The Research On Friction Stir Welding Of Zr-based Bulk Metallic Glasses

At present,the researches on friction stir welding of bulk metallic glasses mainly focus on the influence of welding parameters on the microstructure and mechanical property of weld,but there are few reports on the plastic flow behavior of weld metal which is closely related to the weld formation.The plastic flow behavior of weld metal can directly affect the weld macroscopic morphology and mechanical propert ies of the welded specimens.Therefore,systematic research on the friction stir welding of bulk metallic glasses will be of great significance.Firstly,the "trace material method" was utilized to study the plastic flow behavior of welds with different morphologies during the formation process.Only the feed rate was changed through the control variation method,three kinds of welds with "complete type","hole type" and "tunnel type" were obtained.According to the change of the position and shape of the tracer in the horizontal and vertical directions of the welds before and after welding,the plastic flow behavior of the weld metal and the formation and evolution of weld defects during the welding process were analyzed.Finally,the "plastic flow" model was constructed according to the plastic flow law of weld metal.In the model,with the increase of feed rate,the flash edge gradually increases and the heat absorbed by the per unit length weld decreases,leading to the decrease of plasticized metal involved in the plastic flow,and the flow capacity deteriorates,defects generate.In the model,the weld plasticized metal transfers mass and heat in the form of " annular flow" in the nugget zone.The boundary between the shoulder influenced zone and the needle affected zone is “defect-prone zone”,and the bottom of the advancing side of the needle affected zone is “difficult-filled zone”.When the needle inserts into the BMGs sample,the "volume balance" of the weld metal is destroyed.In order to reach a new equilibrium state,the weld metal will overflow along the place with the smallest resistance,thereby the "instantaneous cavity" forms at the boundary between the shoulder influenced zone and the needle affected zone,making the plasticized metal nearby fill this place,and finally trigger the "annular flow" of the plasticized metal in the nugget zone,which is the original driving force of the plasticized metal flow.Secondly,the mechanical properties of the welded samples were studied.The micro-hardness test on cross section of the "complete type" weld was ca rried out,and the changes of micro-hardness and micro-indentation morphology were analyzed.Finally,the following conclusions are drawn: the mechanical properties of the welded sample are closely related to the free volume concentration in the matrix.Th e severe plastic flow in the nugget zone results in higher free volume concentration,leading to lower hardness and poor plasticity.The free volume of the non-nugget zone is severely affected by temperature,obtaining lower free volume concentration,high er hardness value and worse plasticity.Finally,the fracture behavior of welded sample under different loading modes was investigated.By fine-tuning the welding parameters,the welding specimens broke in different loading modes,the fracture morphology and plastic deformation ability of different regions were analyzed by observing the fracture morphology.The specific conclusions are as follows: the "torsion-like" fracture in nugget zone is hybrid section,and the non-nugget zone is a tough fracture.The "tensile-like" type fracture in nugget zone is hybrid section,and the non-nugget zone is brittle fracture.The plastic deformation ability of welded samples is different under variou loading modes.For the fracture morphology,the "torsion-like" fracture have better fracture plastic deformation ability compared with the "tensile-like" fracture.