Study On The Weldability Of Al-Si(8.4～12.2%)

The distinct advantages of being of low density,high heat resistance,high temperature strength,good thermal stability,low thermal expansion coefficient and good wear resistance distinguish Al-Si alloy from other alloys.The use of the alloy engine pistons, cylinder block,cylinder head,wheel hub,all kinds of machinery parts and electrical products,and its service life than ordinary aluminum alloy significantly.In recent years,the application of Al-Si alloy has developed very rapidly,and Al-Si alloy is mostly applied in the automotive field to make the key auto parts to replace using iron and steel,so Al-Si alloy is the most promising lightweight automotive materials.The main molding way of these parts made of Al-Si alloy is still casting,because the poor weld-ability of Al-Si alloy can simply lead to the defects of collapse,hot cracks,pores, breakout and the phenomenon of aluminum oxidation,over-firing and evaporation of alloying elements and degradation of welding performance in the welding process.Studies show that Al-Si alloy uses lower power density welding method of connection;because of the high contents of Si,needle-like eutectic silicon and large polygonal crystal plate silicon are formed inside the weld metal tissue,seriously splitting matrix to reduce the weld strength and plastic;then the metal near the seam zone is prone to overheated situation of grain growth,causing the reduction of welding strength and plasticity value.When Al-Si alloy uses hard soldering method of connection brazing hard time,Al-Si and the melting point solder is similar to brazing when Al-Si have been easy to bum,so difficult to control the quality of brazing;When Al-Si alloy uses soldering method of connection,Al-Si filler metal and the the existence of a larger inter-electrode potential difference,resulting in joint anti-chemical corrosion worse.In recent years,with Al-Si alloy composition optimization and integrated performance,casting technology,the use of rapid solidification process and short semi-solid processing of Al-Si alloy technology to improve the silicon in the Al matrix of the solid solution and grain refinement tablets so that the welding of Al-Si alloy improved connectivity with advanced technology,make it possible to connect Al-Si Alloy.In this paper,two experiments under contacting reactive brazing and laser welding are used to weld AC4B and ZL109 Al-Si alloy,with the specific connection process, mechanism,distribution of elements analysis of Al-Si alloy AC4B contacting reactive braze-welding adapter after adding mesosphere Cu,as well as the mechanical property at different temperatures,so as to bring up the optimum one.The paper studies on the heat-affected zone,weld fusion zone and the microstructure of the central area of CO₂ laser welding ZL109 Al-Si alloy under the condition of no filler metal;the factors impacting the mechanical properties of Al-Si alloy laser welding with different parameters;the mechanism of welding pores and cracks as well as the way to eliminate pores and cracks to improve performance through technological measures.Experiments show that dense stem can be achieved through using Cu at a temperature of 560℃.AC4B Al-Si alloy can intermediate Cu layer as a transient liquid phase diffusion bonding was dense interface connector..At the connection time of 10min,the joint microstructure is composed ofα-Al solid solution,single crystal Si,inter-metallic compound composed of CuAl₂ and Al₄Cu₉.At the connection time of 120min,the joint microstructure is composed ofα-Al solid solution,single crystal Si,inter-metallic compound composed of CuAl₂.The micro-hardness values at the joint center at the connect time of 10min,20min and 120min are respectively HV213.5,HV228.2 and HV272.3,and significantly higher than that of the parent metal.The micro-hardness of welding seam is very uneven with central parts comparatively higher.The micro-hardness of middle welding seam is respectively HV289.9 and HV149.7 for the gray massive phase and the black one,the precipitated phase enhances the micro-hardness of the seam.Connect time has a significant impact on joint shear strength.As time goes,the strength has an increase to its maximum of 70.2MPa at 120min before its decrease.The surface topography of fracture turns from brittleness to a mixture of co-existence of toughness and brittleness. Besides the relatively brittle low-melting-point and lower seam joint strength,the defects of eutectic formed by Al-Si alloy contacting reactive brazing also involve unsound seam which can impact the quality of eutectic contacting reactive brazing.The use of CO₂ laser welding under the condition of no filler metal to Al-Si alloy ZL109 can lead to continuous weld,dense weld organizations with stripe conchoids. Welded joints include three parts:weld seam,fusion zone and heat affected zone.The rapidly heating and cooling speed of the heat affected zone can severely affect the organization and joints performance;the parent metal crystal grain near the fusion zone later develops into columnar crystals before turning into epitaxial solidification is deemed as the ready phenomenon resulted from the growing of metal bath,the direction of growth alters with the change of metal bath heat dissipation and finally the epitaxial solidification stops growing and becomes dendrite.The seam phases are mainlyα-Al,Si and Mg₂Si.The micro-hardness is smallest in ZL109 Al-Si alloy welded joints near the fusion line,with the weld center distance decreaseing and the microhardness increaseing,when the welding speed of 1.8 m/min,2.4 m/min and 3.0m/min,the average of welding center seam HV 116,HV121,HV125,the average of faz microhardness is HV95,HV90,HV86.The density of heat input has significant impact on mechanical property.The tensile strength increases as the density of heat input decrease;when the density is 44J/mm,the strength reaches 102.5MPa which is almost equal to the parent metal-93.2%.The use of CO₂ laser welding without filler metal to ZL109 Al-Si alloy for welding can lead to macro-defects such as stayed and breakout weld surface,metallurgical defects such as weld hydrogen internal holes and hot cracks inside the seam.