Surface modification of biomedical Mg-Ca and Mg-Zn-Ca alloys using selective laser melting: Corrosion behaviour, microhardness and biocompatibility

Magnesium alloys such as Mg–Ca and Mg–Zn–Ca are good orthopaedic materials;however their tendency to corrode is high.Herein we utilize selective laser melting(SLM)to modify the surface of these Mg alloys to simultaneously improve the corrosion behaviour and microhardness.The corrosion rate decreased from 2.1±0.2 mm/y to 1.0±0.1 mm/y for the laser-processed Mg–0.6Ca,and from 1.6±0.1 mm/y to 0.7±0.2 mm/y for laser-processed Mg–0.5Zn–0.3Ca.The microhardness increased from 46±1 HV to 56±1 HV for Mg–0.6Ca,and from 47±3 HV to 55±3 HV for Mg–0.5Zn–0.3Ca.In addition,good biocompatibility remained in the laser processed Mg alloys.The improved properties are attributed to laser-induced grain refinement,confined impurity elements,residual stress,and modified surface chemistry.The results demonstrated the potential of SLM as a surface engineering approach for developing advanced biomedical Mg alloys.

In-situ alloyed CoCrFeMnNi high entropy alloy:Microstructural development in laser powder bed fusion

In-situ alloying has the potential to combine the compositional flexibility of high entropy alloys(HEAs)and the advanced forming capability of laser powder bed fusion(LPBF).This study fundamentally investigated the elemental homogenisation and grain development in the in-situ alloying process of CoCrFeMnNi HEA,by analysing the basic units,i.e.,tracks and layers,and introducing Mn as an alloying element to the base Co Cr Fe Ni HEA.Different modelling methods were employed to predict meltpool dimensions,and the results indicated the dependence of the modelling on practical meltpool modes.Delimitation of elemental distribution was found in keyhole meltpools since an intensive flow was generated due to recoil pressure.The homogeneity of in-situ alloyed Mn in single tracks was insufficient whether operated in conduction mode or keyhole mode,which required remelting from adjacent tracks and following layers to promote homogenisation significantly.The preferred orientation in single tracks along scanning directions changed from<001>to<101>as the scanning speed increased,although the cross-sections were similar in size with identical linear energy density.Such preference can be inherited during the printing process and lead to different textures in three-layer samples.It was also observed that applying hatch spacing smaller than a half meltpool width could coarsen the grains in a layer.The results from this study provide structure-parameter correlations for future microstructural tailoring and manipulation.

Heat treatment of hot-isostatic-pressed 60NiTi shape memory alloy:Microstructure,phase transformation and mechanical properties

60NiTi alloy is considered to be a promising material for specialized bearing and gear applications due to its high hardness,strength,and low modulus.However,fabricating 60NiTi through conventional processing methods is challenging due to the brittleness and poor workability.In this study,60NiTi with high relative density was successfully fabricated directly from pre-alloyed powder through hot isostatic pressing.The effects of solution and aging treatments on microstructure and mechanical properties were systematically studied by advanced characterization techniques.The hot-isostatic-pressed 60NiTi showed low average hardness and elastic strain due to the formation of a soft Ni_(3)Ti phase and B2 NiTi matrix.Solution treatment above 1000℃dissolved the Ni_(3)Ti phase and promoted the formation of nanoscale Ni_(4)Ti_(3)precipitates,which significantly improved the hardness,strength,and elastic strain of 60NiTi.The formation of the Ni_(4)Ti_(3) phase can be mainly attributed to the driving forces induced by the chemical supersaturation and mechanical stress concentration.Finally,the phase transformation mechanisms during heat treatment and compression test were discussed.