Simultaneously minimizing residual stress and enhancing strength of selective laser melted nano-TiB2 decorated Al alloy via post-uphill quenching and ageing

https://doi.org/10.1016/j.matchar.2021.111242Get rights and content

Highlights

  • Develop a new strategy to reduce residual stress and enchance strength of SLMed parts.

  • Residual stress is reduced due to newly produced opposite stress and recovery of dislocations.

  • Strength is enhanced due to precipitation strengthening by dispersed nano-Si particles.

Abstract

Selective laser melting (SLM) of aluminium alloys is of research interest to produce customized or complex-shaped metal components for functional or structural applications. In order to palliate the undesirable residual stress developed in the SLM process, traditional post-annealing heat treatment has been widely used to lower residual stress in selective laser melted (SLMed) Al alloys. However, the traditional post-annealing method usually leads to a decrease in strength which is another concern for applications. In this study, we developed and validated a new strategy combining post-uphill quenching and subsequent ageing to simultaneously minimise residual stress and increase the strength of SLMed nano-TiB2 decorated Al alloy parts. The results show that the high as-built residual stress was partly counteracted by the newly-produced residual stress in the opposite direction during the uphill quenching stage and synergistically reduced by dislocation recovery during the ageing stage. The microstructural features of grains and cell structures remain unchanged after the treatment. While substantial new nano-Si particles with dot-like or needle-like morphology precipitate inside the cells. The tensile strength was improved mainly due to effective precipitation strengthening by dispersed nanosized Si precipitates. The proposed strategy is expected to be useful in other materials and components fabricated by SLM in which residual stress is also unwanted.

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. Alsingle bondSi series alloys, especially the hypo-eutectic and eutectic Alsingle bondSi 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 Alsingle bondSi 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 Alsingle bondSi 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.

References (37)

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