Simultaneously minimizing residual stress and enhancing strength of selective laser melted nano-TiB2 decorated Al alloy via post-uphill quenching and ageing
Graphical abstract
Introduction
Selective laser melting (SLM), as an emerging additive manufacturing (AM) technology for metals, alloys, and composites, has shown enormous potential in the aerospace, automobile, and medical industries, which demand complex geometries and individualised production [1,2]. The SLM process has been developed to produce high-quality parts with excellent mechanical properties [3,4]. However, one significant problem that must be solved urgently is the undesirable residual stress developed in the as-built state due to the unique thermal conditions (i.e., violent heating and cooling cycles inducing steep thermal gradients) [[5], [6], [7]].
Residual stress can be found in almost all SLMed alloys, including aluminium [8,9], superalloy [10,11], titanium [12,13], and stainless steel [14,15]. The presence of residual stress leads to significant design errors, makes processability difficult, and can even cause severe geometry distortions and cracks [6]. Thus, residual stress is a critical obstacle to the full-scale adoption of the SLM process in industrial applications. Owing to the above detrimental effects, residual stress and distortions in as-built parts must be reduced within a low range acceptable for applications. AlSi series alloys, especially the hypo-eutectic and eutectic AlSi alloys, are the most commonly studied low-cost and attractive choices of Al alloys for weight-saving SLM applications. To date, a vast number of studies on SLMed AlSi samples have reported the existence, magnitude, and distribution of residual stress [[16], [17], [18]] and its formation mechanism [14] to find practical solutions to residual stress reduction. For example, adjusting the scanning strategy can reduce residual stress to a certain extent, but the remaining residual stress is still high [14]. Pre-heating the base substrate, changing process parameters, post-solution heat treatment [[19], [20], [21], [22], [23]], and post-annealing [[22], [23], [24], [25], [26], [27]] could also lower residual stress, but they also lead to strength loss, which is another concern for applications. To date, no effective approach to relieving the high residual stress in SLMed AlSi series alloys without sacrificing strength has been presented.
Therefore, we propose a new approach combining post-uphill quenching and ageing (hereafter referred to as ‘U + A') to relieve residual stress while simultaneously promoting precipitation strengthening in as-built parts. This U + A process reduces residual stress by introducing a new reverse residual stress (i.e., in the opposite direction to the as-built residual stress) and dislocation recovery. In this paper, we applied this approach to an in-situ nano-TiB2 decorated Al alloy fabricated by SLM [[28], [29], [30]]. The mechanisms for explaining residual stress reduction and strength enhancement are elucidated with the aid of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD).
Section snippets
Materials
In the present study, we used a home-made in-situ nano-TiB2 decorated Al-7Si-Mg (TiB2/Al-7Si-Mg) alloy powder, fabricated by the salt-metal reaction and gas atomization, because in-situ TiB2 particles have various advantages as a reinforcer of Al alloys (details can be found in our previous studies [[30], [31], [32], [33]]). The alloy powders are spherical in shape and have a particle size in the range of 15–53 μm. The chemical compositions of the alloy powder and SLMed sample were determined
Microstructure characterization
The EBSD results of longitudinal-section along the building direction (BD) of the SLMed TiB2/Al-7Si-Mg alloy under as-built and U + A treated conditions are presented in Fig. 2. From the inverse pole figure (IPF) map in Fig. 2a, it can be seen that the as-built alloy exhibited equiaxed grain structures due to the heterogeneous nucleation effect of nano-TiB2 particles (detailed explanation can be found in our previous study [29,30,33]). The corresponding grain size distribution statistic of
Conclusions
In conclusion, U + A processing has been demonstrated as an effective approach to reduce residual stress while improve the strength of SLMed TiB2/Al-7Si-Mg samples. After U + A treatment, the highest residual stress was reduced to an acceptable level (<30 MPa) because of the newly produced stress opposite to the original stress and recovery of dislocations. Meanwhile, the samples exhibited a higher yield strength of 410 MPa, ultimate tensile strength of 531 MPa, and reliable elongation of 8.3%.
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.
Acknowledgements
This work was financially supported by the National Key Research and Development Program of China (Grant No. 2016YFB1100100) and the Natural Science Foundation of China (Grant No. 51971137). The authors thank TESCAN China for EBSD characterizations.
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