Experimental Study On Fracture Repair Of Silicon Phase In Deformed Al-15Si Alloy

Hypereutectic Al-Si alloy has been widely used because of its low density,low thermal expansion coefficient,high wear resistance and good stability.However,it is easy to crack on the silicon phase when the alloy was subjected to additional load,leading to the damage of the workpiece.In this paper,DC casting and P modified Al-15Si alloy was used as the research object,and the fine and uniform micro structure was obtained by hot extrusion deformation.And the extruded samples were heat treated,in order to study the influence of 24h,48h and 72h heat treatment at 560? on the silicon phase,then the internal cracks were prepared by tensile test on the sample which was heated at 560? for 72h,and finally studied the healing process of the internal cracks in different holding time and heating temperature.The process of internal crack healing and the change of internal crack morphology were observed by means of optical microscope and scanning electron microscope.Finally,the influence factors and the healing mechanism of internal crack were analyzed.The following conclusions can be obtained:(1)The microstructure of DC casting Al-15Si alloy consists of bulk primary silicon,?-Al dendrite and(?-Al+Si)eutectic structure.After hot extrusion,the acicular and fibrous eutectic silicon in the cast microstructure became fine particles which were uniformly distributed on the Al matrix.Meanwhile,the well-developed ?-Al dendrites were replaced by equiaxed grains.(2)When the extruded microstructure was treated at 560? in different holding time,the sharp corners of the primary silicon and eutectic silicon became rounded.The size of bulk silicon changed little,which was about 28?m;while the silicon particle size increased,the average size of extrusion state silicon particle was 2.62?m,the average size of silicon particle after 24h,48h,72h heat treatment respectively was 5.38?m,15.89?m,16.18?m.(3)The internal crack of Al-15Si alloy occurs on the second phase silicon particles after tensile test at room temperature,and the internal crack size is affected by the distribution of silicon particles,the size and morphology of silicon particles.The closed fracture cavity presents a vacuum state,and the conventional heating method can make the internal crack heal.(4)The internal crack healing is affected by the holding temperature,holding time and crack size.The internal crack healing degree of specimen at low temperature is much less than that at high temperature;at a certain temperature,the healing effect of internal crack increases firstly and then decreases with holding time;and the larger the volume of the crack cavity,the more difficult the internal crack healing is.(5)There are three distinct stages of internal crack healing in Al-15Si:The crack tip becomes passivated;the matrix was pushed into the crack cavity and the internal crack length was shortened;the crack cavity was completely filled,and the silicon and the matrix were bonded to the structure.The internal crack healing should meet certain thermodynamic and dynamic conditions,and the mechanism of crack healing is the migration of vacancies.