Study Of Residual Stresses In Additive Manufactured Aluminum Alloy Structure

As one of the emerging metal additive manufacturing technologies,Selective laser melting(SLM)has been widely studied because it can produce high-quality parts with complex geometries which are difficult or expensive to produce with conventional subtractive manufacturing technologies,and has been widely applied in industries such as aviation and aerospace.In order to promote the development and industrial application of SLM,understanding the build-up mechanism of residual stresses during SLM processes as well as the distribution of residual stresses in SLM produced parts is necessary.Accurate measurements of residual stresses in SLM produced parts is essential.The main results are as follows:The present study combines theoretical analysis and experimental measurements to determine that the high surface roughness of the sample will cause the residual stress values measured by XRD to be significantly lower,thereby proving the necessity of conducting pretreatment to SLM produced samples before measuring residual stresses using XRD.The present study proposed a pretreatment process combining mechanical grinding and chemical etching for SLM produced aluminum alloy structure,which are characterized by its high surface roughness.Through experimental measurements and data statistics,it is proved that the proposed pretreatment process can effectively improve the accuracy of XRD measurements performed on SLM produced aluminum alloy structure.This improved residual stress measurement method is also applicable to SLM parts produced using other materials,especially for those with high surface roughness.Based on finite element simulation and experimental results obtained using the improved method,the distribution of residual stresses on SLM produced aluminum alloy structure was determined.The residual stresses are unevenly distributed on sample surfaces with a strong anisotropy.The residual stresses are mainly tensile on the surface and compressive inside the SLM produced parts.The major component on the lateral surface is mainly along the building direction,while the major components on the top surface is mainly perpendicular to the building direction.In order to study the build-up mechanism of residual stresses during SLM processes,the experimental measurements of residual stresses are combined with the microstructural morphology and the high-temperature tensile performances of SLM produced aluminum alloy structure.The results showed that the residual stresses generated during SLM processes at the interface between sample and building platform exceed the tensile strength of the material at high temperature,which causes cracks and opening of the melt pool boundaries.The residual stresses are thus partially released.The effect of scan strategy on residual stress distribution and microstructural morphology on the lateral surface of the SLM produced samples was studied.The results show that the scan strategy has no significant effect on residual stress distribution on the lateral surface of the SLM produced samples.The samples produced with different scan strategies present similar microstructural morphology(with melt pool as basic unit)on the lateral surface.The stresses were mainly distributed in the core of the melt pool,where is mainly composed of large and columnar grains.Uneven stress distribution was observed near the melt pool boundaries.The effect of TiB₂ particles on residual stresses in SLM produced samples was studied.The results showed that residual stresses in SLM produced TiB₂/Al Si10Mg samples are generally smaller than the residual stresses in SLM produced Al Si10Mg samples,but share a similar residual stress distribution.The maximum residual stress in SLM produced TiB₂/Al Si10Mg samples was close to the maximum residual stress in SLM produced Al Si10Mg samples and is much lower than its yield strength at room temperature.This is due to its similar high-temperature tensile performances compare to Al Si10Mg.