Fatigue crack growth performance of peened friction stir welded 2195 aluminum alloy joints at elevated and cryogenic temperatures

The effects of various surface treatments on fatigue crack growth and residual stress distributions in friction stir welded 2195 aluminum alloy joints were investigated. The objective was to understand the degree to which residual stress treatments can reduce fatigue crack growth rates, and enhance fatigue life of friction stir welded components. Specimens were fabricated from 12.5 mm thick 2195-T8 aluminum plate, with a central friction stir weld along their length. Residual stresses were measured for three specimen conditions: as-welded (AW), welded then shot peened (SP), and welded then laser peened (LP). Crack growth rate tests were performed in middle-cracked tension specimens under constant amplitude load for each of the three conditions (AW, SP, LP) and at three temperatures (room, elevated, and cryogenic). At room and elevated temperature, crack growth rates were similar in the AW and SP conditions and were significantly lower for the LP condition. At cryogenic temperature, it was difficult to discern a trend between residual stress treatment and crack growth rate data. Laser peening over the friction stir welded material resulted in the fatigue crack growth rates being comparable to those for base material.