The Role of Internal Oxides on the Liquid Metal Embrittlement Cracking During Resistance Spot Welding of the Dual Phase
- PDF / 6,302,167 Bytes
- 12 Pages / 593.972 x 792 pts Page_size
- 117 Downloads / 194 Views
NTRODUCTION
IN recent years, advanced high-strength steels (AHSSs) have been developed to improve crash safety performance as well as to reduce fuel consumption and greenhouse gases emission in automobiles. Dual phase (DP) steels whose microstructures consist of a ferrite matrix with dispersed martensite (or bainite) are one of the most common types of AHSSs.[1–4] It is reported that more than 20 grades of AHSSs such as DP, transformation-induced plasticity (TRIP), twinning-induced plasticity (TWIP), and quenching and partitioning (Q&P) steels are commercially available, of which more than 30 pct are DP steels.[4,5] To maintain the integrity of the vehicle structures, protection of AHSSs from corrosion is required, typically with Zn. In a conventional hot-dip galvanizing process, DP steels are cleaned, annealed intercritically (between Ac1 and Ac3 critical temperatures), and followed by immersion into a molten Zn bath. However, with conventional annealing atmospheres, some elements such as Mn, Si, and Al are A. GHATEI KALASHAMI, C. DIGIOVANNI, M.H. RAZMPOOSH, and N.Y. ZHOU are with the Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada. Contact e-mail: [email protected] F. GOODWIN is with the International Zinc Association, Durham, NC 27713. Manuscript submitted November 26, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
selectively oxidized.[6] The presence of these surface oxides prior to immersion in the Zn (Al, Fe) molten galvanizing bath decrease reactive wetting of the Zn onto the steel surface, resulting in bare spot defects and unacceptable coating quality.[7,8] To reduce the selective external oxidation, the annealing dew point is increased.[9,10] As a result, the partial oxygen pressure of the furnace atmosphere is increased and internal oxides are formed rather than external ones that improved the reactive wetting.[7,11] Although Zn coating is preferable in terms of corrosion protection, it can result in liquid metal embrittlement (LME) cracking in further manufacturing processes such as hot-forming and welding. LME is an intergranular brittle fracture of ductile metals in simultaneous presence of certain low melting point liquid metals and tensile stresses.[12] LME crack formation has been reported in various high-temperature processes such as hot-press forming,[13,14] laser welding,[15,16] and resistance spot welding (RWS) of Zn-coated AHSSs. Furthermore, LME during RSW process is reported in numerous studies in various grades of AHSSs such as twinning-induced plasticity (TWIP),[17–21] transformation-induced plasticity (TRIP),[22–26] quench and partitioning (Q&P),[27] and DP steels.[28,29] Despite much work in this area, an exact mechanism which explains LME crack initiation and growth during the RSW process is not yet clear. Ashiri et al.[18] found that the LME cracks would be located near the contacted area
between the electrodes and steel sheets due to the presence of higher temperature and tensile stresses
Data Loading...
