Abstract
Friction stir processing (FSP) enables surface modifications can likely be applied as a new post-weld treatment for improving fatigue strength. When applying FSP to high-strength materials, tool wear occurring at the interface between the tool tip and the topmost steel layer has been regarded as an unavoidable issue and is related to the tool rotational speed. The present study investigated the relationship between the tool rotational speed and fatigue strength of arc-welded high-strength low-alloy (HSLA) steel joints with weld toes subjected to FSP using a spherical-tip WC tool. FSP was conducted on the weld toe of HSLA steel joints with various tool rotational speeds. Tool wear increased with increase in tool rotational speed, and consequently contents of constituent elements of the WC tool increased in the topmost steel layer of weld toes, leading to large increase in fatigue strength. One reason for the increase with tool rotational speed is significant increase in solid solution hardening due to supersaturated W and C in the topmost steel layer consisting of martensite laths. The hardened topmost steel layer prevented fatigue crack initiation, and the increased fatigue strength depended on the contents of supersaturated W and C. Alloying of the topmost steel layer with tool constituent elements of W and C accompanied with WC tool wear during FSP is unique additive manufacturing to increase the fatigue strength of welded joints, and can be employed locally on structural components susceptible to fatigue.
Highlights
Fatigue strength of arc-welded joints is commonly said to be lower than that of the base metal, because of stress concentration, tensile residual stress, and deterioration of the mechanical properties due to microstructural change at the weld toe
A surface morphological and microstructural feature of the topmost layer of the weld toes after friction stir processing (FSP) varied with tool rotational speed: a notched surface was observed in FSP400 (Fig. 3(e)), while smooth surfaces were observed in FSP600, FSP800, and FSP1000 (Fig. 3(f)–(h))
The white contrast areas correspond to the portions alloyed with constituent elements of the WC tool, and the thicker portion for FSP1000 was provided by the faster tool rotational speed
Summary
Fatigue strength of arc-welded joints is commonly said to be lower than that of the base metal, because of stress concentration, tensile residual stress, and deterioration of the mechanical properties due to microstructural change at the weld toe. The International Journal of Advanced Manufacturing Technology coarse precipitates, which can become crack initiation sites, resulting in the improvement of fatigue properties of cast alloys [24,25,26] Considering these advantages, Costa et al reported that FSP on the weld toes increased fatigue strength of several aluminum alloy joints fabricated by metal inert gas welding [27,28,29,30]. Increase in the tool rotational speed would increase the WC tool wear, occurring at the interface between the tool tip and a topmost steel layer, leading to increasing contents of constituent elements of the WC tool in the topmost steel layer of weld toes This would increase solid solution hardening and increase fatigue strength. The relationship between the tool rotational speed and fatigue strength of arc-welded HSLA steel joints with weld toes subjected to FSP using a spherical-tip WC tool was investigated
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