Influence of working environment and built orientation on the tensile properties of selective laser melted AlSi10Mg alloy

Abstract

AlSi10Mg alloy was fabricated by Selective Laser Melting (SLM) using two different atmospheres, argon and nitrogen, both in vertical and horizontal built directions. The influence of work environment and anisotropy are studied in this work by keeping all other process parameters including shielding gas pressure constant. The meso-structural features revealed scan tracks and melt pool layers while the microstructural features have showed cellular structure with α-Al matrix and its boundary made up of the eutectic mixture of Al and Si. These cells were approximately of 500 nm size as revealed by TEM analysis. The grain morphology and texture were studied using EBSD analysis. The XRD analysis revealed similarity in the phases formed in samples built under argon and nitrogen environment. Both the horizontal as well as vertical built samples contain elongated grains with $〈100〉$ direction oriented along the built direction and $〈100〉$ fibre texture in the XY plane. Tensile test results showed the maximum strength of 385 ± 5 MPa was exhibited by vertically built sample under nitrogen environment while the lowest strength of 338 ± 2 MPa was exhibited by horizontally built sample under argon environment. Surface roughness and porosity measured in all the conditions exhibited minor variation in the average surface roughness and porosity area fraction values. Among the samples tested under same built orientation, the variation in strength is approximately 5% higher in nitrogen environment while among the samples tested under same working environment, the variation in strength is approximately 7.5% higher in vertically built samples. This states that the influence of working environment on strength is lesser than that of the built orientation. The materials exhibited brittle failure with inclined fracture profile, fracture morphology with pores, river patterns and inter-cellular fracture plus approximately 4.5% elongation in all the tested samples, implied isotropic elongation behaviour. The samples built under both the work environments exhibited the similarity in defect structure, microstructure and nearly isotropic behaviour in mechanical properties. It inferred that nitrogen could be preferred over argon as shielding gas in SLM for its cost effectiveness.

Introduction

Additive manufacturing (AM) is an advanced manufacturing technique wherein the production of a part is by building up consecutive layers and operates autonomously on the basis of 3D model. Selective laser melting (SLM) is one of the powder bed fusion type metal AM processes [1] used in aerospace and biomedical applications for manufacturing Ti alloys, stainless steel, Ni based super alloys and Al alloys [2], [3]. Unlike Ni, Ti or stainless steel, the SLM of Al alloys are challenging due to its high thermal conductivity, low melt viscosity and high oxygen affinity [4]. The most commonly used Al alloys for SLM, are hardenable AlSi10Mg alloy and eutectic AlSi12 alloy [5]. Comparatively higher Si content in these Al alloy reduces difference in the solidification temperature, thereby, minimise the hot cracking possibilities [6] which favours SLM. However, high thermal conductivity and low viscosity leads to poor absorption of laser, which demands more laser power. The oxidizing nature of Al alloys require shielding gases to prevent it from getting oxidized. The process parameters together with the shielding gases influences the quality of the solidified laser melt [7].

The SLM process parameters, namely, laser power (P in J/s), scan speed (v in m/s), hatch spacing (h in m) and layer thickness (t in m) influence the mechanical properties as well as surface quality of the fabricated material. The energy density (E in J/m³) of the SLM process is expressed by the Eq.(1) [8] as follows,

$$E=\frac{P}{v.h.t}$$

Read et al. [9] investigated the influence of this energy density on the porosity of SLM fabricated AlSi10Mg alloy. The porosity content has decreased with increasing energy density up to 60 J/m³ and it scattered further up to 120 J/m³ and exhibited keyhole defects. Andani et al. [10] studied the effect of laser power and scan speed individually on the powder spatter during SLM using a high speed camera. The spatters were found to decrease with decreasing laser power and increasing laser speed while the defect percentage was found to increase with decreasing laser power and increasing laser speed. Thus, the defects observed at high energy, where metal melts completely, were due to the spattered powder particles generated due to higher scanning speed. Further, minimum hatch spacing [11], [12] and minimum layer thickness [13], [14], [15], [16] were observed favourable for metal SLM. However, certain studies revealed the similarities in the mechanical properties irrespective of the laser power used and surface roughness obtained. Buchbinder et al. [17] reported that the yield strength and tensile strength are in the same range of 210–240 MPa and 400–450 MPa, respectively, for AlSi10Mg alloy samples built with 240 and 960 W. Shi et al. [18] reported good mechanical properties using high layer thickness of 200 µm during SLM of Ti-6Al-4V but it showed high surface roughness.

The shielding gases used can influence the process. Other than protecting the molten pool from surroundings, the flowing shielding gas can dissipate heat, affect spattering and defects like porosity and surface roughness. Chen at al [19], hypothesised that the infiltrating gas can pass through the narrow gaps between solid-solid contacts and transfer heat. Excessive thermal conductivity of the powder may lead to balling and poor surface finish, but shielding gases dissipates heat from overhanging region which decrease the dross and improve the surface roughness. The shielding gas may be inert, like helium and argon, or reactive like nitrogen. The inert gases does not have any influence on the metallurgical process while nitrogen may affect it, as it can be absorbed by the metal during melting. During laser welding of duplex stainless steel, spattering was observed on all the welds while using argon, but spattering was not observed during high speed welding while using nitrogen as shielding gas [20]. Ol'shanshkii and Morozov [21] evaluated the suppression of porosity during welding of Al alloys by increasing external pressure on the shielding gas, above the molten metal. Argon, nitrogen and helium are the protective gases used during SLM of Al alloys to keep the oxygen level lesser than 0.1%. The infiltration with helium increase the thermal conductivity of powder by 300% when compared to nitrogen and argon, which aids to improve the surface quality [19]. However, Wang et al. [22] reported that the mechanical properties of the AlSi12 parts SLMed under argon and nitrogen environment were superior to those processed under helium environment, as helium formed pore clusters in the microstructure.

Other than process parameters and shielding gas, the layer by layer addition nature of the SLM process can cause anisotropy, due to epitaxial growth along the build direction. The higher laser power tends to transfer the heat in the build direction which gives rise to directional solidification. Inconel and Al alloys had shown anisotropy in yield strength and tensile strength [23], [24] while electron beam melted Ti-6Al-4V alloy exhibited anisotropy in percentage elongation [25]. Prashanth et al. [26] tailored the microstructure suitable for specific mechanical property, in-situ during SLM of Al-Si12 alloy. The anisotropy in this work was controlled by changing the hatch style or/and base plate heating without altering the process parameters such as laser energy and scan speed. Kunze et al. [23] observed that the build direction and scanning direction significantly influence the texture in the material. Hence, different scanning strategies namely, X-scan, XY-scan, rotating scan of 45–70° after every layer and chequered board scans were employed to realise improvement in the microstructure and property of SLMed products [8], [27], [28].

The influence of process parameters were extensively investigated by the recent research works on the SLM of Al alloys. On the other hand, there were only limited literatures available on the influence of protective environment on the microstructure of SLMed material. Further, it will be interesting to study the effect of shielding gases together with the anisotropic nature of the process. Hence, the present work is focused to investigate the defects and anisotropy of AlSi10Mg alloy fabricated through SLM under argon and nitrogen environment and their influence on tensile properties. The microstructural characteristics and textures of SLM fabricated AlSi10Mg alloy were characterised by SEM/EBSD and TEM to substantiate the tensile and fracture characteristics of the alloy observed in the present work.