Rheocast of lead-free bismuth bronze and phosphorus bronze utilizing high fluidity slurry manufactured by repeated extrusion process

以无铅铋青铜和磷青铜为原料，将低过热度熔体通过旋转的规则八角形转子和外接冷铁块之间的开口，制得具有细小固体颗粒的半固态流变铸造浆料。首先，我们主要研究了转速对无铅铋青铜凝固小锭组织的影响。当转速为50rpm时，铸锭组织由无转子状态的枝晶转变为玫瑰花结状，当转速高于100rpm时，铸锭组织转变为平均晶粒度约为50μm的颗粒状组织。如果使用高压铸造机将以100rpm制得的浆料重铸成圆盘形状，则圆盘从中心到边缘具有几乎均匀的颗粒状结构。然而，由于流变铸造过程中液态快速冷却产生的脆性细小枝晶结构区域的存在，导致拉伸伸长率和强度很低。通过降低冷却速度来抑制细小枝晶区的产生，显著提高了铸件的延伸率，特别是延伸率明显高于常规挤压铸件。另一方面，在几乎相同的工艺条件下，转子法还可以生产平均颗粒尺寸小于40μm的半固态浆料磷青铜。当采用半固态浆料在较慢冷却速度下进行流变铸造时，成形的样品具有细小的晶粒组织，平均晶粒度为30μm。我们预计这种磷青铜重铸样品具有良好的拉伸性能，特别是在延伸率远高于传统铸件的情况下。

Semi-solid slurry with fine solid granules for rheocasting was made from lead-free bismuth bronze and phosphorus bronze by passing the low-superheat melt through an opening between a rotating regular octagonal rotor and a circumscribed chill block. At first, we mainly investigated the effect of rotational speed on the structure of a solidified small ingot of lead-free bismuth bronze. The ingot structures changed from the dendrite of rotor-free condition to the rosette at the rotational speed of 50rpm and to the granular with a mean grain size of about 50μm at higher than100rpm. If the slurry made at 100rpm was rheocast to a disk shape by the use of a high pressure casting machine, the disk had almost uniform granular structures from the center to the edge regions. However, the tensile elongation and strength were very low because of the existence of brittle fine dendrite-like structure regions which were generated by rapid cooling of liquid phases during rheocasting. They were drastically improved by lowering cooling rate to suppress the generation of fine dendrite structure regions, especially in the elongation showing much higher than that of conventional squeeze cast sample. On the other hand, the rotor process also produced semi-solid slurry phosphorus bronze with mean granule size of less than 40μm at almost the same processing conditions. When rheocasting utilizing the semi-solid slurry had been carried out at slower cooling rate, the formed sample had a fine grain structure with mean grain size of 30μm. We expect this reocast sample of phosphorus bronze have an excellent tensile property especially in the much higher elongation than that of the conventional castings.