Research Status and Development Trend of Laser Welding of Press-Hardened Steel for Automobiles
Significance The automobile industry has a large volume,a high correlation,an extensive industrial chain,and a high consumption proportion.It is an important pillar industry that promotes the development of the national economy and is also a landmark industry that reflects the level of advanced manufacturing technology in a country.Owing to the continuous increase in the number and usage of automobiles,the depletion of natural resources,environmental pollution,and global warming have become increasingly prominent.To reduce fossil fuel consumption and greenhouse gas emissions,countries worldwide have established strict standards for vehicle fuel efficiency and carbon dioxide emissions.Vehicle lightweighting is one of the most direct and effective methods of saving energy and reducing emissions.For passenger cars,the body weight accounts for as much as 30％-40％of the total vehicle weight,and lightweight bodies can significantly reduce vehicle weight.However,to meet the increasingly stringent automobile crash standard,the collision safety performance of vehicles must be considered,and lightweight bodies have become a very important topic in the automotive industry.The strength of traditional high-strength steel parts is limited,and the manufacturing process is complex,making it difficult to meet the goals of lightweight automobiles and improve automobile safety performance.The development of hot-stamping technology has given rise to press-hardened steel,which offers good formability,high strength,and the ability to form complex-shaped parts.The application of press-hardened steel to a car body can significantly improve the safety performance of the vehicle and reduce the thickness of the steel plate,which is of great significance for lightweight vehicles.Laser welding technology has become the mainstream welding method for automobile body welding owing to its small heat input,small deformation,high precision,and high efficiency.Due to the thinness of press-hardened steel,typically between 0.8 and 3 mm,the automotive industry laser welds press-hardened steel of different strengths or thicknesses to produce customized performance automotive parts,also known as Tailor Welded Blanks(TWBs).Currently,there are two main methods for manufacturing hot-stamped parts in the automotive industry.One is direct laser welding of hot-stamped components;however,this method tempers the martensite in the heat-affected zone,resulting in softening of the heat-affected zone.The other is by initially laser welding blanks,followed by hot-stamping forming.This method prevents the softening of the heat-affected zone and has become the mainstream method for welding press-hardened steel.To prevent oxidation and decarburization of the sheet during the hot-stamping process and to improve the surface quality and forming accuracy of the sheet,the surface of the sheet is usually prefabricated.Owing to the presence of the coating,when laser welding is used,the components in the coating melt and diffuse into the weld,significantly deteriorating the mechanical properties of the welded joint.Progress The conventional coatings for press-hardened steel mainly include Al-Si,Zn-Fe(GA),Zn(GI),Zn-Ni,and other composite coatings.The advantages and disadvantages of different coatings vary(Table 1).The Al-Si coating is the most widely used coating for press-hardened steel because of its high temperature resistance,oxidation resistance,formability,and lower cost than Zn-Ni and other composite coatings.The compositions of the Al-Si coating and the base material differ significantly and are greatly affected by the laser welding process.However,a common problem persists in different laser welding processes.Al segregation occurs in the weld seam,and Al enrichment leads to ferrite formation.This is the fundamental reason for the significant decline in the mechanical properties of the welded joints,which limits the further application of press-hardened steel.Several studies have been conducted to solve the problem of Al segregation in laser-welded press-hardened steel caused by Al-Si coatings.At present,methods to suppress Al segregation in welds primarily include 1)removing the coating before welding,2)optimizing the welding process parameters,3)increasing the fluidity of the molten pool,4)modifying the Al-Si coating,5)adding an interlayer and powder,and 6)filling the welding wire.However,these methods have limitations,such as high cost,low efficiency,complex processes,and unsuitability for large-scale production.The"multi-in-one"press-hardened steel laser-filled wire welding technology,which does not require the removal of the Al-Si coating,was developed by the Yang Shanglu team of the Shanghai Institute of Optics and Fine Mechanics,Chinese Academy of Sciences.This technology does not require the coating to be removed before welding and only fills the"multi-in-one"special welding wire during the welding process,which can achieve efficient and high-quality welding of Al-Si-coated press-hardened steel above 1500 MPa grade.This method has wide applicability,low cost,and broad application prospects.Conclusions and Prospects In this study,the characteristics,applications,and laser welding methods of press-hardened steel are introduced in detail.The advantages and disadvantages of different coatings are summarized.The welding problems of press-hardened steel are noted.In addition,the improvement methods for laser welding defects in press-hardened steel are summarized,and the development trends of press-hardened steel and laser welding technology are discussed.Coating technology provides oxidation resistance in press-hardened steels;however,it also causes problems in laser welding.As press-hardened steel increases in strength and decreases in thickness,the chemical composition of the welding wire must be optimized,and in-depth research must be conducted on the microstructure evolution mechanism of the welding wire composition affecting the coating of press-hardened steel and the mechanical property strengthening mechanism of the joint to explore an efficient,intelligent,widely applicable,and robust laser welding method for lightweight automobiles.In the development of laser welding technology for press-hardened steel,it is also necessary to focus on quality inspection of the welding process to achieve adaptive parameter adjustment of the laser welding process and online quality monitoring after welding.
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