Friction welding of TiAl intermetallic compound

Summary

Recent years have seen TiAl intermetallic alloy attracting research interest as a high specific strength intermetallic compound. Practical applications of TiAl, however, are complicated by the fact that the material shows little ductility at temperatures below 1000 K, features poor plastic workability even at temperatures above 1200 K, to say nothing of its performance at normal temperature, and cannot be worked by the normal methods of forging, extrusion, rolling, etc, as apply to conventional metals. An important area facing new problems in this context is that of developing suitable joining techniques for TiAl intermetallic compound applications. The approach adopted in the present investigation is to address the problem of TiAl joining from the twin perspective of fusion welding (laser welding) and solid-state welding (friction welding) and to examine the weldability and joinability of TiAl intermetallic compound by these two methods. Fusion welding (laser welding) and solid-state welding (friction welding) of TiAl intermetallic alloys were performed, and investigations were made to determine the structures and high-temperature strength properties as well as whether or not any defects occur when the respective welding methods are used to join TiAl intermetallic compound. The results obtained may be summarised as follows:

1. Sound joints are obtained by friction welding across the entire range of welding conditions adopted in this study. In contrast, laser welds exhibit extensive cracking under all welding conditions.

2. The friction-welded joints have a high-temperature strength equal to or greater than that of the base metal. Both the base metal specimens and friction-welded specimens exhibit an inverse temperature dependence. The elongation is as small as around 2% up to 1123 K, increasing at a temperature above 1173 K and showing that the ductility improves.

3. The welds and heat affected zone produced by laser welding show pronounced hardness changes. The friction welds, however, show relatively little hardness change, and the change in the hardness due to the friction welding conditions is also small.

4. X-ray diffraction results indicate the presence of TiAl and small amounts of Ti₃Al and Ti Al₃ in the weld, HAZ, and base metal of the friction-welded joints, whereas no precipitates, such as e.g. oxides, nitrides, etc, are detected in any part of the welds, HAZ, and base metal of these joints.

The foregoing results suggest that fusion welding of TiAl intermetallic compound is difficult to perform. In contrast, sound welds with good high-temperature strength properties can be obtained by friction welding.