| Microsegregation, morphology and distribution of secondary phases could significantly influence the material properties. The coarse secondary phases formed during solidification often become the source of stress and cracks, which would damage the subsequent plastic deformation effect and heat treatment. The effects of ultrasonic melt treatment on microsegregation and secondary phases were investigated at7085aluminum alloy. Conventional ingot and ultrasonic ingot were researched by DSC analysis and solution treatment, to discuss the transformation behavior of secondary phases. The main researches of this paper were shown as follows:1. Ultrasonic field was introduced during solidification of7085aluminum alloy, the effects of ultrasonic melt treatment on microsegregation and solute relative solid solubility were investigated. The results showed that:compared with the ingot casted without ultrasonic field, the ingot casted in the ultrasonic field presented large amounts of finer and more uniform equiaxed grains, and its average grain size was decreased from452um to176um. And ultrasonic melt treatment eased the microsegregation, increased the effective coefficient. The solute relative solid solubility of zinc, magnesium, copper was improved7.4%,15.3%,6.9%, respectively.2. There were mainly three types of secondary phases in the ingots, Al2Cu, T and rich-Fe phase. Conventional ingot included mounts of elongated coarse phases, some10-20μm independent platelet secondary phases also existed in intracrystalline. While morphology and distribution of secondary phases were modified significantly in the ultrasonic ingot, and secondary phases were not found in intracrystalline. The tensile strength of ultrasonic ingot was23%higher than conventional ingot, fracture observation showed that the fracture behavior of conventional ingot was mainly intercrystalline failure and transgranular fracture. Fracture of ultrasonic ingot was mainly expanded along the equiaxed grain boundary. 3. Secondary phases in the conventional ingot and ultrasonic ingot were investigated by DSC thermal analysis. The results indicated that low-melting phases were reduced by21%after ultrasonic melt treatment, image analysis results indicated secondary phases were reduced by19%after ultrasonic treatment. The tensile strength of ultrasonic sample was higher than conventional sample after heat treatment. Fracture indicated that conventional sample remained many insoluble coarse secondary phases, which were less, finer and more uniform in ultrasonic sample. With45figures,12tables, and72references. |
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