Microstructure Of Beryllium Laser Welding And The Welding Process Simulation,

Because of its excellent performances such as lower density,higher melting point,higher specific stiffness,Beryllium has widely applied in aviation,space and nuclear industries. However,the defects,such as brittle phase,cracks and hole,are easy formed during welded beryllium,therefore the welding problem of beryllium and beryllium with other metals is the key to block its wide usage.Beryllium was welded by means of Nd:YAG laser welding,with AlSi alloy,Al,metal glass,Zr+AlSi+Zr as filler materials.By microstructure observation and performance testing for welded joint,the microstructure forming rule composed of beryllium and filler metal was analyzed,the theoretical probability of such four metals as beryllium welding filler material was researched,which could be used to optimize welding technics of beryllium.Based on the deterministic model and probabilistic model,combining the computer simulation and experiments,develop a CAMC macro-micro-model for researching on solidification in welding pool and grain growth in welded heat affected zone.It is successful to simulate the microstmcture evolvement during welding by CAMC.The microstructure and evolvement mechanism in beryllium laser welding joint that AlSi alloy,Al,ZrTiNiCuBe bulk glass,Zr+AlSi+Zr acted as filler metal was analyzed.Application AlSi alloy or Al as filler metal,the result of metallograph observed indicated that the microstructure of weld zone and fusion zone in the beryllium laser welding joint was composed of quasi-composite compound phase formed by beryllium and filler metal.The microstructure that hard,brittle beryllium particles dispersed distribution in soft,ductile filler metal was helpful for avoiding welding defect and improvement integration mechanical performance.For ZrTiNiCuBe bulk glass acted as filler metal,the microstructure shown that alloy at center of weld zone remained amorphous solids,but an intermetallic compounds layer was formed in fusion zone that structure was loose and chemical composition was uniform. The approach of improvement its weldability is decreasing the fusion area by adjustment the welding technics parameters.The weldability of beryllium that Zr+AlSi+Zr is filler material is bad by reason of Be₅Zr,Al₂Zr intermetallic compounds formed in fusion zone that will lead to easily formed cracks.The beryllium grain growth kinetic equation was developed: D=D₀+7.7×10^-1t^0.6215exp(-40985/RT).The grain growth in heat affected zone of beryllium laser welding was analyzed by research on the rule of grain growth in beryllium and measuring temperature circulation curve.Based on the kinetic equation and temperature circulation curve,predicted that beryllium grain would grow about Iμm and the width of heat affected zone was about 0.5mm,it was coincide with the result of metallograph observed.The research on usage weldability of beryllium for AlSi or Al as filler materials indicated that the alteration of welding technics would bring microstructure of welding joint,eventually the change would embody on shear strength,fracture mechanism,microstructure features of fracture.The shear strength of Be/AlSi/Be,Be/Al/Be is location at 188.5MPa～283MPa or 109.6MPa～240.4MPa,which is between filler metal and beryllium.With percent of beryllium in weld zone being bigger and beryllium particle distribution changed,the shear strength of welding joint was higher,the rupture mechanism was transformed form ductile fracture with fractographs of dimples to brittle fracture with mixture fractographs of cleavage feature and local dimples.In a word,for Be/AlSi/Be,Be/Al/Be,it is concluded that percentage of beryllium in welding zone is key factor to controlling welding joint strength and fracture mechanics.Application of ZrTiNiCuBe bulk glass as filler metal,the average shear strength of welding joint is location at 230MPa,the microhardness in fusion zone is lower than that of ZrTiNiCuBe bulk glass and beryllium.Intermetallic compounds,such as Be₂Zr,α-BeO,BeCu,NiZr,Be₁₃Zr formation in the fusion zone of Be/ZrTiNiCuBe/Be welding joint,would block the dislocation movement and lead welding joint to ruptur near the fusion zone in brittle fracture with rock candy surface.Because of the fusion zone area becomes larger with heating input increasing,the welding technics should keep reducing the parameters of welding line energy on condition that welding joints are effectively bonded.CAMC model was developed by coupling macro-thermal transfer and micro-modeling, combing the deterministic model and probabilistic model.The model exploits the fully advantage of MC method and CA model.The skeleton of grain was first described by using CA model,and then,the growth inside of grain was corrected by MC method.The evolution of amount,dimension and appearance of grains can be described quantitatively by using CAMC model.Therefore,it was very successful to simulate the microstructure formation during metal solidification in weld pool by CAMC model.Based on the two factors of energy and curvature,a 2-dimensions cellular automata model for simulating grain normal growth is proposed.The model can reflect some important physical phenomena on grain normal growth,including the relation of grain growth velocity to temperature and curvature and time invariance of grain size distribution.At the same time, a transition rule of cellular automata time(CAS) to real time is put forwarded. Energy-curvature drove cellular automata modeling Normal grain growth in welding heat-affected zone is accomplished in real time.The simulation results correspond well with the rule of grain distribution in welding heat-affected zone.