I. Introduction
Hot-formed steel, as an advanced automotive steel material, has been widely used in the automotive industry in recent years. Formed through a hot stamping process, it significantly enhances the strength and safety of vehicle body structures while achieving vehicle lightweighting. Welding is a critical step in connecting hot-formed steel components, and its quality directly impacts the overall performance and reliability of the vehicle body. Therefore, in-depth research on the welding technology of hot-formed steel is of great significance for promoting the development of the automotive industry.
II. Characteristics of Hot-formed Steel
(1) Ultra-high Strength
During the hot stamping process, hot-formed steel undergoes a transformation from austenite to martensite, resulting in extremely high strength. Its tensile strength typically reaches 1500 MPa or higher, effectively improving the vehicle body's resistance to deformation in collisions and ensuring the safety of occupants.
(2) Good Dimensional Accuracy
Due to the high temperature and mold constraint during the hot forming process, hot-formed steel achieves good dimensional accuracy. The shaped parts have small deviations, facilitating subsequent assembly and welding work and improving the overall manufacturing precision of the vehicle.
(3) High Hardness
The martensitic structure of hot-formed steel imparts high hardness, which not only enhances the material's wear resistance but also improves the fatigue resistance of the parts to some extent. However, the high hardness also poses challenges for subsequent processing and welding.
(4) Relatively Poor Weldability
Compared with conventional steels, hot-formed steel has relatively poor weldability due to its chemical composition and microstructure. Welding issues such as cracks and joint softening are prone to occur, necessitating stricter control of the welding process.
III. Key Technical Points for Welding Hot-formed Steel
(1) Selection of Welding Materials
Welding materials that match the chemical composition and strength grade of the base material should be selected based on the composition and properties of hot-formed steel. For example, for hot-formed steels with higher carbon content, low-hydrogen welding materials with good crack resistance should be chosen to reduce the occurrence of welding cracks.
(2) Optimization of Welding Process Parameters
Precise control of welding current, voltage, welding speed, and other parameters is essential. Appropriate heat input is crucial; excessive heat input can lead to overheating of the joint structure, reducing strength and hardness, while insufficient heat input may result in incomplete penetration or poor weld formation. Optimal process parameters for different thicknesses and joint configurations need to be determined through extensive testing.
(3) Welding Sequence and Direction
The welding sequence and direction should be arranged reasonably to minimize welding stress and deformation. For complex hot-formed steel structural components, symmetric welding, segmented welding, and other methods should be employed to ensure uniform distribution of welding stress and avoid localized stress concentration.
IV. Main Welding Methods for Hot-formed Steel
(1) Resistance Welding
Technical Features: Resistance welding utilizes the resistive heat generated by the current passing through the contact points of the workpieces to heat them up, forming a welded joint under pressure. It offers advantages such as fast welding speed, high production efficiency, small welding deformation, ease of automation, and no need for filler material, effectively preserving the original properties of hot-formed steel.
Application Scope: Suitable for welding hot-formed steel sheets with a thickness of approximately 0.8 to 3 mm. Commonly used for connecting hot-formed steel components in automotive body manufacturing, such as spot welding and seam welding of door inner panels, body frames, and other parts.
Operating Points: Strict control of welding current, welding time, and electrode pressure is required. Adjust the welding current according to the hardness and thickness of the hot-formed steel to ensure sufficient heat for the workpieces to reach the welding temperature; the welding time should ensure adequate heating without overheating; the electrode pressure should be moderate, ensuring tight contact of the workpieces without causing surface damage or deformation due to excessive pressure. Additionally, electrodes should be cleaned regularly to prevent adherence of metal to the electrode surface, which can affect welding quality.
(2) Laser Welding
Technical Features: Laser welding employs a high-energy-density laser beam as the heat source, featuring high energy density, fast welding speed, narrow weld seams, small heat-affected zones, minimal welding deformation, and excellent joint performance. It minimizes the impact on the microstructure and properties of hot-formed steel, ensuring high strength and toughness of the welded joints.
Application Scope: Suitable for welding high-precision, high-quality hot-formed steel components, especially for the welding of thin and ultra-thin sheets (less than 1 mm). In automotive manufacturing, it is commonly used for butt welding and lap welding of hot-formed steel, such as welding of engine hoods, trunk lids, and other parts.
Operating Points: The equipment's precision and stability are crucial, ensuring the focusing accuracy and energy stability of the laser beam. The assembly precision of the workpieces is also strictly required, with tight control over gap and misalignment, typically keeping the gap within 0.1 mm and the misalignment no more than 10% of the sheet thickness. Additionally, protective measures during welding should be taken to prevent metal vapor from contaminating the laser optical path.
(3) Arc Welding (Exemplified by Mixed Gas Shielded Welding)
Technical Features: Mixed gas shielded welding typically uses a mixture of argon and carbon dioxide as the shielding medium, relying on the arc generated between the welding wire and the workpiece to melt the metal for welding. It offers advantages such as relatively low cost, adaptability to various welding processes, and the ability to perform welding in all positions.
Application Scope: Capable of welding hot-formed steel of various thicknesses, it is widely used for welding medium-thick plates (3 to 10 mm). It is commonly used in non-critical structural parts of automotive manufacturing or in repair applications.
Operating Points: Effective gas shielding is essential to prevent air intrusion into the weld. Reasonable selection of welding wire diameter, welding current, voltage, and gas flow rate parameters is necessary. Adjust parameters according to the thickness and welding position of the hot-formed steel to ensure good weld formation. Additionally, wind protection measures should be taken during welding, and welding should be suspended or protected if the wind speed is too high. Furthermore, the welding area should be thoroughly cleaned to remove impurities such as oil and rust to ensure welding quality.
V. Technical Challenges and Solutions for Hot-formed Steel
(1) Welding Cracks
Technical Challenge: Hot-formed steel is prone to cold cracks and hot cracks during welding. Cold cracks are mainly caused by hydrogen diffusion, welding stress, and the formation of hardened structures; hot cracks occur due to the presence of low-melting eutectics in the weld metal during solidification, under tensile stress.
Solution: For cold cracks, preheating before welding is recommended, with a preheating temperature generally between 100 and 200°C, adjusted according to the steel thickness and composition. Strictly control welding process parameters to reduce hydrogen sources, such as using low-hydrogen welding materials and drying welding materials. Post-weld heat treatment for hydrogen removal should be conducted promptly, with a temperature generally between 200 and 350°C, and the holding time determined based on the workpiece thickness. For hot cracks, adjust the welding material composition to reduce the content of low-melting eutectics in the weld; optimize welding process parameters to reduce welding stress, such as using a smaller welding current and faster welding speed.
(2) Joint Softening
Technical Challenge: In the heat-affected zone during welding, due to thermal cycling, the microstructure of hot-formed steel changes, leading to reduced strength and hardness, known as joint softening. This significantly affects the load-bearing capacity and overall performance of the welded joint.
Solution: Select appropriate welding methods, such as laser welding and electron beam welding, which have high energy densities and can effectively reduce the width and heat exposure of the heat-affected zone, minimizing joint softening. Simultaneously, optimize welding process parameters to control heat input and minimize the impact on the heat-affected zone's microstructure. For cases where joint softening has already occurred, post-weld heat treatment, such as tempering, can be used to restore some of the joint's properties.
(3) Porosity Issues
Technical Challenge: During welding, gas intrusion into the weld or gas generated by metallurgical reactions that fails to escape in time can form pores in the weld. The presence of pores reduces the density and strength of the weld, affecting the quality of the welded joint.
Solution: Ensure the welding materials are dry to avoid moisture absorption; enhance gas shielding during welding, select appropriate shielding gases and gas flow rates, and ensure effective gas shielding. Reasonably control welding process parameters such as welding current, voltage, and welding speed to allow sufficient time for gas to escape. In mixed gas shielded welding, attention should be paid to the purity and mixing ratio of the gases, and the gas supply system should be regularly inspected to prevent impurities from entering the gas. Simultaneously, the workpiece surface should be thoroughly cleaned to remove impurities such as oil and moisture, reducing the sources of gas.
The advancements in hot-formed steel welding technology are not limited to the automotive industry; they also exhibit enormous application potential in aerospace, machinery manufacturing, and other fields. By continuously overcoming technical challenges and optimizing welding processes, we will be able to fully leverage the performance advantages of hot-formed steel, providing a solid guarantee for the high-quality development of various industries. I believe that with the joint efforts of numerous researchers and technical workers, hot-formed steel welding technology will surely usher in a brighter future and contribute to the progress of the global manufacturing industry.
Contact Person: Christina Liu
Tel: 86 20 87813325 / 86 20 87819588 / 86 20 87815075
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