameter, scan strategy, can result in significantly different microstructural and mechanical properties of the printed parts. Therefore, it is very important to understand the effects of the different SLM process parameters on the part quality and properties.
Aluminium (Al) and its alloys are characterized by their light weight, high strength, corrosion resistance, and good weldability, making them suitable for a range of applications in industries such as automotive, aerospace, machinery and tooling, defence, and construction. Of the different alloy combinations, aluminium-silicon-based alloys (Al-Si), specifically AlSi 10 Mg, AlSi 12 , A356 (AlSi 7 Mg 0.3 ), and A357 (AlSi 7 Mg 0.7 ), have been extensively used in the SLM process owing to their fabricability. This paper presents a comprehensive review on latest research conducted on mechanical properties of Al alloys processed by SLM. A large number of articles have been published on process optimization, anisotropy effects due to build orientation, and the improvement of SLM built part properties by heat treatment. This paper presents a classification and discussion of these published papers into various research categories to identify research directions and research gaps for different types of mechanical properties and performance under static and dynamic loading conditions.

3D-Printing of Aluminium Alloys by SLM
Aluminium alloys processed by SLM are attracting attention due to their light weight, high strength to weight ratio, corrosion resistance, and good mechanical properties, and due to the unique advantages offered by SLM, such as tool-less fabrication, geometric freedom, customized design, and intricate shapes. As a result, there is an increased interest from researchers to fabricate Al-Si alloy components using SLM with desirable properties.
Even though Al-Si alloys find extensive applications in the cast form, they are difficult to process using SLM because it is a laser-based process with rapid melting and solidification. Al powders are lightweight, have poor flowability, high reflectivity, and high thermal conductivity. Moreover, Al powders have low laser absorption and are prone to oxidation and balling [8]. Montero-Sistiaga et al. [9] have stated that, besides high reflectivity, high reactivity with oxygen, and high conductivity, there are potential challenges in processing Al alloys using SLM. Despite these drawbacks, aluminium can be alloyed with other metals to overcome some of these challenges. Therefore, the two most common Al-Si alloys used in SLM systems are AlSi 10 Mg and AlSi 12 . The addition of silicon to aluminum improves its fluidity and reduces its melting temperature. The addition of magnesium to aluminum increases its strength through solid solution strengthening and also improves its strain hardening ability [10]. Although significant amount of works are published on AlSi 10 Mg, the research work on AlSi 12 and other Al-Si alloys, including A356 and A357, is still evolving [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. Table 1 presents a compilation of key published literature on the SLM of these Al alloys, which are reviewed in detail in this study, capturing the main findings of each article, considering the process conditions (build orientation, heat treatment, and process optimisation), types of mechanical properties, and type of SLM machine used. It was also noted that most of the researchers have used commercial SLM machines produced by leading SLM manufacturers, such as SLM Solutions, EOS, Concept Laser, Realizer, and Renishaw. More detailed discussion on some of these publications is presented in later sections of this review. After heat treatment, the hardness and the tensile strength of the printed parts reduced by about 20% and 12%, respectively, whereas the ductility increased by a factor of 2.8.

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The compressive yield strength of the heat-treated parts was 169 ± 6 MPa which was approximately half the strength of the as-built parts.
Renishaw AM250 [11] Aboulkhair et al. Microhardness along the plane parallel to the build plane was found to be higher (109.7 ± 0.9 HV) than the plane perpendicular to the build plane (99.07 ± 2 HV).

Density
• Using the optimised process parameters, a relative density > 99% was achieved.

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The AlSi 12 powder with good flowability and apparent density leads to good processability in SLM, while highly fine and spherically shaped powder particles result in poor processability.

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The particles with nearly spherical shaped morphology exhibited very poor flowability, leading to high porosity levels. The building orientation has a significant influence on the properties of the printed samples even after post heat treatment is applied.

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After hot isostatic pressing the samples, the porosity reduced considerably, and the fracture toughness of the specimens improved. Porosity, microhardness, tensile testing and fracture morphology • Microstructure of the samples printed using optimised process parameters consisted of extremely fine Al-Si eutectic dispersed within primary α-Al dendrites.

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Hardness in the transverse direction was higher than that along the longitudinal direction.

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The engineering stress-strain curve at different orientations also shows anisotropy in the strength and ductility, which is possibly due to the orientation between the tensile stress and crystal growth direction. Density, Roughness • Narrow process window to obtain optimal density and surface quality for printed parts.

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Higher scan speed of 1400 mm/s was used for high density/productivity demands, whereas lower scan speeds of 1100-1200 mm/s were used for parts with a high demand in top surface quality.

Hardness, Density
• Hatch distance is the most influential process parameter affecting the print part density.

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There is a close correlation between the geometry of scan tracks and macroscopic properties of the printed parts.

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The optimal energy density was found to be 1. The microstructure revealed fish-scale morphology along the Y-build direction and oval shaped structure in the Z-build direction.

Dynamic-Compression
• The dynamic anisotropic properties were insensitive to variation in strain rates. • Anisotropic differences were considerably reduced by applying T5 heat treatment.

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The as-built samples failed after SHPB tests which was observed in the T5 heat treated samples.
EOSINT M280 system [94] [138] It is evident from the information presented in Table 1 that there is a lot of interest in understanding the SLM processability of various aluminium alloys. Olakanmi et al. [139] reported in detail the influence of process parameters, powder properties, and laser types on the densification of aluminium alloys. Moreover, they also reported the potential issues arising during the SLM of aluminium alloys, such as metallurgical defects, porosity, and oxidation. The primary causes for these processing challenges include high reflectivity, high thermal conductivity, and poor flowability of aluminium alloy powders, which, as reported by Aversa et al. [140], can be addressed by the development of new alloys specifically designed to improve the SLM-printability of various aluminium alloys. This idea was corroborated by Sercombe and Li [141] who reported improved microstructures and enhanced mechanical properties of various aluminium metal matrix composites fabricated via SLM.
The primary research focus areas of most of the published literature on SLM of aluminium alloys can be broadly classified into four categories, viz, process parameter optimization, effects of build orientation, in-situ and post heat treatment strategies, and modelling and numerical analysis of print-part performance as listed in the overview of publications in Table 2. It is clear from this overview that AlSi 10 Mg alloy has received maximum attention of research papers for all four research focus areas for mechanical properties, with a large number of researchers paying attention to the effects of build orientation and heat treatment of this alloy. More research needs to be done on process optimization and effects of the build orientation and heat treatment of AlSi 12 alloy and other two less popular A356 and A357 alloys processed by SLM. As reported by Rometsch et al. [142] and Zhang et al. [143], these studies aim to improve the performance of SLM-printed aluminium components via: (a) optimizing powder characteristics, (b) optimizing process parameters to obtain 100% dense parts, (c) minimizing the defects such as residual stresses, distortion, and cracking, (d) optimizing post heat treatment techniques to achieve desired mechanical properties, (e) understanding the microstructural characteristics of the print-part, and (f) anisotropic mechanical properties as a result of varying build orientation.

Mechanical Properties of SLM-Printed Aluminium Alloys
Characterisation of the mechanical properties of any additively manufactured part is generally very demanding. Although ASTM F3122-14 presents standard guidelines to evaluate most of the mechanical properties of 3D-printed metallic materials, there is no set standard developed for this purpose, making it difficult to benchmark and/or compare the properties of the same material printed using different AM printers with varying geometries. Therefore, extensive research has been published to characterize various types of mechanical properties of SLM processed Al alloys under static, dynamic, and heat treatment conditions. Table 3 is a compilation of the various mechanical properties investigated by researchers for four main types of SLM-printed aluminium alloys, namely AlSi 10 Mg, AlSi 12 , A356, and A357.   Table 3 shows that maximum research effort has been directed to investigate the tensile properties of SLM processed aluminium alloys. In most of these publications, effect of build orientation and heat treatment on microhardness, yield strength (YS) and ultimate tensile strength (UTS) has been studied. Many of these publications have also discussed the microstructure variation in the samples arising due to rapid heating and cooling encountered in the SLM process and relate them to the mechanical property relationships. Table 3 also reveals that some important mechanical properties such as quasi-static compression, dynamic (high strain rate), fatigue, impact, flexural and wear have received very little attention of researchers. These properties are of significance in many automotive, aerospace, and biomedical applications.
Heat treatment is one of the essential processes to achieve the desired property such as the ductility of the part produced by SLM. Ma et al. [148] have applied the heat treatment of hyper-eutectic alloy AlSi20 to investigate the mechanical properties as well as microstructure. In the heat treatment process, Si particles became coarser and the morphology changed from fibrous to plate like structure. Also, as anticipated, the changes in the microstructure during heat treatment affects the mechanical properties. A significant decrease in strength and, on the other hand, a very high increase in ductility were observed.
A detailed description of research carried out on each of the above-mentioned aluminium alloys is discussed below.

SLM-Printed AlSi 10 Mg
AlSi 10 Mg is one of the most common aluminium alloys and finds wide applications in various industries of 3D printing applications such as automotive and aerospace. The effect of build orientations in horizontal (H), inclined (I), and vertical (V) directions, and the effect of post heat treatment on the tensile mechanical properties of SLM-printed AlSi 10 Mg are summarized in Table 4. In most of the studies, the tensile samples built in horizontal direction on the build plate seem to provide higher tensile strength than the sample build in vertical direction. In most studies, it was observed that heat treated (HT) SLM samples display reduced tensile strength and increased ductility than the as-built (AB) SLM samples. Very few studies have been conducted to assess the effect of build orientation on heat treated SLM samples. Table 4 also presents results of some studies carried out on fatigue and fracture toughness of the SLM-built parts. Very few studies have been undertaken on flexural behaviour of the printed aluminium parts. Table 4 shows results of work carried out on the quasi-static and dynamic compression of SLM processed aluminium samples. Again, a great deal of studies are required to understand the quasi-static compression and high strain rate dynamic tensile and compressive behaviour of SLM-processed aluminium parts for different orientations and heat treatment conditions. Moreover, the mechanical performance of SLM-processed aluminium alloys operating at elevated temperatures have also not received the attention of researchers.    According to Aboulkhair et al. [149], when the process parameters and scan strategy were optimised, it would affect the density of AlSi 10 Mg parts produced by SLM directly. Therefore, the porosity component was evaluated, and the part was found to be 99.8% dense. In a study by Read et al. [108], the mechanical properties of the SLM built AlSi 10 Mg samples were found to perform better than as-cast alloys of the same composition. The mechanical properties tested were tensile strength, creep, and the porosity. Similarly, Li et al. [80] reported that the solution treatment of as-built AlSi 10 Mg samples produced by SLM yielded better ductility (from 5% to 24% approximately), while the tensile strength was significantly reduced from 434 MPa to 168 MPa approximately. Li et al. [81] investigated the as-built SLM AlSi 10 Mg samples and tested them at −70 • C for mechanical properties.
The fish-scale morphology along the build direction and oval structures on the direction perpendicular to the build direction were observed.
A study by Hitzler et al. [68] focused on the anisotropic compressive behaviour of SLM AlSi 10 Mg. They found that compressive Young's modulus was significantly higher than the Young's modulus of tensile loading as well as Young's modulus of bulk base material. The compressive Young's modulus found to be about 82 GPa. Furthermore, they reported that the compressive yield strength was found to be similar to tensile yield strength, however the ultimate compressive strength was obtained to be significantly higher than the tensile strength.
Wang et al. [150] studied the effect of energy density on densification behaviour and surface roughness of SLM AlSi 10 Mg. The densification behaviour was analysed using X-ray and CT scanning. High relative density was obtained through the point distance of 80-105 µm with an exposure time of 140-160 µs. Furthermore, energy density was stated to have a significant influence on the surface morphology. On one hand, the increased energy density could lead to a balling defect and, on the other hand, reduced energy density could cause defects, such as porosity and micro-cracks.
Everitt et al. [54] investigated the hardness through nano-indentation and found that AlSi 10 Mg samples produced by SLM have higher hardness than cast counterpart. Furthermore, the microstructural studies revealed that grain sizes were increased at the melt pool edges, which were associated with the homogenous distribution of Si particles throughout the sample.
A study by Leon and Aghion [151] evaluated the effect of surface roughness on SLM processed AlSi 10 Mg followed by stress-relief heat treatment. They focused on corrosion resistance and corrosion fatigue performance. Furthermore, the results of SLM processed AlSi 10 Mg samples were compared with the cast counterparts. Polishing was found to improve the corrosion fatigue life span of SLM samples, whereas the unpolished SLM samples caused an increase in surface roughness and other surface defects.
Ch et al. [43] studied the SLM AlSi 10 Mg alloy under two different environments namely argon and nitrogen, while building the parts in both horizontal and vertical orientations. Their microstructural findings include cellular structure showing α-Al matrix displaying Al and Si eutectic mixture. Tensile strengths were found to be 385 MPa and 338 MPa for the nitrogen and argon atmosphere respectively. Furthermore, the samples built in the horizontal orientation showed a variation of about 5% in the nitrogen environment, whereas in the vertical orientation, 7.5% variation of strength was found. Based on their findings, nitrogen was inferred to have preference over argon as the shielding gas.
Takata et al. [121] studied the microstructure and mechanical properties of SLM processed AlSi 10 Mg samples heat treated at temperatures 300 • C and 500 • C. They reported that, at elevated temperatures, fine Si phase precipitation occurred resulting in coarse eutectic Si particles. The as-built part was found to exhibit a tensile strength of 480 MPa. Furthermore, the tensile strength was reported to be isotropic, whereas the tensile ductility was found to be anisotropic. Even, the anisotropic property was found to disappear at a heat-treated temperature of 530 • C.
Girelli et al. [152] investigated the effect of temperature, solution treatment, and ageing on the microstructure, microhardness, and density of SLM AlSi 10 Mg. They also investigated the AlSi 10 Mg samples produced by gravity casting under the same heat treatment conditions. The as-built SLM samples were said to exhibit superior mechanical properties than gravity casted samples due to the refinement of grains and nano-sized Si particles. The as-built AlSi 10 Mg samples had a fine microstructure and the heat treatment was found to decrease the ultimate tensile strength but not the yield strength.
Liu et al. [85] investigated the as-built AlSi 10 Mg samples produced by SLM and reported the issues with respect to surface roughness and mechanical properties. Sandblasting had shown a difference of 80% in surface roughness values. Furthermore, the results of tensile tests revealed superior outcome than the die-cast counterparts. A similar behaviour was found in terms of hardness and density.
In a study by Raus et al. [107], mechanical properties namely microhardness, tensile strength and impact toughness were investigated. The results were then compared with conventional high pressure die cast A360 alloy. The results revealed that microhardness and yield strength were higher for the SLM AlSi 10 Mg by 42% and 31% respectively than the die cast counterparts. They used the SLM process parameters as laser power of 350 W, scanning speed of 1650 mm/s, and hatching distance of 0.13 mm to attain the part density of 99.13%.
Recently, Hadadzadeh et al. [61] studied the strain rate behaviour for AlSi 10 Mg alloy processed by SLM under dynamic loading in two build orientations, one horizontal and the other being vertical. The strain rate was said to increase rapidly in both the cases, while the strain rate at vertical orientation was~1400 s −1 . Both the flow stresses follow a similar trend, whereas the flow stress in the vertical orientation were stated to progress further with increase in strain.
Nurel et al. [95] have also reported the dynamic properties of SLM processed AlSi 10 Mg at strain rates of 7 × 10 2 -8 × 10 3 s −1 using SHPB. The SLM AlSi 10 Mg was found to have been affected by build orientation and heat treatment. They also investigated the anisotropic nature of SLM parts using ellipticity after the SHPB tests, but they have not observed any strain rate sensitivity. The focus of this research was on the significance of the build orientation at different strain rates. No significant correlation of build orientation with respect to strain rate was found in the range of 1000-3000 s −1 .
Asgari et al. [31] have also investigated the dynamic behaviour and texture of AlSi 10 Mg with build plate heating at 200 • C. Their findings showed that, in vertically built SLM samples, shock loading did not have an influence on texture and in horizontally built samples, transition of texture was observed (at a strain rate of 1600 s −1 ). The dynamic mechanical behaviour was investigated using split Hopkinson pressure bar at a strain rate ranging from 150 s −1 to 1600 s −1 .
Trevisan et al. [10] reviewed the process, microstructure and mechanical properties of SLM AlSi 10 Mg. The focus was on mechanical properties coupled with very fine microstructure, which is primarily due to rapid melting of AlSi 10 Mg powder and fast solidification. The review analysed the effect of main process parameters, namely laser power, scan speed, layer thickness, and scan strategy. In addition, the effect of heat treatment on tensile as well as fatigue properties associated with the SLM AlSi 10 Mg samples were discussed.

SLM-Printed AlSi 12
The AlSi 12 alloy is increasingly gaining focus because it is a near eutectic alloy characterised by good casting ability and high specific strength. The low melting point of this alloy provides good castability due to its low density and good wear properties, AlSi 12 is quite attractive for applications in automotive and aerospace industries. It can be used for cryogenic applications as it can retain its strength at low temperatures such as at high altitudes [106]. The mechanical properties of SLM-printed AlSi 12 are summarized in Table 5. It is noted that SLM processed AlSi 12 alloy has received less attention for mechanical characterization compared to AlSi 10 Mg alloy in terms of heat treatment and build orientation effects. Most of the comments mentioned earlier concerning the need for further research on different properties mentioned for the AlSi 10 Mg alloy also apply for SLM printed AlSi 12 alloy.   Siddique [14,115] reported that in automotive and aerospace industries, there is trend to replace cast iron with Al-Si alloys to achieve light weighting in components such as engine blocks, cylinder heads, pistons, power trains, and intake manifolds. Many of these parts are moving and operating under dynamic loading conditions with elevated temperatures. Therefore, extensive work needs to be done to understand the mechanical behaviour of Al-Si alloys processed by SLM under dynamic loading, impact, wear, and elevated temperature conditions. Prashanth et al. [17] investigated mechanical behaviour of SLM AlSi 12 and reported that the yield strength and tensile strength were 260 MPa and 380 MPa respectively. The effect of annealing on the microstructure and on the tensile properties revealed that mechanical properties could be fine-tuned, and wide range of strength and ductility could be achieved. In another study, Prashanth et al. [16] evaluated the effect of annealing on tribological and corrosion properties of SLM processed AlSi 10 Mg through sliding and fretting wear tests. The sliding wear showed less wear rate when compared to the cast parts. The studies revealed that wear properties and corrosion behaviours have a correlation with each other, as both are strongly associated with a change in microstructure. The change in microstructure was due to the heat treatment, as the Si particles grow, and the density decreases with increasing annealing temperature.
In another investigation, Prashanth et al. [153] carried out compression tests on both AlSi 12 -Titanium, Niobium, and Molybdenum (TNM) composites and AlSi 12 matrix produced by SLM and studied their behaviour. The compression tests revealed that TNM composites-AlSi 12 matrix had significantly better compressive strength when compared to the AlSi 12 matrix. However, the TNM composites had revealed less plasticity. Prashanth et al. [101] also analysed the tensile behaviour of SLM AlSi 12 , where they applied the base plate heating and four different hatch styles. The samples showed similar crystallite sizes but different textures. In addition, the samples produced with base plate heating had lesser residual stress. The results reported were the yield strength ranging between 235 MPa and 290 MPa, while the ultimate tensile strength varied between 385 MPa and 460 MPa. Furthermore, the ductility varied between 2.8% to 4.5%. Since the base plate heating produced desirable results, they carried out base plate heating at three different temperatures namely 473 K, 573 K, and 673 K, which yielded the tensile ductility of 3.5%,~3%, and 9.5% respectively.
Siddique et al. [116] used the computer tomography for investigating the fatigue performance of SLM AlSi 12 samples. The samples revealed better or at least comparable yield strength or ultimate tensile strength when compared to cast parts. Surface roughness was found to affect the fatigue strength, which was improved by post processing. Furthermore, a porosity study was essential to employ the samples for high cycle applications. The computer tomography was applied to study the stress concentration due to porosity. In another study Siddique et al. [15] determined the effect of process parameters and post processing on the microstructure and mechanical properties of SLM processed AlSi 12 . They performed quasi-static tests and fatigue tests for mechanical characterisation. Furthermore, they observed extra-ordinary eutectic microstructure using scanning electron microscope. By changing the build rate and keeping the process in control, mechanical properties obtained were found to be better for SLM parts than cast counterparts. Moreover, Siddique et al. [14] focused on the design considerations and mechanical properties of SLM processed AlSi 12 parts. From the application perspective, the parts were designed to perform in the high cycle environment. Therefore, the study was more focused on very high cycle fatigue and the corresponding crack propagation behaviour in the cyclic environment. They further investigated the base plate heating and post processing, especially stress relieving, which are critical for the fatigue loading of AlSi 12 parts. The base plate heating was found to influence the fatigue crack initiation mechanisms.
Wang et al. [131] studied the effect of build chamber atmosphere on the SLM AlSi 12 samples. The effect of three different atmosphere namely argon, nitrogen and helium were studied for their influence on the mechanical properties. Hardness, density, and relative density were analysed and found to have similar attributes for all the atmospheres. However, the mechanical properties of parts produced in argon and nitrogen atmospheres were found to be better than the helium atmosphere. Furthermore, the mechanical properties in SLM parts were better than those produced by conventionally made AlSi 12 counterparts.
Lykov and Baitimerov [154] investigated the SLM AlSi 12 to determine the process parameters to ensure the least porosity. The samples were found to have varied microstructures and the porosity was reported to be about 0.5%. The AlSi 12 powders were found to have poor flowability. The surface morphology revealed that samples have an uneven surface. They recommended that to reduce the surface roughness and porosity, it was essential to use AlSi 12 powder with fine fraction of about 17 µm and the scanning strategy include double pass laser scan strategy.
Mendřický and Keller [155] focused on the precision, geometry and dimensional accuracy of the AlSi 12 parts produced by SLM. The part orientation was planned to minimise the internal stress. The SLM part was digitised to collect the sample data. The collected data was compared with the CAD data to evaluate for dimensional and geometrical accuracy. The deformation of the model was caused by the internal stress on the as-built AlSi 12 parts.
Šafka et al. [156] analysed the build orientation and the placement of part in the SLM build substrate, which could reduce the amount of support structures required and improve the flexural strength of SLM AlSi 12 parts. They observed that the least energy and therefore less heat would lead to little deformation of the printed final part.
Chou et al. [45] proposed a new approach of using pulsed SLM over the conventional SLM for AlSi 12 and achieved greater control of the heat input. In the SLM AlSi 12 , Si refinement below 200 nm was attained. Furthermore, 95% of dense parts and the hardness of 135 HV were achieved. The part produced by pulsed SLM method yielded better hardness than the conventional cast alloy counterpart. The cooling rate and thermal gradient had an influence on solidification phase and the microstructure.
In a study by Vora et al. [13], a novel method had been discussed to produce aluminium parts with the anchorless SLM. This anchorless method is opposed to the regular method that uses anchor or support for the SLM parts. Furthermore, the anchorless SLM (ASLM) could produce SLM parts that can remain in the stress-free state. The ASLM was found to be suitable for processing eutectic alloys, as well as hypo eutectic and hyper eutectic alloys. In addition, they studied the in-situ alloy formation within SLM chamber. The residual stress was also found to be less with the ASLM processed AlSi 12 alloy.
Suryawanshi [120] investigated the tensile strength and toughness of SLM AlSi 12 and found that mechanical properties were relatively better than the cast counterparts, except the ductility. The scan strategy especially linear vs checker-board hatch style was analysed and the later was found to have significant effect on the tensile strength. Furthermore, the meso-structure due to the laser hatch tracking leads to improved fracture toughness. In addition, to attain higher fatigue strength, the residual stresses, porosity, and un-melted powder particles needed to be removed. The SLM AlSi 12 was found to give more avenues for material design and fabrication with enhanced strength and toughness.
In a study by Li et al. [82], the SLM AlSi 12 was reported to have controllable and ultrafine microstructure. The excellent mechanical properties were achieved through solution heat treatment. They introduced a novel approach for refining the eutectic alloy of AlSi 12 , which yielded better tensile properties than cast counterparts. The solution treated AlSi 12 was found to produce the tensile ductility of~25%. Furthermore, the tailoring of the mechanical properties according to the applications was possible by controlling the solution heat treatment time.
Louvis et al. [8] investigated the process parameters of SLM AlSi 12 to achieve uniform relative density. The process parameters investigated were laser power and the laser scanning rate. The investigation was extended to understand the difficulty of processing AlSi 12 when compared to stainless steel and titanium alloy. The major factor that affected the relative density was the oxidation factor. The process parameters were varied using two different SLM machines, one with 50 W laser power and the other with 100 W laser power. Even with the optimum process parameters combination, the machine with 100 W laser power yielded only the relative density of 89.5%. Furthermore, it was recommended that to produce SLM AlSi 12 parts with 100% relative density, it was essential to develop methods to disrupt the oxide formation or to prevent oxidation.
Ackermann et al. [157] studied the fabrication of thin structures and their applications using SLM AlSi 12 parts. In service, the applications of SLM AlSi 12 parts were found on microelectronics, fine mechanical structures, and automotive mechanisms. The powder characteristics such as mean particle size and powder shape, the process parameters such as laser spot size and scanning speed were found to influence the thickness of fine structures. They arrived at 0.21 mm as the optimum thickness for thin structures, for sufficient strength.
Rathod et al. [106] investigated the effect of scanning strategy and heat treatment on the tribological properties of the SLM AlSi 12 parts and compared with cast alloy. The process of annealing lead to Si precipitation resulting in the reduction of hardness. The as-built AlSi 12 SLM samples were reported to have less wear rate when compared to heat treated SLM samples and cast samples.

SLM-Printed A356 and A357
There are two other types of Al-Si alloys, A356 and A357, which have been processed by SLM. These two alloys are also Al-Si-Mg alloys. They both contain 7%Si by weight, but they have a slightly different Mg content. The alloy A356 is AlSi 7 Mg 0.3 and alloy A357 is AlSi 7 Mg 0.7 . Very few studies have been made on mechanical characterisation of these two alloys processed by SLM. The mechanical properties of SLM-printed A356 and A357 are summarized in Table 6.  Kimura and Nakamoto [20] optimised process parameters to arrive at dense SLM A356 (AlSi 7 Mg 0.3 ). The relative density attained was 99.8% with the laser irradiation conditions. The mechanical test results found the ultimate tensile strength of~400 MPa, the yield strength of~200 MPa and the elongation from 12-17%. The heat treatment revealed the difference in microstructure and mechanical properties of SLM A356 samples and as-cast samples. After annealing, the SLM samples became elongated to 30%, while the tensile strength of A356 samples reduced by half the value and becamẽ 200 MPa.
Rao et al. [25] investigated the mechanical properties and microstructure of SLM A357. The processing parameters were optimised to achieve dense A357 samples, along with fine microstructure. The porosity was also analysed based on relative density and laser parameters. The anisotropy of SLM A357 Al alloy samples were investigated in the horizontal and vertically built tensile samples. Fractographic studies revealed that horizontal orientation was better for printing the A357 tensile samples. In another study Rao et al. [24], the tensile behaviour of SLM A357 in the as-built and heat treated condition was investigated. The as-built sample displayed ultrafine microstructure. The tensile samples of SLM A357 were found to have better properties than their cast counterparts. The Al grains in the as-built parts had eutectic nano-sized Si particles, contributing for higher strength. However, the nano-sized Si particles did not favour the ductility. After the heat treatment of SLM A357, as anticipated, the tensile ductility improved and was reported to be about 23% with reduced tensile strength.
Yang et al. [26] investigated the effect of heat treatment on SLM A357 by focusing on stress relief to analyse the mechanical property and microstructure. For the as-built A357 samples, the rapid melting and rapid cooling affects the intermetallic phases such as Mg 2 Si precipitates. During the heat treatment process, the breaking up of the Si network was said to occur, which lead to high ductility. Si particles at grain boundaries were observed to coarsen. Furthermore, anisotropy was observed to disappear in terms of affecting yield strength and ductility when the microstructure became more homogenised.

Other Aluminium Alloys Processed Using SLM
From the research perspective and for the applications with desired mechanical properties, new Aluminium alloy are being developed and processed using SLM. These new aluminium alloys have been developed by alloying them with new elements, such as Cu, Ni, Sc, Zr, V, and Zn, to impart improved properties and better processing by SLM.
Aversa et al. [158] developed Al-Si-Ni alloy samples for SLM with a combination of AlSi 10 Mg and Ni powders. This combination was found to be near eutectic by composition. Furthermore, this combination of AlSi 10 Mg and Ni were reported to have better hardness than AlSi 10 Mg. The increase in hardness was mainly attributed to the addition of Ni, which was demonstrated through nano-indentation measurements. The SLM process parameters were optimised to reduce porosity. The results obtained with the combination of AlSi 10 Mg and Ni had also yielded Al3Ni agglomerates.
Spierings et al. [159] developed Sc and Zr-modified AlMg alloys for SLM processing. The Sc and Zr-modified Al alloy is commercially known as "Scalmalloy" and offers more advantages over traditional 4xxx as-cast alloys. These advantages include high strength and high ductility at low anisotropy. Furthermore, fine grain microstructure and weak texture along the build direction causes low anisotropy. They also analysed the influence of larger scan speeds on the mechanical properties of Scalmalloy and found that grain sizes were reduced from 1.1 µm to 600 nm.
Zhang et al. [160] investigated the manufacturing of Al-Cu-Mg alloys using SLM. They studied the effect of process parameters on the density of SLM Al-Cu-Mg alloys. The energy density was found to have significant influence on the densification of Al-Cu-Mg alloys. The threshold value of energy density was 340 J/mm 3 , that yielded the sample density of 99.8%. Furthermore, fine microstructure was obtained without any imperfections and micro-cracks. A high value of ultimate tensile strength of 400 MPa and the yield strength of~275 MPa were obtained for the SLM Al alloy part. In addition, they found fine grains and solid solution strengthening mechanism for the higher mechanical strength. In another study, Zhang et al. [161] investigated the microstructure and mechanical behaviour of SLM Al-Cu-Mg alloy and Zirconium modified SLM Al-Cu-Mg alloys. The addition of Zr had the effect of reducing the hot cracking in the SLM part. The comparison of Zr modified Al-Cu-Mg alloy and Al-Cu-Mg alloy was found to show an ultrafine grain with Zr addition. Furthermore, Zr modified Al alloy had significantly higher yield strength close to its ultimate tensile strength of~450 MPa.
Nie et al. [162] studied the effect of Zr on the formability, mechanical properties and microstructure of SLM Al-Cu-Mg-Mn alloys. The addition of Zr to SLM Al-Cu-Mg-Mn alloy was observed to be effective in crack controlling mechanism, grain refinement and in the controlling of mechanical properties. The addition of Zr was found to cause the transformation of grain type from columnar to equiaxed type. Furthermore, they investigated crack inhibition and enhancement of mechanical properties due to the addition of Zr with SLM Al alloy.
Karg et al. [163] investigated the laser beam melting of EN AW-2219 and observed that laser processing of EN AW-2219 (Al-Cu alloys) was very challenging. Porosity and tensile tests were conducted for the SLM Al-Cu alloy. Then, T6 heat treatment was applied to some built samples and the elongation was observed. The elongation was found to exceed in the build direction by a factor of two when compared to traditional cast counterpart.
Martin et al. [164] stated that several types of aluminium alloys cannot be processed using SLM due to the complex melting and solidification dynamics. The issues were found to be microstructures with larger columnar grains and periodic cracks. However, they suggested methods to resolve these issues by introducing nanoparticles that would control the solidification. Then the powder particles were found to be qualified for SLM. Furthermore, the technique of solidification control could be applied for the conventional process as well, where hot cracking and hot tearing occurs commonly.
Montero-Sistiaga et al. [9] studied the SLM Al7075 (Al-Zn-Cu-Mg), a wrought alloy, to improve its density. The SLM Al7075 initially displayed poor density and micro-cracking, but it was improved by adding 4% Si. Heat treatment was carried out to improve the hardness of SLM Al7075. Furthermore, the focus was more on producing high strength aluminium alloys.
Aversa et al. [165] investigated the mechanical, metallurgical and the thermal properties for SLM of Al-Si-Zn-Mg-Cu alloy. The introduction of Si was analysed with respect to Al-Zn-Mg-Cu alloy and the crack density was found to be reduced. This crack density reduction was due to improved molten phase fluidity and the reduction of the co-efficient of thermal expansion. Furthermore, the SLM Al-Si-Zn-Mg-Cu alloy was found to be promising in terms of microstructure, microhardness, and the tensile properties.
Zheng et al. [166] reported the variation of microstructure and hardness of an SLM Al-8.5Fe-1.3V-1.7Si. Among the melt pool, three different zones namely laser melted zone, melting pool border and heat affected zone were identified. Microhardness results of the SLM process exceeded the as-cast counterpart. Furthermore, with the decrease in laser scanning speed, laser melted zone was found to reduce significantly.
Croteau et al. [167] investigated the microstructure and mechanical properties of SLM Al-Mg-Zr alloys. The energy densities applied for the fabrication of SLM parts ranged between 123 and 247 J/mm 3 and the resultant relative density was verified using X-ray tomography. The alloying elements have a significant role, where Mg acted as a solid solution strengthener and Zr contributed towards metastable precipitates resulting in grain refinement and prevention of hot tearing. Furthermore, Zr alloy had improved the mechanical properties.
In a study by Maamoun et al. [88], the influence of process parameters of SLM AA 6061 and AlSi 10 Mg were investigated, based on the relation between microstructure and mechanical properties. AA 6061 is also an Al-Si-Mg alloy with a higher coefficient of thermal expansion (CTE). The process optimisation was carried out on both the aluminium alloys to reduce the defect in microstructure. The mechanical behaviour of the aluminium alloys was analysed through the design of experiments and the results were presented for hardness, yield strength and ultimate tensile strength. The results obtained was useful in improving the part quality that had reduced the post processing requirements.
Uddin et al. [168] analysed AA 6061 using high temperature heating in an SLM environment to yield crack-free parts. The hardness was found to be 54 HV, the yield strength as 60 MPa, ultimate tensile strength of 130 MPa and the elongation of 15%. Furthermore, they revealed that AA 6061 could be successfully manufactured with SLM without displaying the cracking phenomenon.

Summary and Research Gaps
Although aluminium can be with Zn, Cu, Mg, Mn, and Si to produce age-hardening alloys, casting alloys, and work-hardening alloys, the area of focus in this review was the Al-Si alloys processed by selective laser melting (SLM) metal additive manufacturing technique and their mechanical properties. The Al-Si alloy has the cast-ability and weldability, which forms the unique combination and a suitable candidate for processing in SLM. As mentioned, the most common SLM aluminium alloys are AlSi 10 Mg and AlSi 12 based on the literature review. This review has focused on classification of published research on SLM-processed aluminium alloys in terms of research carried out on the effects of process optimisation, build orientation and heat treatment on four main types of Al-Si aluminium alloys. The published papers have also been categorised for various types of mechanical properties considered in SLM parts to identify research gaps on which mechanical properties needs more in-depth investigation from the point of view of industrial applications. This review has revealed that even though many studies are reported for the tensile mechanical properties of SLM aluminium alloys, very few studies were carried out on the dynamic behaviour in tension and compression, fatigue, impact, wear, and flexural response of the printed parts. Specially very few studies were published on high strain rate loading behaviour of parts made by any metal based additive manufacturing processes like DMD, EBM, and SLM.
One of the other research gaps found in this review is the effect of build orientations on the quasi-static and dynamic compression properties of SLM-processed various other aluminium alloys. It was also observed that some important mechanical properties such as fatigue, impact, flexural, bending, and wear have received very little attention. Moreover, the mechanical performance of SLM-processed aluminium alloys operating at elevated temperatures has also not received much attention. These properties are of significance in many automotive, aerospace, and biomedical applications.
Another important research avenue is to understand the effect of machine-specific characteristics on the properties of the printed components. It was found in this review that researchers have used different SLM machines supplied by various machine manufacturers globally. It is interesting that the same material when printed using different machines yield different mechanical and microstructural characteristics. This can be attributed to the differences in the laser systems used, inherent characteristics of the machine itself, and various other parameters which are not quite understood. Moreover, the repeatability of the mechanical performance of printed parts using same and/or different machines is to be further explored.
Lastly, the post heat treatment plays a crucial role in tuning the microstructural characteristics of the printed parts and render them suitable for desired applications. Most researchers have investigated the heat treatment protocol typically used for conventionally fabricated aluminium alloys, which may not be ideal for SLM-printed parts, as they have different inherent properties. Therefore, further research is required to be carried out to assess the effects of different heat treatment strategies on the performance of printed parts.