Defect-Correlated Vickers Microhardness of Al-Si-Mg Alloy Manufactured by Laser Powder Bed Fusion with Post-process Heat Treatments

Abstract

Laser powder bed fusion is an additive manufacturing process characterized by different advantages like the manufacture of samples with complex geometry without the use of tools and/or molds. Generally, the manufactured samples are characterized by high tensile strengths which, however, can be affected by the presence of defects due to the unoptimized process parameters. In a large applications field, a low density of the as-built AlSi10Mg samples is a very important parameter to considered, e.g., due to both the loss of the tensile strengths correlated with a premature failure of the samples and the increase in time and costs associated with the manufacturing process. In addition, different post-process heat treatments can increase these effects leading to an ineffective manufacturing process. In this scenario, the present work shows the analysis of spherical and lack-of-fusion pores induced by the laser powder bed fusion process on the AlSi10Mg samples and their variations after different heat treatments (direct aging and T6). At the same time, the influence of pores on the Vickers microhardness and the tensile properties has been studied in the same AlSi10Mg samples (bars and billets) that were printed with single- and double-laser machine setup. Different process parameters were also analyzed and compared. The study was supported by the microstructural and pore analysis performed by optical microscopy along the XZ plane (build direction) and the XY plane. Finally, the greatest effects of pores were observed on the Vickers microhardness values; in fact, two different relationships between microhardness and density variation are discussed. The T6 heat treatment leads to a rounding of the pores already formed in the as-built samples and to a formation of new small pores. Graphical Abstract

Aging Profiles of AlSi7Mg0.6 and AlSi10Mg0.3 Alloys Manufactured via Laser-Powder Bed Fusion: Direct Aging versus T6

“…The microstructural changes induced by the T6 HT lead the UTS and YS at values lower than 300 MPa and 250 MPa, respectively, without a satisfactory improvement of the elongation values. In addition, the high temperatures reached during the SHT induces a pore size increment as widely reported by [ 23 , 24 ] and discussed in [ 25 ] for the AlSi10Mg0.3 alloy. Tonelli et al [ 26 ] added that the T6 HT can reduce the microstructural anisotropy and the correlated mechanical properties, in addition to the completely relieve of the residual stress after 10′ at 540 °C in AlSi7Mg0.6 subjected to LPBF.…”

“…In the former zone, only one laser scans the powder bed; in the latter one, the powder bed is scanned simultaneously by two lasers. The design of the printed samples, where the yellow and red areas of the build platform represent the SL and DL zones, is shown in [ 25 ]. For both aluminium alloys, bars (10 × 10 × 300 mm 3 ) and billets (10 × 100 × 300 mm 3 ) were manufactured with the same process parameters ( Table 2 ) but in different processing times.…”

“…The temperatures chosen for the DA, SHT and AA heat treatments were based on [ 18 , 22 ], where AlSi10Mg0.3 alloys manufactured with a layer thickness of 50 μm were analysed. The top and bottom samples listed in Table 3 were cut by the bars (see Figure 1 in [ 25 ]) and the different heights were chosen in relation to the both the HV measurements performed on the same bars in as-built conditions and to the results obtained in [ 18 , 22 ]. The T5 (i.e., DA) and T6 HTs were carried out following the conditions dictated by the standard specification of aluminium alloy casting [ 27 ].…”

“…For each couple of indentations at the same height, the 60 average values of HV were used to plot the HV profiles for the SL and DL bars. Focusing on xy planes, the 3 × 3 matrices were always carried out on the centre of the bar cross-section to avoid the contribution conferred by the skin part ( Figure 1 ) as shown in [ 25 ]. The 9 indentations were spaced 1 mm from each other as highlighted by the δ in Figure 2 b.…”

“…Firstly, one OM micrograph at 500× and three at 1000× of magnification were considered to analyse the Si-eutectic on solution heat-treated samples at 1 h (SHT 1 h), 2 h (SHT 2 h) and 4 h (SHT 4 h). Secondly, the pores analysis was carried out following the methodology used in [ 25 ] because the density values of the heat-treated AlSi7Mg0.6 samples will be compared to those discussed for the AlSi10Mg0.3 samples in the same research article. For this reason, the chosen HTs are: DA at 175 °C, 200 °C and 225 °C × 8 h, and T6*.…”