Effect and Fracture Behavior of Weld Root Gap on Fatigue Performance in Dual Phase (DP590) Steel

Abstract

AHSS (Advanced High Strength Steel), which is widely used as a material for automobile body and structure parts for vehicle weight reduction, is a steel which recovers ductility loss and formality degradation due to high strength increase of conventional HSS (High Strength Steel). In addition, the use of automobile body and frame material has been increasing for the purpose of improvement for fuel economy and crash energy absorption in terms of safety. Dual Phase Steel, one of advanced high strength steels, has both high tensile strength and low yield strength due to the coexistence of two types of metal structures, a strong martensite structure and a soft ferrite structure. It also has excellent weldability and corrosion resistance required by the automotive industry. The DP (Dual Phase) Steel has been applied to the vehicle body and frame through spot welding. Recently, on the other hand it is being tried use through GMAW (Gas Metal Arc Welding) as a component for suspension parts such as a front lower control arm and a rear tubular beam type axle because it has sufficient high strength and formability with good weldability. However, fatigue fracture from suspension parts with DP steel occurs early in the overlap welded area of the GMAW compared to the designed fatigue life. It is necessary to understand the cause and the factors affecting fatigue life degradation. So far, the main factors affecting the fatigue performance of welded parts of high strength steels for body have been investigated by the shape of weld bead and weld length as welding dimension perfective. In case DP Steel has a relatively thick plate material when they are joined by overlap welding of suspension parts unlike those used for car body plates, and weld root gaps necessarily occur at overlapping welds between the upper and lower sheets due to the complex and rounded shape. The purpose of this study is to investigate the effects of overlap weld root gap on fatigue performance and fracture behavior of GMAW overlap joints using Dual Phase (DP590) Steel which is one of AHSSs. The tensile shear strength test was performed before the high cycle fatigue test, and it was investigated that the tensile shear maximum strength of the overlap weld test specimen was dramatically reduced compared to the base metal tensile test specimen. The yield strength and elongation rate also decreased as the weld root gap increased. However, in the case of the 1.5 mm weld root gap test specimen, unlike the specimens of smaller weld gap length, the final fracture location was changed from the weld toe of lower sheet to the weld root gap of upper sheet. The results of the high cycle fatigue test showed a decrease in fatigue strength due to the effect of overlap weld shape and the presence of a weld root gap between the upper and lower sheets. Compared to the base metal specimen, the fatigue limit of the overlap weld specimens decreased by 80%. And as the length of weld gap increased fatigue life also decreased. The fatigue limit of 1.5 mm weld root gap test specimen was further reduced by 27% compared to 0 mm test specimen without weld root gap. Four types of fracture modes were identified according to the weld root gap length and applied load stress level. Depending on the presence or absence of plastic deformation and the applied stress level was divided into high and low relatively, additionally four modes were classified according to the final fracture location when they were broken. Under the relatively high fatigue stress area, the fatigue fracture mode with plastic deformation in the low cycle area was observed, but in the case of low fatigue stress area, no deformation occurred and the final fracture occurred in the high cycle life. The final fracture positions varied depending on the length of the weld root gap. When the root gap was small, the final fracture occurred at the weld toe area of the lower sheet on overlap weld specimen , on the other hand, when the root gap is large, the weld root gap area of the upper sheet is finally broken. The hardness distribution showed the highest values is located in the weld metal area and gradually decreased toward the base metal side from center area the lowest hardness values inspected in the region of the boundary between the coarse grain and fine grain heat affected zone, regardless of the type of specimen. Scanning electron microscopic examination of the fractured specimens showed that crack initiations occurred at several multi points in the weld toe and root area, and striations and micro cracks were also found in the propagation area. Finally in the final fracture zone, as ductile fracture aspects, dimples and voids were observed and occurred with the necking phenomenon at the same time. The larger the applied stress, the larger the area of the final fracture surface tends to be wider than the lower stress.