Ultrahigh strength steels are frequently used within the automotive industry. The driving force for use of these materials is to exchange thicker gauges to thinner and lighter structures. To get excellent strength and beneficial crash performance, the steel is microalloyed with boron which contributes to the 1500 MPa tensile strength. Increasing the carbon content will give superior tensile strength up to 2000 MPa. Welding of these components is traditionally done by resistance spot welding, but to get further productivity and increased stiffness of the structure, laser welding can be introduced. Welding of boron alloyed high strength steel is in general a stable and controlled process, but if increasing the carbon content quality issues such as cracking could possibly be a problem. In the present study, weldability of two different hardened boron steels with tensile strengths of 1800 and 1900 MPa, respectively, has been evaluated. Laser welding has been done in a lap joint configuration with 3.8–4.7 kW and varying welding speed between 3.5 and 5.5 m/min. As reference, results from more conventional 1500 MPa boron steel have been compared to 1800 and 1900 MPa boron steels to show the influence of the carbon content. Metallographic investigation, hot crack test, cold crack test, shear tensile, and cross-tension strength tests have been done. The results show that a weld quality similar to that for conventional boron steel can be achieved. Cracking and other defects can be avoided. As expected when welding martensitic steels, the failure mode in tensile testing is brittle. No weld defects have been found that influence strength. The sheet interface weld width, which together with stack-up thickness correlates with strength of the joint, could be increased by increasing the heat input and defocusing the laser beam. The effect of increased carbon content on weldability will be discussed more in detail, as well as the risk of cracking.
Pores in the weld metal lower the mechanical properties of the weld. It is therefore important to understand the pore formation mechanisms and find procedures that could reduce porosity. This study focused on laser welding of 3 mm thick magnesium alloy AM50, investigating how different parameters affect porosity formation. Low levels of porosity content were achieved by either increasing the welding speed or using a two-pass welding approach. It was found that higher welding speeds did not allow pores, which were pre-existing from the die-casting process, to have sufficient time to coalesce and expand. In the two-pass welding technique, pores were removed as a result of a degassing process which occurred through the second pass.
Ultra high strength steels are frequently used within the automotive industry for several components. Welding of these components is traditionally done by resistance spot welding, but to get further productivity and increased strength, laser welding has been introduced in the past decades. Fusion welding is known to cause distortions due to built in stresses in the material. The distortions result in geometrical issues during assembly which become the origin of low joint quality due to gaps and misfits. U-beam structures of boron steel simulating B-pillars have been welded with laser along the flanges. Welding parameters and clamping have been varied to create different welding sequences and heat input generating a range of distortion levels. The distortions have been recorded dynamically with an optical measurement system during welding. In addition, final distortions have been measured by a digital Vernier caliper. The combined measurements give the possibility to evaluate development, occurrence, and magnitude of distortions with high accuracy. Furthermore, section cuts have been analyzed to assess joint geometry and metallurgy. The results show that final distortions appear in the range of 0–8 mm. Distortions occur mainly transversely and vertically along the profile. Variations in heat input show clear correlation with the magnitude of distortions and level of joint quality. A higher heat input in general generates a higher level of distortion with the same clamping conditions. Section cuts show that weld width and penetration are significantly affected by welding heat input. The present study identifies parameters which significantly influence the magnitude and distribution of distortions. Also, effective measures to minimize distortions and maintain or improve joint quality have been proposed. Finally, transient finite element (FE) simulations have been presented which show the behavior of the profiles during the welding and unclamping process.
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