Quench-partitioning(QP) steels are extensively employed to reduce automobile weight for its ultra-high-strength and ductility. As an example, QP1180 steels have the total elongation of 15% and the strength of more than 1200 MPa. They are often welded employing laser beam to achieve a low heat input, narrow heat-affected zone, and high production efficiency. Generally, the QP steel laser-welded joints are tensile fractured at the region of the base metal, but the testers fail at the softening zone. The extreme narrow softening zone contains tempered martensite, which is restricted and strengthened by the neighboring hard heat-affected zone and base metals. Thus, the deformation is evenly distributed along the gage length. However, the fracture position varies according to compositions, tensile strain rates, and various welding parameters. For other advanced high-strength steels, the softening zone are not always fracture positions. QP980, DP1000, and MS1500 sheets of steel have softened zone, but CP800 and TRIP980 steels have no softening. Most of failures occur at the weakest point. Recently, the oscillation scanning technique is coupled with the laser welding process for the advantages of high efficiency, low gap requirement, low crack and porosity, and fine grains. We hope to examine the effects of this scanning technique on the microstructure and mechanical properties of QP1180 steel welded joints.

QP1180 steel sheets with the 1.2 mm thickness are welded at the laser power of 3 kW, travel speed of 3 m/min, and zero defocusing distance with various scanning strategies. There are three linear scanning rotation angles of 90°, 45°, and 0°. The weld surfaces are compared and the weld cross-sections after etching are observed and measured employing an optical microscope and scanning electron microscope. Using electron back scattered detection, the microstructure and grain information are characterized. The microhardness and tensile properties are also measured under the three linear scanning rotation angles. Finally, the fracture surfaces are characterized to explain the deformation and fracture mechanism.

The weld formations of QP1180 high-strength steel welded joints under different linear scanning rotation angles demonstrate various weld penetrations, fusion zone widths, the distances between two intercritical heat impacted zone, and widths of coarse grain heat affected zone (CGHAZ) and fine grain heat affected zone (FGHAZ). Only the samples under the linear scanning rotation angles of 45° and 0° obtain full penetration at the same total heat input. The samples under the linear scanning rotation angle of 45° achieve the widest heat impacted zone and the similar fusion zone width with that of the sample under the linear scanning rotation angle of 90°. The sample microstructure under the linear scanning rotation angle of 45° reveals the phase compositions that are identical to those of the normal laser welds: the dendritic grain lath martensite in the fusion zone, coarse grain lath martensite in CGHAZ, fine grain lath martensite in FGHAZ, mixed phases of lath martensite and original base metal in intercritical heat affected zone (ICHAZ), and the tempered martensite in subcritical heat affected zone (SCHAZ). The grain orientation map and band contrast map reveal the difference between lath martensite and ferrite in morphology and gray scale. The hardness result reveals that there are slightly higher hardness in FGHAZ and lower hardness in SCHAZ than that of the base metal. The hardness distribution is unaffected by various rotation angles, but the average hardness in fusion zones is respectively (482±6)HV, (471±5)HV, and (472±7)HV under linear scanning rotation angles of 90°, 45°, and 0°. The full penetration welds under linear scanning rotation angles of 45° and 0° demonstrate ultimate tensile strength of 1271 MPa and 1245 MPa, respectively. The sample’s tensile failure position is the same as the previous investigation result: the fracture occurrs in the softening zone. It may be explained by the widening of softening zone. The variation of rotation angle during linear scanning laser welding can change the maximum and minimum absolute speeds of the moving heat source and the energy transfer direction, and it finally leads to the difference in energy distribution. The overlap ratio in remelting plays a role in enlarging the fusion zone and heat-affected zone. A large rotation angle can increase the transportation heat in the horizontal direction. The linear scanning with various rotation angles then results in various weld formation and mechanical properties.

In this research, three linear scanning strategies with various rotation angles are employed in laser welding of QP1180 steel. Through the rapid and space-limited scanning, the weld formation significantly changes and is adaptive to a large gap. A large rotation angle results in a small weld depth and a large fusion zone/heat affected zone. Here, the rotation angle of 45° is appropriate for large weld width and weld penetration. The microstructure and tensile properties of QP1180 high-strength steel welded joints using oscillation scanning are not significantly altered compared with those of normal QP1180 high-strength steel welded joints, while the failure position moves to the softening zone in QP1180 steel, which normally occurs with a high heat input. The deformation and fracture mechanisms of QP1180 steel welded joints are altered by the scanning strategy.