Premature failure of an additively manufactured material

Abstract

Additively manufactured metallic materials exhibit excellent mechanical strength. However, they often fail prematurely owing to external defects (pores and unmelted particles) that act as sites for crack initiation. Cracks then propagate through grain boundaries and/or cellular boundaries that contain continuous brittle second phases. In this work, the premature failure mechanisms in selective laser melted (SLM) materials were studied. A submicron structure was introduced in a SLM Ag-Cu-Ge alloy that showed semicoherent precipitates distributed in a discontinuous but periodic fashion along the cellular boundaries. This structure led to a remarkable strength of 410 ± 3 MPa with 16 ± 0.5% uniform elongation, well surpassing the strength-ductility combination of their cast and annealed counterparts. The hierarchical SLM microstructure with a periodic arrangement of precipitates and a high density of internal defects led to a high strain hardening rate and strong strengthening, as evidenced by the fact that the precipitates were twinned and encircled by a high density of internal defects, such as dislocations, stacking faults and twins. However, the samples fractured before necking owing to the crack acceleration along the external defects. This work provides an approach for additively manufacturing materials with an ultrahigh strength combined with a high ductility provided that premature failure is alleviated.

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

“…Generally, the as-built samples are characterized by an increase of n values with increasing distance from the preheated build platform. This behaviour can be explained through the possible increase of high-density defects or dislocations as previously reported by the argumentation of the residuals stress [75]. Sjögren et al [76] study the effective correlation between the dislocation movements and the strain hardening exponent (n), which increases with the increase of the multiplication and entanglement of dislocations.…”

“…On the other hand, the mobile dislocation easily traverses the grains if the microstructure presents the fragmented and coarsened Si eutectic (non-cellular structure) as obtained after the T6 HT (Figure 4). Figure 8b reports the strain hardening rate normalized with the true stress [75] of the samples (4), ( 1) and ( 2) as representative of the as-built, direct aged and T6 heat-treated conditions, respectively. Focusing on as-built and direct aged samples, it is worth repeating that the failures occur before the necking formation, and these can be considered as premature failures caused by the mechanisms previously discussed.…”

Effect of Substrate Plate Heating on the Microstructure and Properties of Selective Laser Melted Al-20Si-5Fe-3Cu-1Mg Alloy

“…In addition, other compositions related to the present composition (Al-20Si-5Fe-3Cu-1Mg) like Al-20Si-5Fe [ 65 , 66 ] and Al-20Si-5Fe-2X (X = Cu, Ni, Cr) [ 10 ] irrespective of the form present (powder or melt-spun ribbons or hot consolidated specimens) show the presence of three phases (α-Al, Si, and δ-Al 4 FeSi 2 ) like the gas atomized Al-20Si-5Fe-3Cu-1Mg powder. Hence, in the present case, the non-equilibrium nature of the SLM process and the presence of anisotropic heat extraction conditions [ 67 ] lead to the formation of the other two phases, Al 2 CuMg and β-Al 5 FeSi.…”

“…The Vickers microhardness of the Al-20Si-5Fe-3Cu-1Mg gas atomized powder is observed to be 234 HV, which is very close to the hardness of the SLM sample, suggesting that SLM and gas atomization process exhibit similar cooling rates, however different is the solidification conditions [ 64 ]. However, similar compositions like Al-20Si-5Fe, and Al-20Si-5Fe-2X (X = Cu, Ni, Cr) [ 10 , 66 , 67 ] processed via powder metallurgical route (gas atomization and subsequent hot consolidation) show hardness of the samples varying in the range 140–200 HV. The hardness is significantly lower than the present SLM sample owing to the presence of Cu and Mg, which promotes the formation of the intermetallic phases Al 2 CuMg and β-Al 5 FeSi.…”

Additive Manufacturing of Aluminum‐Based Metal Matrix Composites—A Review

“…The dislocation density of the PBF‐L alloys is high, of the order of 1 × 10 14 m m −3 . [ 20,122 ] The dislocation interactions with each other will give rise to strengthening, where the dislocations impede with each other. The dislocation strengthening is given by [ 123,124 ] where m is the Schmid factor, α is a proportionality constant, b is the Burger's vector, and ρ is dislocation density.…”