Laser shock processing of magnesium alloys: microstructure evolution and enhanced mechanical properties
Authors
Mao, Bo
Issue Date
2020
Type
Dissertation
Language
Keywords
Deformation twinning , Laser shock peening , Magnesium alloys , Mechanical properties , Microstructure
Alternative Title
Abstract
Laser shock peening (LSP) is an advanced laser-based surface processing technique which has been widely utilized to enhance the engineering performance of metallic materials. Among a variety of metallic materials, magnesium (Mg) and its alloys have attracted tremendous research interests due to their low density and high specific strength. However, the applications of Mg alloys is often restricted by their poor mechanical properties. Recently, LSP has been utilized to tailor the microstructure and crystallographic texture of Mg alloys for enhancing their surface hardness, wear resistance, corrosion resistance, fatigue durability, and stretch formability. Despite these promising experimental results of enhancing the engineering performance of Mg alloys by LSP, the fundamental processing-microstructure relationship has not been fully understood. Microstructure evolution of metallic materials during LSP is of great practically importance for the LSP process development and control for optimized mechanical performance. Moreover, the effectiveness and efficiency of LSP for enhancing the engineering performance of Mg alloys have not been fully explored, which is mainly due to a lack of fundamental understanding of the deformation mechanism of Mg alloys during LSP process. The complex deformation modes of Mg alloys render their microstructural and mechanical response to thermal-mechanical processing significantly different and complicated from their high symmetry counterparts, like steels and aluminum alloys with a cubic structure. Specific scientific knowledge gaps for the LSP processing of Mg alloy are: (1) how would the twinning behavior of Mg alloys subjected to LSP be different from it is under quasi-static loading condition? (2) how do the activation and distribution of twins affect the mechanical properties of Mg alloys? In this study, LSP experiments are conducted on a rolled AZ31b Mg alloy. The microstructure before and after laser processing are characterized. A focus is placed on understanding the deformation twinning mechanism. The effect of laser intensity on the twin volume fraction is investigated. The surface hardness as associated with the twin density is measured. The mechanism responsible for the formation of gradient twinning microstructure and the twinning-induced hardening effect are discussed. The anisotropic response to LSP in terms of microstructure and hardness improvement in Mg samples is discussed. Twin-twin interactions during LSP is examined and a non-dislocation based mechanism is proposed. Tribological testing and room temperature stretch-formability testing are carried out on the LSP processed Mg alloys to further explore the opportunities of using LSP for enhancing the mechanical properties. Through this study, it has been found that: (1) Deformation twinning, in particular, the {101 ̅2}〈101 ̅1 ̅ 〉 tension twinning mode, plays a critical role during the LSP processing of Mg alloys. A gradient twinning microstructure in which the density of twins decreases with depth was introduced to an AZ31B Mg alloy plate by LSP. The gradient surface hardening effect was accompanied with the gradient twinning distribution. (2) Twin-twin interactions profusely exist in Mg alloys as processed by LSP. Interfaces between different tension twin variants shows that these interfaces present abnormal morphologies that cannot be accounted for by twinning dislocation theories. Patches of one variant can be completely surrounded by another variant. Such an abnormal behavior of twin-twin interaction can only be explained by non-twinning-dislocation theories that fundamentally differ from the classical twinning theory. (3) The tribological performance of Mg alloys can be improved by LSP processing. Both the surface friction coefficient and wear rate decreases with the volume fraction of twins introduced by LSP. The improved tribo-performance of Mg alloys by LSP are attributed to the twinning-induced hardening effect, twin growth and saturation phenomenon, and twinning-induced surface crystallographic texture change during sliding. (4) The room temperature-stretch formability of Mg alloys can be enhanced by LSP processing. A combination of texture weakening by extension twinning and grain refinement induced by LSP may account for the improved stretch formability of the Mg alloy.