The segregation control of coating element for pulse fiber laser welding of Al-Si coated 22MnB5 steel

Highlights

• The segregation of Al element in fusion zone was reduced greatly due to the introduction of pulse oscillation.

• Nearly full martensite microstructure of fusion zone was obtained at the appropriate pulse parameters.

• The average microhardness of fusion zone in hot-stamped condition could reach to 523HV at the optimized pulse parameters.

• Tensile properties in hot-stamped condition improved greatly with 3.7 % total elongation and the fracture surface was mainly ductile feature.

Abstract

In this study, pulsed wave (PW) fiber laser welding of 1.4-mm-thick Al-Si coated 22MnB5 steel sheets was adopted to investigate the effect of pulse modulation on reducing the segregation of coating element in fusion zone (FZ) and improving mechanical properties of welded joints. It was found that the segregation of Al element in FZ was reduced greatly due to the introduction of pulse oscillation, which weakened the formation of δ-ferrite in FZ. The experiment results indicated that nearly full martensite microstructure of FZ was obtained at appropriate pulse parameters. The average microhardness of FZ in hot-stamped condition could reach to 523 HV under the condition of optimized pulse parameters, which was comparable to the BM. Meanwhile, the tensile strength and total elongation increased up to 1600 MPa and 3.7 %, respectively. In addition, lots of small dimples on the fracture surface also suggested ductile tendency. However, the failure still occurred in FZ and the crack initiated from the soft fine α-ferrite phase near fusion line, which could be improved in further study.

Introduction

As the demands for vehicle weight and fuel consumption reduction become more urgent, the press-hardened steel (PHS) with ultrahigh tensile strength (>1500 MPa) has been developed to improve the safety quality of anti-collision structural components such as bumpers, A-pillars and B-pillars. Karbasian and Tekkaya (2010) and Yao et al. (2013) both have indicated the hot stamping process of PHS with heating at 900∼950 °C for 3∼10 min and then press-forming to martensite in a cooling equipment, thereby the high-strength components could also have high forming accuracy and complex structures. For further weight reduction and design flexibility, PHS has always associated with laser-welded blanks (LWBs) technique. Al-Si coating is commonly applied to 22MnB5 boron steel by hot dipping due to the outstanding antioxidant at high temperature, while causes serious issue about mechanical properties reduction of welded joints.

Coating elements segregated in fusion zone (FZ) lead to the phase evolution and affect the mechanical properties. Yoon et al. (2017) have observed intermetallic compound as Fe₃(Al,Si) with high Al content (8.24∼15.10 wt.%) along fusion line of Al-Si-coated boron steel, which seriously weakened the strength of weld joints. However, Saha et al. (2016a) have revealed by TEM analysis that the Al-rich second phase in fusion zone was non-equilibrium δ-ferrite in as-welded condition and transformed into α-ferrite after hot stamping. Saha et al. (2016b) have also investigated the mechanical properties of Al-Si-coated 22MnB5 welded joints. Caused by Al segregation with ferrite formed in FZ, the strength and ductility of joints in hot-stamped condition both decreased severely (1300 MPa, 1%). Lin et al. (2018b) have compared the coated and de-coated welded joints of boron steel. It was found that the tensile property of coated joint was comparable to the de-coated joint in as-welded condition but reduced heavily after hot stamping, resulted from the ferrite segregation in FZ.

Recently, many efforts have been put into the approaches about reducing the adverse effect of aluminized coating and enhancing the weld mechanical properties of Al-Si-coated PHS. To eliminate the introduction of coating elements, Li et al. (2017) have reported a nanosecond laser ablation technique of removing the coating layer previous to welding procedure. Some other de-coating methods such as plasma ablation (Yao et al. (2018)), electron beam ablation (Fang et al. (2018a)) and water jet cleaning (Fang et al. (2018b)) were also confirmed feasible by correlative patents. However, the de-coating procedure costs expensive device investments and reduces the production efficiency in general. Other approaches without coating removal have been put forward too. Kang et al. (2015) have proposed a novel method of arc-pretreatment with TIG, which could transform the Al-Si coating into intermetallic before welding and reduce Al-segregation in FZ, thereby enhanced the strength to base metal (BM) level. Kang et al. (2016) have found that the ferrite fraction in FZ could be lessened with the increase of beam diameter, heating temperature and holding time, but mentioned no related mechanical property results of Al-Si coated joints. Lin et al. (2018a) have adopted carbon-steel filler wire (ER70S) in laser welding for Al-Si-coated boron steel. The dilution effect of filler wire contributed to less and uniform δ-ferrite distribution in FZ, but the improvement of ductility was insufficient. Sun et al. (2019) have promoted a novel idea to insert Ni foil as an interlayer to Al-Si coated PHS sheets when laser welding. Full martensite was obtained in FZ and the tensile samples ruptured in BM. Wang et al. (2020) have investigated the different effect on microstructure evolution with varied Ni foil thickness (30∼100 μm). The results indicated that the thickness of 60 μm was most appropriate to obtain full martensite in FZ.

Pulse modulation has played a significant role in porosity reduction during laser welding, which mainly caused by the oscillation effect on molten pool. Kuo (2005) and Kuo and Jeng (2005) have found that micro-porosity decreased as the peak-to-base power ratio W_r increased due to the periodic pulse agitation. Jiang et al. (2016) have compared the effects of different wave mode when laser welding Inconel alloy. The results indicated that the pulsed wave mode increased the welding pool agitation and assisted the bubbles float upward during solidification process, leading to the porosity decline. In addition, Kumar et al. (2017) and Meng et al. (2019) both have reported that the appropriate pulse modulation helped to effectively reduce the welding distortion and suppress the porosity formation of thick stainless steel laser welding. Liang and Luo (2017) have revealed that the periodic laser vibration of pulse laser welding caused dynamic surface ripples and fish-scale pattern, furthermore simulated by three-transient numerical modeling. Based on the above analyses, the pulse laser welding has the potential application for Al-segregation controlling of Al-Si-coated boron steel by pulse oscillation in molten metal, but this method has not been discussed previously.

Without removing the Al-Si coating, pulse fiber laser welding of 22MnB5 steel sheets is adopted in this study to investigate the effect of pulse modulation on reducing the segregation of Al element in FZ and improving mechanical properties of welded joints. The pulse parameters such as pulse base power or peak to base power ratio W_r are optimized to obtain the desired weld formation and microstructure. The ferrite fractions of pulsed wave (PW) and continuous wave (CW) welded joints are presented to compare the difference of weld microstructure. The chemical compositions of ferrite and martensite in different welds are analyzed to study the segregation control of coating elements. Meanwhile, the mechanical properties as well as fracture characteristics of PW and CW welded joints in hot-stamped condition are studied comparatively.