ABSTRACT
Many statistical data show that 90% failure accidents of the welded structures are caused by fatigue failure of welded joints. Therefore, it is remarkable in social benefits and economic benefits to improve the fatigue strength of welded joints of welded structures. In this paper, it is systematically studied on the improvement of the fatigue strength of welded joints using plasma spray.
Based on the volume expansion principle, a kind of new dusty spray (LTTDS) has been developed. Test results show that the residual stress of the new dusty spray coating was compression stress, and the mean value is -31.7 MPa.
Test results show that type of metal powder spray has great influence on the improvement of fatigue properties by plasma spray. When △σ=80MPa, the fatigue life of LF6 cruciform joints is equal to that of those plasma sprayed with AlSi, but the fatigue life was decreased by 72.3% when sprayed NiAl. For the 1Cr18Ni9Ti cruciform joints welded by TIG, the increase in fatigue strength and fatigue life of plasma sprayed specimens were 20.4%~25.9% and 1.58～9.62 times respectively.Comparison with the specimens sprayed with low carbon steel, the increase in fatigue strength was 3.6% when the new dusty spray was used. The increase in fatigue strength and fatigue life of plasma sprayed cruciform joints welded by MAW were 22.5% and 1.98~4.52 times respectively. Comparison with the specimens sprayed with low carbon steel, the increase in fatigue strength was 5.3% when the new dusty spray was used. For the longitudinal weld joints, the increase in fatigue strength and fatigue life of plasma sprayed specimens were 10.1%~13.9% and 1.34~1.70 times respectively. The increase in fatigue strength and fatigue life was 3.8% and 0.21~0.32 times, when the new dusty spray was used.
Test results show that type of spray process has great influence on the improvement of fatigue properties, when low carbon steel was sprayed, the increase in fatigue strength of flame sprayed and plasma sprayed specimens were 9.7% and 20.4~25.9% respectively, and the increase in fatigue life were 1.55~1.97 times and 1.58~9.62 times respectively. When △ σ=180MPa, the fatigue lives of plasma sprayed and spray welded specimens are equal. the flame sprayed and as welded specimens failed at the weld toe, while the plasma sprayed specimens failed in the parent plate remote from the weld. And part of the sprayed coating of the flame sprayed specimens peeled off during the fatigue tests. Those sprayed with low carbon steel were failed at the weld toe, but the specimens sprayed with the new dusty sprays were failed from the weld metal.
Finite element analysis results show that the stress concentration factor at the weld toe of the plasma sprayed specimens is decreased from 2.75 to 1.1. Plasma sprayed coating are effective to decrease the stress intensity factor of weld toe. The stress at the toe perpendicular to the coating-substrate interface is greater than the coating-substrate adhesion, so part of the coating near the toe peeled off during the fatigue tests. This is the reason why the fatigue life of the flame sprayed specimens is lower than that of plasma sprayed ones.
KEY WORDS：Welded structure, Welded joint, Plasma spray, Fatigue strength,Fatigue life, Stress concentration
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