Work Hardening of Heat-Treated AlSi10Mg Alloy Manufactured by Selective Laser Melting: Effects of Layer Thickness and Hatch Spacing

Abstract

The present study analyzed the microstructure and the mechanical properties of AlSi10Mg SLMed bars (10 × 10 × 300 mm) and billets (10 × 100 × 300 mm) before and after the direct aging at 200 °C for 4 h and the T6 heat treatment. The discussed results are compared to those obtained by the AlSi10Mg samples manufactured with the same geometry but using different process parameters (layer thickness higher than 40 μm and a hatch spacing lower than 100 μm) and also through the Quality Index (QI). These work conditions allow the obtaining of a microstructural variation and different tensile properties in as-built top samples. In both batches, the cycle time was 45 h and together with the preheated build platform at 150 °C, induced an increase of UTS (Ultimate Tensile Strength) and yield strength on the bottom rather than the top samples due to the aging phenomena. Upon completion of the direct aging heat treatment, the effects induced by the platform were cancelled, keeping a full cellular microstructure that characterized the as-built SLMed (Selective Laser Melted) samples. Moreover, the Considère criterion and the work hardening analysis showed that the failure occurs after the necking formation in some of the T6 heat-treated samples. In this last case, the Si eutectic network globularized into Si particles, causing a decrease of UTS (from around 400 MPa to 290 MPa) in favour of an increase of ductility up to 15% and reaching a QI in the range 400 ÷ 450 MPa. These values place these samples between the high-quality aluminium cast alloy and T6 heat-treated ones.

Additive Manufacturing of AlSi10Mg and Ti6Al4V Lightweight Alloys via Laser Powder Bed Fusion: A Review of Heat Treatments Effects

“…The principal process parameters ( P , v , h , t ), which define the ED function (Equation (1)), influence microstructure and mechanical properties of AlSi10Mg and Ti6Al4V as-built samples manufactured via L-PBF process [ 9 , 14 , 68 ]. As shown in Figure 4 , they are the only parameters that can be fully controlled, together with the laser source parameters, to obtain a high-quality sample.…”

“…Shi et al [ 128 ] showed an increase of layer thickness up to 200 μm, maintaining a Ti6Al4V sample density δ > 99.73% (fully dense). On the other hand, varying the layer thickness of AlSi10Mg and Ti6Al4V samples manufactured via L-PBF changes the mechanical properties obtained [ 9 , 32 , 103 , 128 ]. The use of double or quadruple lasers, which work in parallel on the same layer powder bed, or the use of skin-core scan strategy are other examples concerning the increase in the BR [ 120 , 129 ].…”

“…De facto, by decreasing the t and h , the grain size decreases. Ghio et al [ 9 ] showed, instead, an increase in the amount of the equiaxed grains with the increase in layer thickness and decrease in the hatch spacing.…”

“…Despite this equation describing the cast alloy, different research study validates the same equation to describe the AlSi10Mg alloy manufactured via L-PBF process considering the SDAS as the cellular island of α-Al [ 136 , 139 ]. Moreover, in this case, if the MPC is characterized by a higher cooling rate (~10 5 ÷ 10 6 K/s, [ 9 , 136 , 139 ]) than the MPB, the adjacent solidified material is exposed to annealing temperatures that generated the HAZ as expressed in Section 2 . These three different zones are characterized by an increment of the grain size due to the Si diffusion [ 7 , 14 , 136 , 139 , 142 , 143 ].…”

“…AlSi10Mg is an age-hardening alloy based on the Al-Si-Mg ternary system and is characterized by low density (2.67 g/cm 3 , [ 6 ]), good mechanical properties, excellent castability and capability of being heat-treated [ 7 , 8 , 9 ]. The Al-Si phase diagram, shown in Figure 1 a, highlights the fact that AlSi10Mg presents a short solidification range (ΔT ~ 40 K), making it the most common aluminium alloy used in additive manufacturing (AM) [ 10 , 11 ].…”