Effect Of Friction Stirs Welding On Microstructure And Properties Of Bulk Metallic Glass Composites

Bulk metallic glass composites?BMGCs?not only exhibit the special mechanical,chemical and physical properties of bulk metallic glasses?BMGs?,but also overcome the brittleness problem of single-phase BMGs at room temperature,which can meet the requirements of engineering applications.However,it is a problem that people have been paying more attention to how to make the BMGCs bigger.As a typical solid-phase welding process,friction stir welding?FSW?h as less heat input during the welding process,it is beneficial to maintain the metastable structure of BMGCs.The BMGCs have excellent superplastic flow behavior in the supercooled liquid region.Therefore,the BMGCs are very suitable for connection with the FSW process.Therefore,the connection between BMGCs with FSW can solve the problem that small dimensions and simple shape of BMGCs.It is of great practical significance to give full play to the performance advantages and to improve the application ra nge of BMGCs.In this article,La₆₆Al₁₄Cu₂₀ BMGCs were selected for friction stir welding,the effect of processing parameters on the joints molding,microstructure and the micro-hardness of the joints have been studied.The effects of different loading rates on the creep properties of as-cast specimens and the specimens obtained under optimum welding parameters have been investigated by the nanoindentation technique.The room temperature creep mechanism of two kinds of s pecimens was discussed.The difference of creep behavior between the as-cast specimens and the welded specimens was compared.The results of the study are shown as follow:Welding parameters?the tool rotation speed,the welding speed,the plunge depth?have a significant effect on joints molding.After the tool rotation speed is changed,grooves are appeared when the tool rotation speed is low;the joints surface out of flatness and the formation of the overlap when the tool rotation speed is large.After the welding speed is changed,when the welding speed is low,the middle position on the back joints surface is rough,there is a little crack on t he joints surface;the minitype tunnel holes and overlap are appeared when the welding speed is large.After the plunge depth is changed,minitype holes and groove defects occurred when plunge depth is low;A lot of overlap emerged and there is a little crack on the joints surface when plunge depth is large.The optimum welding parameters are as follows:the heating temperature is 80?,the tool rotation speed is 1000 rpm,the welding speed is15 mm/min,and the plunge depth is 0.3 mm.The distributions of?-La,La Al₃ in the as-cast specimen,while the crystallized phases at the joints are?-La,La Al₃,La₂Cu Al₇.With the increase of the tool rotation speed,the size of the crystallized phase in the matrix decrease d gradually,and the agglomeration of grain would have appeared.However,at 1200 rpm,the size of the crystallized phase increased.With the increased of welding speed,the size of crystallized phase decreased gradually,and the distribution of crystallize d phase is more uniform,a few crystallized phases became more bigger at the lower welding speed.The size of the crystallized phase decreased with the increased of the plunge depth.Through analysis the micro-hardness shows:the micro-hardness of all the specimens is lower than the micro-hardness of the specimens that heated but unstirred.The micro-hardness of all welded specimens gradually decreases from the edge of the joints to the nugget zone.When changing the tool rotation speed,the order of the specimens'micro-hardness is 1200 rpm<800 rpm<1000 rpm.The microstructures around the indentation were observed by SEM,and the results show s:there are many cracks appeared around the indentations in addition to the as-cast specimen.As the tool rotation speed increases,the cracks gradually increasing,lengthing and are parallel to each other.The cracks originate from the oxide of the La and the interface between the matrix and the crystallized phase,and gradually extend from the edge of the indentation to the center of the indentation.Through nanoindentation test found that with the increase of the loading rate,the creep displacement of the holding stage increases,and the creep curve consists of two parts.The creep displacement of the as-cast specimen at the same loading rate is greater than the welded specimen.The EVEV model was used to fit the creep displacements-times of the two types of specimens during the holding at 10 m N,and the creep compliances,retardation spectras and the creep stress exponents of the two specimens were calculated.After the analysis found that serious structural relaxation occurred in the welded specimens.