Fatigue behaviour of laser repairing welded joints

Abstract

This paper presents a fatigue study in Nd-YAG laser surface repairing welded joints in specimens of two base materials used in mould production. The tests were carried out in a servo-hydraulic machine in tension, under constant amplitude loading, with two stress ratios R = 0 and R = 0.4. Welded specimens were prepared with U notches and filled with laser welding deposits. The fatigue results are presented in the form of S-N curves obtained in welded and non-welded conditions. Complementary measurements of hardness and residual stresses profiles were carried out along the surface of laser welded specimens to understand the observed fatigue behaviour. The melted material was the weaker region, with lower values of hardness and higher tensile residual stresses, presenting also a high number of defects that are potential failure sites. The presence of such defects can explain the relatively poor fatigue strength of the laser repairing joints in comparison to base materials.

Residual stresses analysis of ND-YAG laser welded joints

“…In a previous work [1,2], the authors observed that welded specimens, tested at R = 0.4, presented a similar fatigue resistance to the correspondent non-welded specimens tested for the same stress ratio and also very close to the curve obtained with welded specimens for R = 0. Residual stresses are probably the main factor affecting this fatigue behaviour.…”

“…In a previous work [1,2], the authors observed (see Figs. 3 and 4) that welded specimens, tested at R = 0, present a significant lower fatigue resistance than the correspondent non-welded specimens tested for the same stress ratio.…”

Mould steels repaired by laser welding

“…10 compares the S-N fatigue curves for non-welded specimens and welded specimens in P20 steel at R = 0. Two series of tests performed with repaired specimens are show: one for the new welding parameters optimized as indicated above and using two directions for the welds (0°and 45°); the other was previously obtained [8] using the currently welding parameters used in the mould industry. It can be clearly observed an important increase of the fatigue resistance with the optimized welding procedure previously described.…”

“…A fatigue analysis of the H13 and P20 mould steels with surface defects repaired by the laser process will be performed with several filler materials, welding parameters and welding geometry. The fatigue data obtained with the new welding conditions will be compared with the previously obtained S-N curves in welded and plain specimens [8]. Finally, microhardness analysis and evaluation of the residual stress state of the laser welded regions as well as SEM observations will also be performed after laser repair.…”

“…In recent work [8], the authors obtained the fatigue strength of H13 and P20 mould steels, for both nonwelding and surface laser welded specimens with U notches. Filler material and welding parameters used in the preparation of the welded specimens were the ones currently used in the mould industry.…”