Thermal Modeling of Resistance Spot Welding and Prediction of Weld Microstructure
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TRODUCTION
RESISTANCE spot welding (RSW) is one of the most prevalent welding processes in sheet metal joining and the automotive industry. The weld properties are affected by many parameters involved in the RSW process. Thus, many investigations have so far been carried out to examine these relationships.[1–4] It has been demonstrated that weld strength, impact resistance, and formability of the produced weld by the RSW process mainly depend on the nugget dimensions, i.e., diameter and penetration.[1,2] The mechanical properties of the produced joint also depend on the weld integrity and its microstructure.[1,2,5] It seems important to determine how operating conditions influence the preceding factors. Numerical simulation is the best alternative method to understand the RSW mechanism, helping to determine the parameters of the process so that the appropriate weld can be produced. Despite considerable investigations on numerical simulations of RSW,[6–13] the development of analytical models still requires close attention focused on this process. An analytical approach can help researchers gain a better understanding of the RSW
M. SHEIKHI, M. VALAEE TALE, and ARASH FATTAHALHOSSEINI are with the Department of Materials Engineering, BuAli Sina University, Hamedan 65178-38695, Iran. Contact e-mail: [email protected] GH.R. USEFIFAR is with the SAIPA Corporation, Tehran, Iran. Manuscript submitted February 8, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
process by finding the relations between the weld properties and the governing process parameters.[4,14] Gould et al. proposed a simple one-dimensional analytical model to predict the cooling rate of the RSW _ at process.[14] Based on this model, the cooling rate (T) any temperature (T) could be obtained as follows: 2 3 2 ap T T 4 T_ ¼ Tmax 5; Tmax 1 þ 2 kE h cos p x 4h2 p
kS
hE
2h
½1 where Tmax is the nugget peak temperature; h is the sheet thickness; kE and kS are the thermal conductivities of the electrode and sheet, respectively; and hE is the distance between the water-cooled surface of the electrode and the contacting electrode surface. Determining Tmax and ignoring the nugget dimensions are the most important problems associated with this model. The purpose of this survey was to put forward a more accurate thermal model to be used during the cooling step to obtain a deeper understanding of the systematic relation between the welding variables, the cooling rate, and the resultant weld microstructure. To evaluate the analytical model, a quarter of the symmetric model as the solution domain was analyzed by the finite element method. Due to the symmetry of the process, only a quarter of the welding zone was modeled and subsequently analyzed using the finite element method. To assess the calculations, the effect of the welding current in the spot resistance welding of carbon steel sheets was experimentally investigated.
Table I.
Chemical Composition of Used Steel Sheet (Weight Percent) Chemical Composition
C
Mn
Si
Ni
Cr
Fe
0.
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