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，通过旋转正八边形转子和外接冷铁之间的开口，制备了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小锭凝固组织的影响。当转速为50 rpm时，铸锭组织由无转子条件下的枝晶转变为玫瑰花状，当转速高于100 rpm时，铸锭组织转变为平均晶粒尺寸约为50 μ m的粒状。如果通过使用高压铸造机将以100 rpm制成的浆料流变铸造成盘形，则盘从中心到边缘区域具有几乎均匀的粒状结构。然而，拉伸延伸率和强度是非常低的，因为存在脆性细枝晶状结构区域，这是由流变铸造过程中的液相快速冷却产生的。通过降低冷却速度抑制细小枝晶组织区域的产生，特别是在延伸率方面表现出比常规挤压铸造样品高得多的性能。在相同的工艺条件下，转杯法也可获得平均粒度小于40 μ m的半固态磷青铜浆料。当采用半固态浆料在较慢的冷却速率下进行流变铸造时，所形成的样品具有平均晶粒尺寸为30 μ m的细晶组织。我们期望这种磷青铜再铸试样具有优异的拉伸性能，特别是在延伸率方面比常规铸件高得多。

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.