Joining of Shape Memory Alloys
Year: 2010
Type: book-chapter
Abstract: In the present report, the joining of shape memory alloys to themselves or to other metals is discussed. The joining processes included are arc welding, beam welding, brazing, adhesive bonding and some other techniques like friction welding, resistance welding, explosive welding, transient liquid phase diffusion bonding and soldering. Welding of SMA by arc welding or beam welding processes is possible but tends to produce brittle joints. Brittleness can be reduced by stress relieving, but degradation of the shape memory effect may occur due to the high temperatures that are required. Welding should also be carried under inert gases to prevent passive oxide build-up. Welding is seen to change the phase transformation temperatures, which may reduce the application window. Post welding training cycles may enhance the as welded shape memory effect and superelasticity. Laser welding tend to give narrow welds, and may thus be preferable to arc welding due to possible finer microstructure and lower thermal stresses and strains left from the welding cycle. Welding of SMA to other metals may be challenging due to the extensive formation of intermetallic compounds. These tend to be very brittle. In such cases, it may be required to use additives that prevent such formation, but very little information is available. Such information would be of great interest to expand the application of SMAmetal couplings. Several brazing processes have been studied, including infrared furnace, laser heating and microwave heating. The main advantage of brazing is that the base metals do not melt, and that there are commercially available brazing alloys, mainly based on Ag-Cu-mixtures. Like welding, certain reduction of the initial base metal properties should be expected. Formation of intermetallic compounds at the interface between the base metal and brazing alloy will take place. This is a necessity to achieve strong bonds, but may also be negative due to the brittle nature of many of these phases. The XTi2 (where X = Cu, Ni and/or Ag) phase is regarded to be very brittle. Brazing of SMA to other metals will give complex interdiffusion across the joint with significant reaction layer formation. The use of proper, ductile interlayer may be good choice to prevent the excessive diffusion and to accommodate thermal strains due to mismatch in thermal expansion.
Author: Odd M. Akselsen
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W1500273962