Fatigue and bending behaviour of friction stir welded DH36 steel

Abstract

Friction stir welding presents many advantages over conventional welding techniques; however, there is limited published data with regards to the fatigue and bending performance of friction stir welded steels. Hence, this investigation aims to evaluate friction stir welded DH36 steel subjected to these loading conditions. A comprehensive fatigue and bending programme has been implemented to assess the impact of process related features, such as weld root flaws, on the welds' performance. Strain gauges located on the top and bottom surfaces of fatigue samples allowed the secondary bending stresses to be quantified when clamped in the fatigue test machine. Bend test samples were completed to a 180° Ubend for as-welded and ground samples. The bend testing programme demonstrated satisfactory performance of friction stir welded DH36 steel. Despite the presence of surface flaws, cracks did not propagate in bending indicating adequate levels of toughness. Fatigue performance was poor in comparison with results from similar welds however, it was found to be acceptable in terms of class recommendations for fusion welding. This lower performance was predominantly attributed to a weld root flaw. Strain gauge measurements indicated that the local stress at the weld root was up to 25% lower than the nominal stress determined prior to testing, thus artificially improving fatigue performance. Welds of good quality and refined microstructure were found, however process related flaws on the top and bottom surface emphasise the need for optimisation of the tool material and welding parameters.

Investigation of Nondestructive Testing Methods for Friction Stir Welding

“…In case of FSW, flat dog-bone shape samples are being used. Tensile, bending and fatigue performance of the FSW material are subjected to different loading conditions and the resulting data are used to understand the mechanical behavior of the joints [75][76][77]. Metallographic examinations can also reveal details of microstructure and some types of defects in FSW material such as lack of penetration and kissing bonds [64].…”

Determination of stresses in tooth roots of gears by Integral Strain Gauges

“…2. The specimens are subjected to cyclic deforming on a fatigue testing machine [14] in conditions of bend with rotation or in conditions of pure bending (Fig.3).…”

A mathematical and experimental analysis of friction stir welding of steel

“…Compared to sample W8, the cooling rate in W10 was higher as the microstructure includes more bainite and the α all was absent The probe end (weld root) of samples W8 and W10 are shown in figure 4.105 -a-and -brespectively, a microstructural examination of the weld root of W8 revealed fine ferrite grains (4 µm). As described earlier this is attributed to the effects of high strain rate and slower cooling rate (dynamic RX) [76] [8] [94] . A similar examination of the microstructure of FSW sample W10 (EH46) probe end revealed larger ferrite grains (10µm) with mix of nodular ferrite and short plates of cementite.…”

“…Lienert 2004 [93] investigated the feasibility of welding 0.18 inch (4. Fowler et al 2016 [94] investigated the effect of defects such as weld root flaws on fatigue and bending strength of FSW DH36 steel with welding speed of 300mm/min and tool rotational speed of 400RPM. They found a slight reduction in the three point bend test results but this was small because the cracks associated with the weld flaw did not propagate.…”

“…2013 also reported the same grain refinement after welding 4mm, 6mm and 8mm thick DH36 plates as a result of DRX. [94] Fowler et al 2016 reported that the weld root microstructure consisted of fine grains of ferrite which were attributed to the lower temperature and slower cooling rate compared to the top surface of the SZ. The grain size of ferrite in the weld root was finer than those in the PM due to the effects of high strain rate coming from material stirring.…”