Elliot Biro

Effects of Heat Input and Martensite on HAZ Softening in Laser Welding of Dual Phase Steels

M. Xia, E. Biro, Z. Tian, Y.N. Zhou
Laser welds were made in three dual-phase (DP) alloys with ultimate tensile strengths ranging from 450–980 MPa and varying microstructures to investigate effects of heat input on heat affected zone (HAZ) softening. To compare the total heat transferred into the HAZ of all the welds, heat input was normalized using the Rosenthal Equation. It was found that HAZ softening experienced in a DP steel was a function of both martensite content and heat input. Maximum HAZ softening was proportional to the martensite content, and the heat input controlled the completion of softening. Material softening was normalized by martensite content, which showed that the contribution of martensite to material hardness from the three materials is the same; however the materials had different transformation kinetics.

Spot Welding Different Sheet Metal Grades and Gauges

E. Biro, L. Cretteur, T. Dupuy
In order to design lighter automotive structures, to improve fuel economies and carbon dioxide (CO2) emissions, the use of new advanced high-strength steels (AHSS) is expanding, allowing weight savings through thinner sheet gauges. Spot welding remains the main joining process for body-inwhite construction. Most spot welds in automotive structures are dissimilar configurations (i.e., different sheet thicknesses and grades are welded together), but AHSS-to-AHSS configurations have become more common.

Softening Kinetics in the Subcritical Heat-Affected Zone of Dual-Phase Steel Welds

E. Biro, J.R. McDermid, J.D. Embury, and Y. Zhou
Welds in dual-phase (DP) steels exhibit heat-affected zone (HAZ) softening in which the tempered or subcritical HAZ exhibits a lower hardness vs that of the parent material. The rate of this softening reaction with respect to welding heat input was determined for four DP steels by making several bead-on-plate laser welds using a variety of heat inputs and measuring the resulting minimum hardness. The reduction in hardness was then fit to the Avrami equation, enabling a comparison of the relative heat needed to soften each steel. It was found that the heat input required for HAZ softening decreased as the C content of the martensite within the DP structure increased. However, the presence of carbide forming alloying elements such as Cr and Mo was able to increase resistance to softening.

Decoupling of the softening processes during rapid tempering of a martensitic steel

E. Biro, J.R. McDermid, S. Vignier, and Y. Zhou
The increased adoption of martensite-containing advanced high strength steels, such as martensitic and dual-phase steels, into automotive applications has led to concerns among practitioners with respect to softening during rapid tempering cycles such as those experienced during laser welding. Past studies on rapid tempering have successfully modeled the rapid tempering process; however, the activation energies and softening rates calculated did not match the classic literature values associated with martensite tempering. The present study examined rapid tempering data for a martensitic steel and separated the softening process into two stages: carbide nucleation and carbide coarsening or growth. The activation energies calculated for each process were found to be consistent with classic literature values for diffusion controlled nucleation and growth of carbides during martensite tempering.

Predicting Transient Softening in the Sub-Critical Heat-Affected Zone of Dual-Phase and Martensitic Steel Welds

E. Biro, S. Vignier, C. Kaczynski, E. Lucas, J.R. McDermid, J.D. Embury, and Y.N Zhou
To improve vehicle fuel economy and crash worthiness the automotive industry has been redesigning parts from advanced high strength steels such as dual-phase and martensitic steels. These steels have high strengths with the higher formability characteristics when compared to lower strength conventional steels of similar ductility. These steels derive their unique properties from their complex microstructures containing ferrite and martensite. During assembly welding, the martensite within the sub-critical region of the heat-affected zone tempers, which locally reduces mechanical properties. Although this phenomenon is well studied, it has yet to be quantified. The present work proposes a technique to measure the softening kinetics of dual-phase and martensite steels using rapid isothermal tempering. The resulting model was then validated by predicting the heat-affect zone softening that occurs in laser and resistance spot welds as well as by comparing the microstructures of the rapid tempered samples to the microstructures found in the sub-critical heat-affected zone of welded samples.

Pulsed Nd:YAG laser welding of copper using oxygenated assist gases

E. Biro, D.C. Weckman, Y. Zhou, and K.J. Ely
The effects of using oxygenated assist gases on the weldability and weld properties of Nd:YAG, pulsed laser welds in copper (Cu) have been evaluated. It was found that the effective absorptivity of the Cu increased as the oxygen content of the Ar assist gas was increased. This facilitated laser welding of Cu at much lower laser powers and increased weld penetration. The use of oxygenated assist gas promoted nucleation and growth of submicroscopic oxide particles within the weld metal. These particles dispersion-strengthened the weld metal, thereby increasing both weld metal hardness and strength. However, when O2 concentrations in the assist gas were greater than 90 pct, weld metal
embrittlement due to excessive volume fractions of oxides was observed. The use of oxygenated assist gas also led to excessive cold lapping and poor bead quality. The bead quality was improved, however, by ramping-down the laser power before terminating each pulse.

The effects of Ni and Au/Ni platings on laser welding of thin sheets

E. Biro, Y. Zhou, D.C. Weckman, and K.J. Ely
The effects of Ni and Au/Ni plating on laser seam welding of 200 mm thick aluminum, nickel, Kovar, and cold-rolled steel sheet in the lap-joint configuration has been studied. Seam welds were made using a pulsed Nd:YAG laser and a range of weld process conditions. The strength of the welds was characterized using tensile shear tests and all welds were examined metallographically. All material combinations were found to be weldable. The platings were not found to affect the range of process conditions that produced acceptable welds in the nickel, Kovar, and steel specimens. However, the Ni and Au/Ni platings reduced the power density required to form a joint in the aluminum specimens due to the higher absorptivity of Ni. In all cases, the power density which resulted in blowthrough was unaffected by the platings. The strength of the majority of the
joints was equivalent to that of the annealed base material. The aluminum weld metal hardness was increased fourfold by alloying from the Ni plating and Au/Ni plating, but this did not affect the tensile shear strength of the joint because failure of the Ni and Au/Ni-plated Al specimens always occurred in the unalloyed softer heat affected zone. Not all of the higher melting point Ni and Au/Ni plating was melted during welding of the Al specimens and significant gas porosity was observed at the interface between the Al weld pool and the unmelted Ni and Au/Ni plating layers. However, this did not affect the tensile shear strengths of the Al welds. A Au/Ni braze joint was observed at the sheet interface adjacent to the fusion boundary of the Au/Ni-plated Ni, Kovar, and cold-rolled steel specimens. This did not affect the strength of the Kovar and cold-rolled steel specimens; however, the Au/Ni braze increased the strength of the Au/Ni-plated Ni specimens to that of the base material.

Impact of liquid metal embrittlement cracks on resistance spot weld static strength

C. DiGiovanni, E. Biro, and N.Y. Zhou
Advanced high strength steels used in automotive structural components are commonly protected using zinc coatings. However, the steel/zinc system creates the potential for liquid metal embrittlement (LME) during welding. Although LME cracks are known to form, limited research has found any detrimental impact of LME cracks on weld strength. In this work, a comparison of zinc coated and uncoated advanced high strength steel joints showed LME decreased strength in welds from transformation induced plasticity type microstructures and an 1100 MPa ultimate tensile strength by 43.6%. LME cracks were observed to propagate until final fracture. However, only cracks located in the periphery of the weld area were found contribute to a loss in strength.

Experimental and Numerical Analysis of Liquid Metal Embrittlement Crack Location

C. DiGiovanni, X. Han, A. Powell, E. Biro and N.Y. Zhou
Advanced high-strength steels used in automotive structural components are commonly protected using zinc coatings. However, the steel/zinc system creates the potential for liquid metal embrittlement during welding. Although liquid metal embrittlement cracks are known to form, the current literature does not include crack location when assessing crack severity. In this work, TRIP1100 joints showed that LME cracks decreased strength from 7.8 to 42.2%, depending on location, between the coated (cracked) and uncoated (non-cracked) condition. Liquid metal embrittlement cracks in critical locations were observed to propagate until fracture from lap shear testing. However, cracks in non-critically loaded areas were not part of the fracture path and did not result in a significant loss in strength. This shows LME crack location can be controlled to improve joint performance and vehicle safety. In addition, a model of lap shear testing in a cracked sample showed how the presence of a crack can affect the internal stress field depending on its location.

Reduction in liquid metal embrittlement cracking using weld current ramping

C. DiGiovanni, S. Bag, C. Mehling, K.W. Choi, A. Macwan, E. Biro, and N.Y. Zhou
Advanced high-strength steels (AHSS) used in automotive structural components are commonly protected from corrosion using zinc coatings. However, the steel/zinc system creates the potential for liquid metal embrittlement (LME) during welding. Although adjustments to the welding process have been studied, the present literature has not examined the use of variable input currents for LME reduction. In this work, a ramp down welding current showed LME severity decreased by 60% compared to a standard constant current. Reductions in both crack size and number of cracks were observed for high currents (Imax + 20%). A numerical model of the resistance spot welding process was constructed, which showed when welding with a ramp down welding current, the electrode-to-sheet interface spends less time at an LME-sensitive temperature compared to the standard welding current. Furthermore, it avoids a large jump in tensile stress when the electrodes are removed.

Effects of Electrode Degradation on Electrode Life in Resistance Spot Welding of Aluminum Alloy 5182

S. Fukumoto, I. Lum, E. Biro, D.R. Boomer, and Y. Zhou
Electrode endurance tests were conducted to investigate the effects of electrode degradation on electrode life in resistance spot welding of 1.5-mm-thick sheet aluminum Alloy 5182 using a medium-frequency direct-current welding machine and electrodes with tip-face diameter of 10 mm and radius of curvature of 50 mm. The observed electrode life ranged from about 400 to 900 welds even though all the process conditions were intentionally kept constant. However, despite the large variation, distinct patterns were found to correlate electrode life to electrode degradation in terms of the change in nominal electrode tip-face area and contact areas at both electrode/sheet (E/S) and sheet/sheet (S/S) interfaces. The reduction in joint strength occurred because of undersized nugget formation due to increased contact areas and hence reduced current density. The electrode degradation may be monitored by the increase in all three areas (nominal tip-face area, and E/S and S/S contact areas), but the E/S contact area is believed to be the most suitable because a minimum of extra work is needed to measure it. The button diameter, measured from peel testing, is affected by nugget diameter (current density) and possiblyother factors, such as weld expulsion and porosity distribution as well.

Advanced Characterization of HAZ Softening of AHSS for Crash Modeling

H. Ghassemi-Armaki, E. Biro, and S. Sadagopan
Advanced characterization of the mechanical properties in the softened HAZ is vital for designing welded AHSS components, as it is needed to predict post-weld performance, such as during crash. The weld heat affected zone (HAZ) of advanced high strength steels (AHSS) containing significant volume fractions of martensite exhibit considerable softening because of tempering effects. HAZ softening of two AHSS; Usibor 1500 (press hardening steel) and M1500 with nominal ultimate tensile strength (UTS) of 1500 MPa has been characterized in this study. Samples were subjected to various rapid tempering cycles using the Gleeble to simulate the microstructures found in different locations of the sub-critical HAZ. Mechanical properties of simulated weld areas have been measured with different geometries, representing different levels of stress triaxiality. Furthermore, Digital Image Correlation (DIC), to determine fracture strain of HAZ, measured the fracture strain of a spot weld and base metal for Usibor 1500. Uniaxial tensile and notch sample geometries were used to investigate the influence of different strain path. The results show fracture strain is a weak function of strain path for samples that fail in the HAZ, whereas fracture strain of the base material considerably depends on strain path.

Suppression of liquid metal embrittlement in resistance spot welding of TRIP steel

L. He, C. DiGiovanni, X. Han, C. Mehling, E. Wintjes, E. Biro, and N.Y. Zhou
Third-generation advanced high strength steels are typically given a zinc coating that provides excellent resistance to corrosion. During the resistance spot welding process, the melted zinc coating enables liquid metal embrittlement (LME) that causes cracking in the weld indent. In this study, LME in TRIP 1100 and TRIP 1200 steels was suppressed by placing aluminium interlayers added between the electrode and steel contact surface. Compared to welds exhibiting LME, TRIP 1100 with aluminium interlayers showed complete strength recovery while TRIP 1200 with aluminium interlayers resulted in a recovery of strength by 90%. Aluminium interlayers suppress LME by the formation of iron aluminides that hinder liquid zinc from coming in contact with the steel substrate, thus preventing LME.

Study and Applications of Dynamic Resistance Profiles During Resistance Spot Welding of Coated Hot-Stamping Steels

O. L.-R. Ighodaro, E. Biro, and Y.N Zhou
This work compares the role of press hardened steel coating type (Al-Si and GA) on resistance spot welding by analyzing the dynamic resistance curves measured during the weld cycles of the respective materials. It was seen that the dynamic resistance profiles for GA- and Al-Si-coated steels are similar. But the GA specimens exhibited higher resistance than Al-Si-coated specimens in the as-received condition, while the Al-Si-coated specimens exhibited higher resistance after hot stamping. From the early stages of the dynamic resistance profiles, data were obtained and applied for computing the values of components of resistances associated with the different coatings since each coating exhibits characteristic value at the early stages. The results revealed that at the start of the welding cycle, the resistance of the electrode/sheet interface was significantly higher than that of the faying surface or the bulk resistance regardless of whether the steel was Al-Si- or GA-coated. The possible uses of these resistance values in studying welding current requirement and electrode tip life were discussed.

Effects of weld line position and geometry on the formability of laser welded high strength low alloy and dual-phase steel blanks

J. Li, S.S. Nayak, E. Biro, S.K. Panda, F. Goodwin, and Y. Zhou
Formability of laser welded blanks (LWBs) was measured in the biaxial stretch forming mode using the limiting dome height (LDH) test. High strength low alloy steel and two dual-phase (DP600 and DP980) steels were used for fabricating LWBs. The failure location and the LDH values of the formed blanks were correlated to the hardness across the welds. The effects of weld line position and geometry on the formality was evaluated by investigating LWBs with three different weld line positions (0, 15 and 30 mm offsets from the blank center) for both linear and curvilinear geometry. The formability was found to be dependent on the weld line position and increased when the weld was located farther from the blank center due to more uniform strain developed during LDH tests. Interestingly, weld line geometry was observed to have a stronger influence on the formability DP600 steel. In addition to weld line position and geometry, heat affected zone softening was observed to be the dominant factor in controlling the formability of all the DP980 LWBs and the curvilinear welds of DP600 with failure consistently occurring in the region with more severe softening.

Electrode pitting in resistance spot welding of aluminum alloy 5182

I. Lum, E. Biro, Y. Zhou, S. Fukumoto, and D.R. Boomer
Electrode pitting was investigated in resistance spot welding of 1.5-mm-thick sheet aluminum alloy 5182 using a medium-frequency direct-current welder and electrodes with a tip face curvature radius of 50 mm and tip face diameter of 10 mm. Detailed investigation of the metallurgical interactions between the copper electrode and aluminum alloy sheet was carried out using scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX) and X-ray diffraction (XRD). The results indicated that electrode degradation, which eventually leads to weld failure, proceeded in four basic steps: aluminum pickup, electrode alloying with aluminum, electrode tip face pitting, and cavitation. Since pitting and cavitation result from Al pickup and alloying, periodic electrode cleaning could extend electrode tip life by limiting the buildup of Al on the tip face. This work is part of the effort to improve electrode tip life in resistance spot welding of aluminum alloys for automotive applications.

Expulsion monitoring in spot welded advanced high strength automotive steels

C. Ma, S.D. Bhole, D.L. Chen, A. Lee, E. Biro, G. Boudreau
Although there have been a number of investigations on monitoring and controlling the resistance spot welding (RSW) of low carbon galvanised steels, those of advanced high strength steels (AHSS) are limited. A data acquisition system was designed for monitoring weld expulsion via the measurement of voltage, current, electrode force and displacement and the calculation of resistance. The dynamic resistance, electrode force and tip displacement were characterised and correlated with the phenomenon of expulsion during RSW of dual phase (DP) steel using an ac welder. Two control strategies for DP600 spot welding were proposed on the basis of the rate of change in the dynamic resistance and the electrode force.