An oxide film is easily formed on the surface of magnesium and aluminum powders due to the high chemical reactivity of these metals. The sinterability of these powders is extremely poor for conventional powder metallurgy. At present, the behavior of the warm compaction method, which has been done using iron powder, is practically very interesting, because high density and strongly sintered materials are efficiently obtained. However, there are only a few reports on the sinterability of magnesium and aluminum powder mixtures by this method. In this study, to consider the sinterability of such mixtures, we examined the effect of the compaction temperature. The compacts that consist of these mixtures were consolidated by the warm compaction method, which was conducted in the temperature range from 301 to 423 K. The compacts were sintered in an argon atmosphere. As a result, the transverse rupture strength for the warm compaction was from 20 to 30 MPa higher than for the cold compaction. The increasing compaction temperature causes sufficient contact between the powder particles, and at the same time, plastic deformation of the powder particles readily occurs. This behavior induced by the warm compaction method would lead to a break down of the oxide films. As a result, sufficient bonding between the powder particles occurred, and the transverse rupture strength increased.

Generally, magnesium powder is sintered by a large plastic deformation and surface oxide destruction is caused during the hot extrusion process. However, in this study, the binary mixtures of a low-melting-point metal powder (Sn, Bi, Sb) and the pure magnesium powder with surface oxide were sintered by transient liquid phase (TLP) under vacuum of about 3 Pa after they were degassed during heating. The size of the intermetallic compound formed at the old powder particle boundary and grain boundary influenced the strength characteristics of the sintered mixtures. As a result, the transverse rupture strength and bending strain of Mg-Sn type sintered compacts were superior to those of as-cast AZ91. This sintering technique without extrusion, the traditional liquid phase sintering, leaves room for improving strength characteristics by controlling the size of an intermetallic compound.

Friction stir welding (FSW) is a solid-state joining process which employs a lower joining temperature than that used in fusion welding. In this study, the weldability and the effects on the interfacial microstructure of alloying elements in dissimilar metal lap joints between commercial pure titanium and nickel-based alloy (Inconel 625) were examined by friction stir welding. The thickness of the stir zone on the titanium side decreased with increasing tool rotation speed. At high rotation speeds, the titanium adhered to the surface of the FSW tool's shoulder and probe. Fracture occurred in the region in the stir zone of titanium where its thickness decreased by tensile share test. A thin reaction layer was formed at the joint interface under suitable joining conditions. Transmission electron microscopy revealed two types of layers in this reaction layer. The layer on the titanium side was less than 100 nm thick and contained nickel, titanium and a small amount of iron; the other layer, on the Inconel 625 side, was less than 50 nm thick and contained all the alloying elements in Inconel 625, plus titanium. A quantitative analysis of the titanium-side layer using TEM-EDX showed its composition to be 68.9% Ti and 29.2% Ni (by atomic percentage), suggesting that the layer is comprised of the intermetallic compound Ti2Ni.

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Magnesium is promising as a light weight material because its density is lower than that of aluminum. In order to enhance this advantage, various alloying elements have been studied. However, embrittlement caused by the simple addition of alloying elements is problematic. In this study, an atomized powder obtained by adding an element to improve the properties of the magnesium was prepared, then sintered by pulse current pressure. The strength property of magnesium sintered compacts was assessed by a transverse rupture test. Based on the results of the transverse rupture test and microstructure observations, the effect of the mean size of the intermetallic compounds on the strength and ductility was discussed. KEY WORDSpowder metallurgy, magnesium, transverse rupture strength, intermetallic compound