A modified heat treatment process has been developed to maintain high strength levels of the base-metal and to enhance fracture toughness of the weld joint in welded materials of Ti-15% V-3% Cr-3% Sn-3% Al alloy used for spherical rocket moter cases of upper stages for scientific satellites. The hardness became relatively high at the weld joint with conventional heat treatment process. This local increase in the hardness results in a decrease of the fracture toughness of the weld metal. In order to reduce slightly the hardness only at the weld joint, the materials were aged at relatively low temperature ( designated as preaging) after solution treatment, and then welded and conventionally aged ( designated as secondary aging). The mechanism to improve the mechanical properties is considered as follows. At first, almost all parts are uniformly hardened with the preaging. Then the hardness of the weld joint is sharply reduced due to TIG welding because of re-solution treatment, while the base-metal maintains the high hardness. At the last step of the secondary aging, the hardness at the base-metal is increased in terms of the second step aging but the weld joint is hardened up to the level slightly down that of the base metal. Consequently this process has made it possible to obtain the desirable mechanical properties.
