A novel route for predicting the cracking of inoculant-added AA7075 processed via laser powder bed fusion

Highlights

• Analysis of cracking through the fabrication of inoculants-added AA7075 alloy.

• Introducing the strain rate and liquid feeding rate in intergranular region.

• Theoretical model to predict the solidification cracks in AA7075 alloy.

• Understanding healing of initiated cracks by liquid backfilling during remelting.

Abstract

Solidification cracks caused by columnar grains in precipitation-hardenable Al alloys such as Al–Zn–Cu–Mg limits the applicability of laser-based additive manufacturing. Recently, cracking has been effectively reduced by introducing equiaxed grains through inoculant addition to Al alloy powder. However, the mechanisms through which equiaxed grains can prevent cracking have not been explained from the viewpoint of both microstructure and process parameters. Thus, the control over the cracking behavior of Al alloys during the laser powder bed fusion (LPBF) process has remained limited because of a lack of theoretical understanding. In this study, a solidification cracking model was proposed using the parametric LPBF results of ZrH₂ particle-added AA7075, where equiaxed grains were formed as a function of energy density. In the proposed model, the cracks in the previously formed layer were healed by liquid backfilling during remelting under appropriate solidification conditions. The synergistic effects of both repeatable melting (layer-by-layer) and equiaxed grain formation on the cracking behavior were revealed. Consequently, the proposed model opens a novel route to cracking prevention in laser-based metal additive manufacturing processes.

Introduction

Metal additive manufacturing (MAM) has shown immense potential in various industries, such as aerospace, automotive, military and shipbuilding, as it provides the direct fabrication of complex and lightweight structures based on computer-aided design [1], [2], [3]. In particular, MAM has been studied extensively for weight reduction in automotive and aerospace industries to improve fuel efficiency [3], [4].

Aluminum alloys are the most commonly used structural materials in automotive and aerospace industries due to their high specific strength [5]. However, there are only a few commercially available alloy systems for LPBF, such as AlSi10Mg, AlSi7Mg, and Al–Mg–Sc–Zr [6]. The performance of additively manufactured AlSi10Mg is limited because of its relatively low intrinsic mechanical properties, although high ultimate tensile strength (UTS) values of 450 MPa were reported [7], [8]. Precipitation-hardenable Al–Cu and Al–Zn–Cu–Mg alloys showed superior mechanical properties, e.g., ASTM standard B209–14 for wrought T6-AA7075 exceeds UTS and elongation values of 530 MPa and 9%, respectively. However, despite their potential, AA7075 provides poor processability for LPBF due to their low weldability and high sensitivity to solidification cracking [9], [10]. Many researches on the fabrication of crack-free AA7075 by additive manufacturing (AM) showed that the approach of adding inoculants in aluminum alloys is one of the efficient ways to eliminate solidification cracks in AA7075 [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24].

Martin et al. [11] showed that the addition of ZrH₂ particles to AA7075 significantly affected the grain size refinement and prevention of solidification cracking in laser-based AM. They elucidated that ZrH₂ particles formed the Al₃Zr intermetallic precipitate during solidification and promoted the heterogeneous nucleation of α-Al, which produced randomly oriented equiaxed grains [12]. Consequently, the effect of adding inoculants or eutectic-forming alloying elements, such as Zr [13], [14], [15], Ti [16], [17], Si [18], [19], Nb [20], Sc [21], Ta [12], ZrH₂ [11], TiB₂ [22], YSZ (yttria-stabilized zirconia) [23], and TiC [24] on the MAM processability of precipitation-hardenable aluminum alloys have been extensively studied. More recently, Tan et al. [25] reported that the lattice misfit between the aluminum matrix and the primary intermetallic phase, such as Al₃Zr, and the growth restriction factor of the solute element were the main factors for evaluating the effectiveness of element or particle addition on processability improvement.

Furthermore, various mechanisms for formed cracks and the prevention of crack formation were proposed using inoculant-added Al alloys [14], [16], [19], [20]. For instance, Zhou et al. [14] proposed the crack healing mechanism using Zr-added Al alloy. The liquid channel length between the grains is shortened due to shorter columnar grains when randomly oriented equiaxed grains are formed by heterogeneous nucleation. Due to the layer-by-layer progress of AM, the previous layer is partially remelted when the next layer is deposited. Thus, cracks are easily refilled, resulting in crack inhibition [14]. Second, as reported by Tan et al. [16], the fully equiaxed fine grain structure leads to a drastic reduction in liquid film thickness compared to the fully columnar grain structure. That is, high capillary force induced by thinning liquid films restricts crack formation by the increased strength of semisolid during solidification with the decreased grain size. They also evaluated the influence of liquid backfilling during solidification on cracking from the viewpoint of grain refinement. According to Tan et al., the period of the mushy zone during which solidification cracking could occur decreased with a decrease in grain size by delaying dendritic growth, resulting in crack inhibition [16]. Third, G. Li et al. [19] described the effect of process parameters on cracking susceptibility using vulnerable zone length and cooling rate. The hot cracking susceptibility decreases at high energy density due to the exponential decrease of cooling rate with increasing of energy density. Additionally, the metallurgical factors, influencing the liquid backfilling, were described using a fraction of the eutectic phase. They assumed a higher eutectic phase fraction in fine equiaxed grain resulted in a lower hot crack susceptibility. Fourth, Zhu et al. [20] proposed crack elimination mechanisms using Nb-added Al alloy. Alloying elements such as Zn and Mg, which have high vapor pressure, would suffer considerable evaporation during LPBF. The amount of evaporation is increased with decreasing in scan speed. The solidification range is changed by the compositional shift of the alloy during the LPBF process, resulting in low crack susceptibility at lower scan speed.

Despite these efforts, the influences of laser processing parameters and microstructure on the cracking behavior in inoculant-added aluminum alloys have been systematically studied as the previously reported cracking mechanisms fail to provide a comprehensive understanding of cracking in AA7075 alloy systems. The microstructure and process parameters are closely correlated to each other. The solidification cracking mechanism, focusing on the microstructure, might overlook the effect of thermal conditions, while the mechanism, focusing on the process parameters, might overlook the effect of equiaxed grain. The effects of the process parameters of AM on the cracking mechanism have not been fully understood because the reported cracking mechanism focused on revealing the effect of each aspect separately. To overcome this issue, a crack prevention model, which considers the effect of microstructure and process parameters simultaneously, should be developed for precipitation-hardenable aluminum alloys to fabricate high-strength AM-processed AA7075 alloys.

Thus, our strategy focused on uncovering the cracking mechanism of ZrH₂ particle-added AA7075 from the perspective of both microstructure and process parameters. Various process parameters were used to fabricate samples under different thermal conditions to engineer their microstructure. ZrH₂ particle-added AA7075 powder for AM was prepared by electrostatic adsorption (EA) [11], and they were successfully fabricated to make bulk samples by laser powder bed fusion (LPBF). To analyze the solidification behavior of AA7075, the finite element method (FEM) was conducted, evaluating the thermal conditions at each process parameter. Finally, the solidification cracking model, which describes the formation and healing of cracks in the LPBF process area of inoculant-added AA7075, was developed.