Full length articleA solution to the hot cracking problem for aluminium alloys manufactured by laser beam melting
Graphical abstract
Introduction
Laser beam melting (LBM), like most additive manufacturing (AM) processes is a layer-by-layer technique. Each powder layer is melted by a focused laser beam according to a desired geometry. The main potential for LBM is the ability to produce complex functional parts with, in most cases, no custom tooling. Additionally, powders can be recycled, which reduces waste.
In recent years, a lot of research has focused on the optimisation of the LBM process parameters in order to obtain dense parts with high mechanical properties. The range of materials that can be processed by this technique is still rather limited, the most relevant today being TA6V, 316 L, Inconel 718 and AlSi10Mg [1]. Only a few Al-Si based cast alloys have been crack free processed [2,3,4,5]. As for wrought Al alloys which are already known for their poor weldability, the application of LBM is considerably limited due to the presence of high thermal gradients that promote columnar growth and thus hot cracking. The level of cracking is particularly high during the LBM of Al6061 [6] and Al7075 [7].
Reasons for hot cracking are globally similar between the different solidification processes of welding and LBM. In both cases, process parameters induce thermomechanical stresses, which are a key ingredient to cracking. However, it appears unlikely that an easy solution can be found along this line in LBM, as thermomechanical stresses can not be controlled in industrial 3D practice. The drastic reduction in terms of thermal gradient that would be necessary to significantly reduce these stresses is hardly attainable by playing on LBM parameters or environment. Second, the alloying elements, selected to produce complex strengthening phases during additional heat treatment, often increase the solidification ranges, which is a priori not favourable for crack sensitive alloys [8]. Last but not least, it is known that high thermal gradients encountered in 3D printing, generally induce a columnar microstructure, elongated in the building direction, which is known to promote hot cracking.
To reduce the sensitivity to hot cracking, it has been clearly demonstrated that fine equiaxed microstructures accommodate more easily strains [9]. In particular, equiaxed growth provide easier grain rotation and deformation during strain accommodation.
Two approaches enable grain refining. The first solution is to modify the thermal load during printing. At the process level, the key variables are the scanning speed, the heat source in terms of both intensity and spatial distribution, and in addition, wherever applicable, the build plate temperature [6]. From a more physical standpoint, the governing parameters are the solidification velocity and the temperature gradient. Along this line, microstructure changes can be obtained [10,11]. A second way is to enhance heterogeneous nucleation, by changing alloy composition or by directly adding a nucleant agent in the base powder [12], [13], [14], [15], [16].
Recently, Montero Sistiaga et al. [17] demonstrated that the addition of 4wt% of pure silicon to Al7075 powder fully avoids cracking during LBM through a change of microstructure. Another success story in this microstructure control line was proposed by Martin et al. [16], by adding a nucleant agent to Al7075 powder, they successfully printed crack free parts. Other attempts have focused on developing new gas atomised alloys dedicated to LBM. The best known example is the ScalmalloyⓇ, an Sc- and Zr- modified 5xxx alloy specifically developed by the Airbus APWorks© for the LBM process. The precipitation of Al3Zr/Al3Sc favourable nucleant phase is the main reason for the elimination of the hot cracking effect. Thereby, many studies focused on this alloy [12], [14], [18], [19], [20], [21]. The Scalmalloy way is efficient but costly as it relies on the expensive Sc rare earth element.
The current work aims at the identification of the solidification control parameters as a way to remove cracks in crack sensitive aluminium alloys using a low cost particles electrostatic coating process to graft nanosize YSZ particles on aluminium powder. Yttrium Stabilized Zirconia (YSZ) powder was selected for its low price and availability due to large scale production, easy handling and the ability to precipitate the Al3Zr phase. Al3Zr is an aluminium nucleant phase well known from casting and welding for αAl FCC, thanks to its small interatomic spacing misfit [22].
In the study, the grain refinement effect of the added YSZ quantity is investigated during processing of Al6061 alloy.
Section snippets
Experimental section
From now, the following abbreviations are used to improve readability: PX% refers to the X volumic% in the blended powder (YSZ + Al6061) and MY% refers to the Y volumic% in the processed material.
XRD
Fig. 6 shows the X-ray diffraction patterns measured on YSZ powder, Al6061 powder, P2%, M2% and M4%. For P2%, M2% and M4%, the (100), (200) and (220) main crystallographic orientations of alpha-aluminium phase (αAl) are observed.
Regarding P2%, peaks related to YSZ powder and peaks associated to αAl phase are present. Once mixtures are 3D printed, peaks related to YSZ powder are no more observed. This means that a large amount of YSZ particles have been melted, dissolved or have reacted. This
Discussion
The objective in this section is to propose plausible scenarios, based on existing solidification models to account for the observed experimental results. The columnar to equiaxed transition has been widely studied in foundry configurations. Hunt et al. [10] took into account nucleant density and chemical undercooling to derive a criterion for the transition in a thermal gradient and solidification speed plane. Later on, Greer et al. [31] added the effect of capillarity undercooling.
Conclusions
Until now, almost exclusively conventional Al Alloys have been processed with the use of LBM. In this work, a new crack free 6xxx aluminium alloy was additively manufactured by LBM using mixing of pre-alloyed gas atomized Al6061 powder with an YSZ particles coating. Based on the results obtained in this study, the following conclusions can be drawn:
- (1)
Mixing conditions enable a good homogeneity and sturdiness regarding YSZ particles grip on aluminium powders, before, during and after the printing.
- (2)
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors gratefully acknowledge CEA-LITENfor financial support, Denis CAMEL, Alexis DESCHAMPS, Fernando LOMELLO, Pascal AUBRY and Nathalie PELISSIER for the constructive discussions and advice as well as Céline RIBIERE for her 3D printing technical assistance. SERMA technologies are also thanked for the FIB lamella preparation.
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