Laser Welding Of High Strength Steel Research Articles

Study on the suppression of solidification cracks in narrow gap laser welding of high-strength steel based on dual-beam gradient control weld solidification

Study on the suppression of solidification cracks in narrow gap laser welding of high-strength steel based on dual-beam gradient control weld solidification

Numerical investigation of sidewall penetration in narrow gap oscillating laser welding process

The lack of sidewall fusion is one of the process challenges in narrow gap laser welding. To solve this problem, beam oscillation has been applied to narrow gap laser welding. Because the laser oscillation behavior had a great influence on the sidewall penetration, a 3D heat transfer model of narrow gap oscillating laser welding of high-strength steel was established for numerical simulation. The effect of oscillation parameters on sidewall penetration was investigated by quantitatively analyzing the sidewall penetration under different oscillation parameters. The results showed that the high-temperature region of the molten pool changed periodically with the laser oscillation. As the oscillation amplitude increased from 0 mm to 1.5 mm, the molten pool cross-section shape changed from tip shape to arc shape, the peak temperature of the sidewall increased from 1857℃ to 2115℃, and the sidewall penetration also increased from 0.82 mm to 0.96 mm. As the oscillation frequency increased from 20 Hz to 100 Hz, the peak temperature of the sidewall decreased from 2036℃ to 1936℃. However, more heat was accumulated in the high frequency mode, and the value of sidewall penetration increased from 0.9 mm to 0.94 mm.

Investigation of Weld Root Defects in High-Power Full-Penetration Laser Welding of High-Strength Steel

The currently available high-power laser shows promising opportunities for the welding of thick plates in a single pass. However, weld-root defect frequently occurs when a high-power laser is used to join thick plates in a full-penetration mode, which has a significantly adverse effect on the serviceability of the weld joint. The purpose of this work is to understand the defect formation mechanism and reduce these defects through controlling welding parameters. In this study, the characteristics of weld root defects were investigated using a 10 kW fiber laser using a program of experiment and theoretical analysis. The corresponding defect formation mechanisms were discussed based on the bottom molten pool behaviors observed by the high-speed camera. The results showed that there were four types of weld-root appearances as follows with an increase of linear heat input from 300 J/mm to 1000 J/mm: weld-root humping (30 mm/s), sound weld (25 mm/s), weld sagging (20 mm/s) and excessive weld sagging. The remedies for reducing weld-root defects were also presented to obtain sound weld bead by optimizing welding parameters. Weld-root humping was formed due to the quasi-full-penetration keyhole. Weld sagging resulted from the imbalance of the hydrostatic pressure and surface tension in the condition of a through keyhole. It was also found that the sound weld was formed when a through keyhole and a proper molten pool size were obtained. Thus, the state of the keyhole and molten pool geometry were the major factors that affect weld-root defects.

Analysis of welding solidification crack in narrow gap laser welding of high-strength steel

Analysis of welding solidification crack in narrow gap laser welding of high-strength steel

Bead formation characteristics in laser welding of high-strength steel under subatmospheric pressures

A series of laser welding experiments were carried out on low-alloy high-strength steel specimens under subatmospheric pressure. The welding penetration and weld bead formation characteristics were evaluated by performing metallographic examination on weld bead cross sections. The effects of technological parameters such as ambient pressure, laser power, and welding speed on weld bead formation were investigated. A multi-factor regression prediction model of welding penetration was established on the basis of the above experimental investigation to evaluate the welding penetration in accordance with the adopted welding parameters under certain ambient pressures. To have more insights into the effect of dynamic behavior of molten pool on the bead formation, the laser welding on “glass-metal” composite samples was performed under varied ambient pressures, and a high-speed digital camera was employed to observe the keyhole profile and the molten pool surface during welding process. A correlation between the welding penetration and the ambient pressure for laser welding of high-strength steel was found that in the range of 101–3 kPa, the decrease of ambient pressure leads to the raise of welding penetration and a smoother weld bead appearance.

Hot cracking investigation during laser welding of high-strength steels with multi-scale modelling approach

ABSTRACTHot cracking during laser welding of advanced high-strength steels is reported to be a serious problem by automotive manufacturers. In this work, hot cracking susceptibilities of transformation-induced plasticity (TRIP) and dual-phase (DP) steels are studied based on a multi-scale modelling approach. Transient temperatures measured from welding experiments are used to validate a finite element (FE) model. The temperature, thermal gradient and cooling rate in the weld fusion zone are extracted from the FE model and pre-defined as boundary conditions to a phase field model. The welding-induced microstructural evolution is simulated considering thermodynamic and mobility data. Results show that, compared to the DP steel, the TRIP steel has a broader solidification range, a greater pressure drop at the inter-dendritic regions, and an increased phosphorus segregation at the grain boundaries; all these make this steel more susceptible for hot cracking.

Numerical Modeling and Experimental Verification of Residual Stress in Autogenous Laser Welding of High-Strength Steel

A three-dimensional finite element (FE) model was developed to numerically calculate the temperature field and residual-stress field in the autogenous laser welding process. The grid independence of the FE model was verified to eliminate the variation of the heat flux between adjacent elements. A cut-off temperature method with combination of the tensile testing was used to consider the effect of high-temperature material properties on the numerical simulation. The effect of the latent heat of fusion and evaporation was also taken into consideration. High compressive initial stress was presented in the selected high-strength steel plates. A subroutine was written to consider the initial stress in the FE mode. Predicted residual stress agreed well with experimental data obtained by an X-ray diffraction technique. Results showed that the transverse and longitudinal residual stresses prevailed in the autogenous laser welding process, and the thermal stress concentration occurred in the molten pool and its adjacent regions. The effect of the welding speed on the distribution of residual stress was also studied. The values of residual stress decreased with an increase in the welding speed.

Feasibility study of using acoustic signals for online monitoring of the depth of weld in the laser welding of high-strength steels

It is a trend to use high-strength steels in the automobile industry because of their good formability, weldability, and high strength—volume ratio. In order to achieve quality control, it is necessary to monitor the welding process online. In this paper, acoustic signals generated during the laser welding process of high-strength steel DP980 were recorded and analysed. A microphone was used to acquire the acoustic signals. A spectral subtraction method was used to reduce the noise in the acoustic signals, and a Welch—Bartlett power spectrum density estimation method was used to analyse the frequency characteristics of the acoustic signals. The results indicate that good welds with full penetration (FP) could be clearly distinguished from bad welds with partial penetration (PP). An algorithm based on the different sound pressures between FP and PP was developed to identify the penetration state in the time domain. Another algorithm based on the different frequency characteristics from 500 to 1500 Hz between FP and PP was also developed to differentiate the penetration state. The results show that these two algorithms can effectively distinguish FP from PP. In addition, the mechanisms of the different characteristics of the acoustic signal generated from different penetration depths and modes were also analysed and discussed. This study shows that it is feasible to use acoustic signals to achieve online monitoring of the penetration states during the laser welding of high-strength steel DP980 in a noisy environment by applying proper digital signal processing methods. The acquired acoustic signal could be used as a feedback signal to control the depth of weld penetration in the laser welding process.