Effect of Laser Processing on Surface Characteristics of Magnesium Based Rare Earth Element Alloys

Abstract

Magnesium (Mg) based alloys have attracted considerable interest in the biomedical field as materials for temporary implants, primarily due to their dissolution in physiological environments and absorption by the human body. However, several problems such as inadequate strength and rapid degradation must be addressed before Mg alloys can be used as bone implants. These inadequacies of Mg alloys can be addressed either by alloying or through surface modification. The first part of this work aims to develop Mg based alloys containing zinc (Zn) and rare earth elements such as gadolinium (Gd), dysprosium (Dy) and neodymium (Nd) for temporary implant applications. Three different alloys with compositions such as Mg-1%Zn-2% Gd, Mg- 1%Zn-2%Dy and Mg-3%Zn-1.5%Gd-1.5%Nd (all in wt.%) are developed via casting route and investigated for microstructure, mechanical properties, in vitro corrosion behavior and cytotoxicity. The microstructure results indicate that apart from major α-Mg phase and few binary phases, Mg-Zn-Gd and Mg-Zn-Dy alloys consist of lamellar long period stacking ordered (LPSO) phases. The in vitro degradation behavior were studied by immersion and electrochemical methods in hanks balanced salt solution (HBSS). The corrosion rate obtained from both the methods are in the order of Mg-Zn-Gd < Mg-Zn-Gd-Nd < Mg-Zn-Dy. The MTT (methylthiazoldiphenyl-tetrazolium bromide) assay study revealed that all the three alloys had no adverse effect on cells and proves the good cytocompatibility of these alloys. The ultimate tensile strength of Mg-Zn-Gd-Nd is better than the Mg-Zn-Gd and Mg-Zn-Dy alloys due to the uniform distribution of the precipitate phase. In order to tailor the surface properties of these alloys, the second part of this work proposes to understand the effect of laser surface melting (LSM) of Mg-Zn-Gd, Mg-Zn-Dy and Mg-ZnGd-Nd alloys on microstructural changes, hardness variation, wettability and in vitro degradation behaviour. Different ranges of laser energy densities have been adopted to alter the surface characteristics of all the alloys and proper correlations have been drawn with respect to the un-altered surface. The surface modified samples showed fine grain structure in the melt zone compared to the untreated substrate. The surface remelting of material at lower energy density conditions promoted the formation of micro cracks in the meltpool zone. Crosssectional microhardness was observed after laser surface melting. Hardness increase of nearly two times was observed in laser melted region due to the grain refinement and solid solution strengthening. In vitro degradation study showed clear improvement in corrosion resistance of LSM samples at certain range of energy densities. At higher energy density, removal of materials from the surface is enhanced, resulting in deeper grooves and higher surface roughness. The wettability studies indicated that the variations in surface chemical composition, grain size and surface roughness of LSM samples strongly influence the surface energy and hydrophilicity