Al-Cu合金薄壁特征件枝晶生长同步辐射成像

Growth Behavior of Dendrites in Al-Cu Alloy Thin-walled Samples by Using Synchrotron Radiation X-ray Imaging Technology

利用同步辐射X射线成像技术研究了Al-20%Cu合金壁厚为2mm和5mm的薄壁特征件枝晶生长演化过程。结果表明:不同厚度薄壁特征件的枝晶生长模式不同。2mm薄壁件的枝晶生长先于两侧形核,并迅速向中心生长,枝晶臂搭接形成毛细网络;5mm薄壁件的枝晶生长由两侧和底部同时形核,侧壁枝晶向中心生长,底部枝晶在上浮过程中长大,中心区存在大面积等轴晶区域。对共晶前沿移动轨迹跟踪发现:2mm薄壁件凝固速度快,厚度方向温度梯度小,共晶前沿以直线向前推进;5mm薄壁件侧壁凝固速度比中心区快,温度场不均匀,共晶前沿按凹形曲线向前生长。壁厚小于2mm时,枝晶网络形成后的毛细作用是影响铸件充型能力的主要因素。提高浇注温度、增加冒口、补液、补缩等方法难以有效改善铸件质量,应考虑采用快速浇铸、压力铸造的工艺方法来减少薄壁内缩松组织的出现。

Synchrotron radiography was employed to study dendritic growth behavior and morphology evolution of Al-20%Cu alloy in 2mm and 5mm thin-walled samples.Graphical results illustrated that dendrite growth patterns depend on the wall thickness of samples.For 2mm thin-walled samples,dendrite nucleated firstly and subsequently grew towards the central,while the dendritic arms impinged together the interdendritic capillary network formed.As the thickness of wall increasing to 5mm,the dendrites nucleated simultaneously across the section.Lateral growth dendrites pierced to the center,centrally borned equiaxed dendrites floated upwards and got stuck in the upper part finally.The consequence was a large eutectic area forming in the lower central part.The contour and proceeding velocity of the eutectic front indicated that thermal gradient across the section and varied growth rate for different thickness walls.When the wall thickness is less than 2mm,capillary effect of interdendritic network contributed mainly to impeding the filling of castings.Therefore,it is difficult to improve thin-walled casting quality by elevating pouring temperature,increasing riser volume and external melt replenishment.The conclusion is drawn that increasing pouring rate and filling pressure will be the effective measures to control the microporosity in thin-walled castings.