| Abstract Scope |
Increasing environmental and safety challenges have brought with them a need to reduce automotive CO2 emissions while improving occupant safety. Greenhouse gas reduction is partially accomplished by thinning the materials used to make vehicles. However, to increase crash performance, these thinner materials are required to withstand higher loads and absorb more energy during crash than past materials. Steelmakers have responded to this challenge by designing multiphase steels that exhibit much higher strain hardening capabilities than past steels, which have been coined 3rd generation advanced high strength steels (3G AHSS). These 3G AHSS exhibit very high strength and elongation, which are beneficial for forming and crash, however these performance improvements have come at the cost of increased alloying and microstructural complexity. Both factors affect steel weldability and weld performance. Specifically, two phenomena have been seen when welding these steels: Liquid metal embrittlement (LME), and a carbon depleted zone at the fusion boundary. This work seeks to explain the material factors responsible for these phenomena, and how the welding process may be modified to minimize or eliminate them though an examination of these phenomena in various steels and under various spot welding conditions.
The chemical compositions of all steels used had high C (>0.20 wt%), Mn (>2.0 wt%) and Si (>1.0 wt%) contents. The steels were examined microstructurally using both optical and scanning electron microscopy to understand the base microstructure, which was a combination of ferrite, martensite, tempered martensite and retained austenite. Then the steels were all welded using an MFDC spot welder, followed by shear and cross-tension strength testing and analysis of the post-welded microstructure.
The excellent combination of strength and ductility of these steels derives from the multiphase nature. During loading, the retained austenite within the structure transforms into strain induced martensite, which leads to increased dislocation density at the ferrite/martensite grain boundaries enabling an addition source of strengthening aside from typical dislocation generation that occurs in the ferrite grain bodies. The high levels of retained austenite necessary for this strengthening mechanism requires that the material undergoes a heat treatment where the martensite is formed and then reheated to stabilize the surrounding austenite phase so that it is stable at ambient temperatures. However, this also requires high amounts of Si to prevent carbide formation and Mn to ensure stability of the austenite.
When welded using typical welding parameters, modelling results have shown that high tensile thermal stresses build in the shoulder and electrode indent due to thermal contraction associated with electrode collapse and electrode withdrawal. These thermal stresses are very high due to the high yield strength of these materials, which stresses the structure. Although this stress is insufficient to cause cracking, grain boundaries are weakened by two factors: Zn penetration from the melted galvanized coating and the high Si, which decreases grain boundary cohesion. The combination of the high thermal tensile stresses, the liquid Zn and the lower strength grain boundaries leads to LME cracking. However, by pulsing the welding current the hot strength of the material may be maintained preventing electrode collapse. As well, by ramping current at the end of the electrode cycle, the intent area is allowed to slowly cool preventing at thermal shock when the electrodes are withdrawn. Both of these techniques have been used to decrease LME crack severity during welding of 3G AHSS.
The high carbon in these steels shown to be affect the local chemistry in the HAZ at the fusion boundary. It is known that the welding lobe of AHSS may be widened by increasing welding time. Increasing welding time slows nugget growth in these highly resistive steels, increasing the current at which expulsion occurs. However, it has been seen that when these high C steels are welded, a white halo may be seen surrounding the nugget on the cross-sectioned and etched weld. This halo has been measured to have reduced hardness and reduced carbon content compared to the surrounding material. When the weld is pulled in tension, fracture preferentially follows this local soft zone, decreasing the fracture energy of the joint. Using thermodynamical modelling, it has been shown that the halo is the result of uphill diffusion of C from the solid HAZ to the molten nugget as the nugget growth slows late in the welding cycle. Therefore, the halo may be reduced in one of two ways. Either welding time can be designed so that the ends before nugget growth ends, eliminating halo growth. Alternatively, a short higher current final pulse may be added to the welding cycle to grow the nugget into the halo before current is terminated.
The current study examined how the chemical and mechanical properties of 3G AHSS interact with the welding process to result in two welding defects: LME cracking and the low C halo at the fusion boundary. It was seen that both phenomena are rooted in how the material changes during the welding process and how the chemical composition, unique to 3G AHSS, contributes to these phenomena. However, it was also shown that an understanding of how these defects form may be used to decrease or eliminate them. It was seen that reducing thermal shock by preventing electrode collapse and the large change in cooling rate associated with electrode withdrawal through current pulsing and slow cooling could decrease LME severity. Furthermore, it was seen how designing the welding cycle to terminate before nugget growth finishes by quickly growing the nugget a small amount at the end of the welding cycle could eliminate the halo. Both examples show how a firm understanding the metallurgy of the welded material and the welding process can be used to better design welding parameters, resulting in the best possible weld properties.
Keywords: 3G Advanced High Strength Steel, Resistance Spot Welding (RSW), Halo, crack propagation, post-weld properties. |