Improving Fatigue Strength And Life Of Single Crystal CMSX-4

The deep penetration of laser peening (LP) and retention of residual stress at high temperature by LP is significant in improving fatigue life and strength of superalloys exposed to high temperature and corrosion. Single crystal CMSX-4, was evaluated for stress relaxation and both fatigue life and fatigue strength of material that was LP-treated compared to non-LP and shot-peened (SP) specimens. LP was done with multiple layers using a 20 J/pulse laser where the laser high energy enabled using a 5 mm spot size on the metal surface. Stress measurements by the slitting technique showed the plastic penetration depth exceeded SP by a factor of 24. Un-peened and peened specimens were exposed to sulphate corrosives at 700°C for 300 hours and then fatigue tested. Five non-LP specimens all failed in low cycle fatigue whereas three identically tested LP specimens all achieved multi-million-cycle runout, indicating a consistent large benefit for life by LP. Tests of fatigue strength gave improvement of 2:1 for laser peened specimens. Measurements post hot-corrosion exposure and fatigue testing showed 5 mm depth retention of residual eigenstress for LP specimens.