A Study on Microstructure and Properties of PHS Fiber Laser Welded Joints Obtained in Air Atmospheres

Abstract

Press-hardened steels (PHS) with a 1.5-mm-thick coated Al–Si layer is welded using an IPG YLS6000 continuous fiber laser in the air atmosphere. An SU5000 scanning electron microscope (SEM) and an Oxford EDS X-Max20 energy spectrometer are used to characterize the microstructure, which consists of delta (δ)-ferrite and lath martensite. It is similar to that of the welding performed in the Ar atmosphere, but the content of δ-ferrite is less. The reason is the formation of Al2O3 inclusions in the molten pool, which reacts with oxygen from the air ambient and the Al from the molten Al–Si coating of PHS. The oxygen content is measured with an ONH-3000 analyzer. An HV-1000 microhardness tester and DNS-100 universal material test equipment are performed to test the hardness and tensile strength. Similar hardness and strength of welded joints are achieved welding in the air atmosphere compared to that of the Ar atmosphere. Fracture was initialed in the fusion line of overlapping zone and propagated along the interface of two plates and fusion line due to the Al segregation.

Microstructural Characterization of Laser Weld of Hot-Stamped Al-Si Coated 22MnB5 and Modification of Weld Properties by Hybrid Welding

“…Increasing heat input usually increases the degree of softening in martensitic steel [ 16 ]. In our set of experiments, heat input variations were relatively low.…”

“…They monitored the welding process with a high-speed camera and studied the formation of oxides on the melt pool surface affecting the dilution of a melt pool, leading to the formation of Al-rich intermetallic phases. Sun et al [16] identified Al-rich regions as delta ferrite and proposed the use of Ni interlayer to reduce its fraction [17]. Lee et al [18] proposed high-carbon filler wire to prevent a brittle fracture of press-hardened laser welds of Al-Si coated 22MnB5.…”

“…Sun et al [ 16 ] identified Al-rich regions as delta ferrite and proposed the use of Ni interlayer to reduce its fraction [ 17 ]. Lee et al [ 18 ] proposed high-carbon filler wire to prevent a brittle fracture of press-hardened laser welds of Al-Si coated 22MnB5.…”

Performance Evaluation and Application of the Laser Welded Joint with Filler Wire for Al-Si Coated Press-Hardened Steels

“…Also they deemed that the decrease of the strength and ductility of the joints in the hot-stamped condition was due to the ferrite formation in the WM rooted at the serious Al segregation [18]. Sun et al [19] also considered that the soft α-ferrite produced in the WM could deteriorate the tensile strength of the joint.…”