Microstructure and mechanical properties of friction stir processed Zn-Mg biodegradable alloys

Magnesium, iron and zinc-based alloys are potential metallic biodegradable alloys for bioimplant applications. The issues of hydrogen evolution along magnesium's surface and the release of bulky corroded products of iron alloys during its usage necessitated to look for an alternate alloy system for biodegradable implant applications. Zn-Mg alloy is one such alternate alloy that is widely explored by several researchers. The typical cast microstructure of Zn-Mg alloy contains pro-eutectic alpha-Zn and eutectic mixture of alpha-Zn and Mg2Zn11. The cast alloy exhibits poor ductility as the intermetallic Mg2Zn11 phase exists only in eutectic mixture. Therefore, the cast alloys were subsequently processed through hot rolling, hot extrusion and rarely through severe plastic deformation to enhance the mechanical properties. In the current study, probably for the first time, friction stir processing (FSP) has been used to refine the microstructure of Zn-Mg alloys with an intention to enhance the mechanical properties. Indeed, it was a challenge to friction stir process the Zn alloys through friction stir processing, as they have poor ductility (< 10 %) in as-cast condition. By choosing tool rotational speed that can impart significant heat input to the processed regime, this study demonstrated successful FSP of Zn and Zn-(0.5, 1.0 & 2.0) wt% Mg alloys. The pure-Zn and Zn-Mg alloys are FSPed using constant tool traverse speed of 20 mm/ min and variable rotational speeds of 560, 710 and 900 rpm. Hardness of as-cast Zn was 32 VHN whereas addition of 2 % Mg increased it to 105 VHN. Hardness of zinc increased to 39 VHN after FSP using 560 rpm; at the same time, further increase of rotational speed retained the hardness around 40 VHN. On the other hand, hardness of Zn-Mg alloys significantly improved after FSP. Interestingly, hardness of alloys increased with the rotational speed. Usually, increase of rotational speed increases heat input in to the processed regime thus assists the development of coarse microstructure results in hardness degradation. An opposite trend has been observed in the current study. The reasons for this notable observation are found to be refinement of microstructure and redistribution of Mg(2)Zn(11 )phase throughout alpha-Zn matrix that happened during FSP.