按需喷射CuSn合金液滴的冷却速率及组织演变

摘要

采用按需喷射方法制备不同粒径的Cu-6％Sn合金粉末,测量不同粒径条件下的二次枝晶间距,并且推导出半经验公式,建立二次枝晶间距与液滴冷却速率之间的数学关系,并利用光学显微镜和背散射电子显微镜观察液滴组织形貌.结果表明:枝晶特征取决于凝固速率、冷却介质以及飞行距离;液滴温度处于液相线与固相线之间,当液滴彼此发生碰撞时,由于碰撞引起热传导率提高,经过数学模型推导,发生碰撞的液滴冷却速率超过4×104 K/s,枝晶组织和晶粒明显细化.由于液滴内部温度梯度作用,枝晶沿碰撞方向生长.%Different sized single droplets of Cu-6％Sn alloy were prepared by drop on demand (DOD) technique.The secondary dendrite arm spacing was measured and correlated with the droplet cooling rate by a semi-empirical formula.The microstructure of droplets was observed by optical microscopy (OM) and electro backscatter diffraction (EBSD).The dendrite feature of single droplets depends on solidification rate,cooling medium and flight distance.When droplets collide with each other at temperatures between solidus and liquidus,the dendrites and grains are refined obviously possibly because the collision enhances the heat transfer.The cooling rate of colliding droplets is estimated to be more than 4×104 K/s based on a Newton's cooling model.The dendrites grow along the colliding direction because of the temperature gradient induced by the internal flow inside the droplets.