This document summarizes research on developing robust spot welding processes for joining advanced high-strength steels used in automotive manufacturing. Spot welding tests were conducted on DP600 steel and HSLA 350 steel combinations. Different pulse types were used to widen the welding lobes and improve process flexibility. A novel two-pulse cycle removed zinc coating with the first pulse and controlled nugget growth with the second pulse at lower current, significantly increasing the welding lobe width. The research aims to enable increased use of advanced high-strength steels for building lighter, safer, and more fuel efficient vehicles.

1. Sheet Metal Welding Conference XI Sterling Heights, MI May 11-14, 2004
ROBUST SCHEDULES FOR SPOT WELDING ZINC-COATED
ADVANCED HIGH-STRENGTH AUTOMOTIVE STEELS
G.K.C. Tawade
Ryerson University
A. P. Lee and Gary Boudreau
Dofasco Inc. Canada
S. D. Bhole
Ryerson University
Abstract
Increased use of advanced high-strength steels (AHSS) is necessary for manufacturing safe,
affordable, and environmentally responsible vehicles in the future. Spot welding is still the
fastest and most affordable joining process for assembling these steels in vehicles. More
knowledge about spot weldability of these steels is necessary for their quick implementation.
Spot weldability is measured in terms of weld lobes and wider (robust) lobes represent better
weldability in production environments. The aim of this work is to develop a robust spot welding
process for joining AHSS. Spot welding tests were conducted on selected material
combinations currently used in a production vehicle. Welding lobes were plotted as per
Auto/Steel Partnership (A/SP) standard. Different types of welding pulses were used to
increase the width of the established weld lobes. Nugget growth study for each type of pulse
was conducted to understand the phenomenon of nugget growth in these steels.
A novel welding cycle was designed. This consists of two pulses with reduced current intensity
on the second pulse. It was found that the first pulse can effectively remove zinc and the
second pulse can control the nugget growth. Significant increase in the weld lobe was achieved
with this novel welding cycle.
Introduction
AHSS are multi-phase steels that contain appropriate amounts of martensite, bainite, ferrite,
and/or retained austenite to produce unique mechanical properties (through transformation
hardening). AHSS exhibit a combination of superior strength and good formability than
conventional high-strength steels and micro-alloyed steels.(1) The better combination of
formability and strength is due to their lower yield strength to ultimate tensile strength ratio.
Dual-phase (DP), transformation-induced plasticity, complex phase, and martensite steels are
some of the AHSS.(1) Dual-Phase 600 [(DP600) minimum UTS 600 MPa] has a soft ferrite
phase and a hard martensite phase. The former is generally continuous and gives excellent
ductility while the hard martensite phase gives strength.(1) AHSS offer improved crash
performance (passenger safety) and fuel economy for next-generation vehicles while being cost
competitive.(2) This is the principal reason why car manufacturers want to increase the use of
AHSS from current 4 to 43% (35% DP + 8% martensite steel) in the near future.(3)
More integration of body components is necessary for vehicle weight reduction which can be
achieved through spot welding different steel chemistry and sheet thicknesses. The spot
weldability of given material is determined with the welding lobe diagram. For a specific
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2. Sheet Metal Welding Conference XI Sterling Heights, MI May 11-14, 2004
material, the lobe diagram provides appropriate welding parameters to produce an acceptable
weld nugget diameter. Each automobile manufacturer may have different acceptance
standards (current ranges) for a specific AHSS. Steel suppliers may have a slightly different
chemistry for the specific AHSS grade. Developing a weld schedule with a large lobe width will
allow greater flexibility for a production-welding environment. The focus of the current work is to
develop robust (large) current ranges for welding DP600 to itself and other grades of HSLA
steels.
S. A. Gedeon(4) examined the weldability and electrode tip life previously. In this study weld
schedules, coating effects, and pulse design were examined. It was suggested that, up- and
down-sloping can increase the current ranges for welding steels which have free zinc in the
coating. Moreover, small variations in coating thickness do not significantly affect the current
ranges. Milititsky et al.(5) showed that multiple pulsing and higher electrode forces can increase
current ranges for DP600 steels. However, there was no discussion on the phenomenon
contributing to the wider current ranges. Issues related to heat-affected zone (HAZ) softening
and liquid metal embrittlement were addressed. Lobes with the C-gun and pedestal-welding
machine were compared. M. Kamura et al.(6) showed that strength of welded joints of DP800 is
not inferior to the conventional steels. Slight softening in the HAZ does not affect performance
of weld joints.
AHSS has richer chemistry than low-carbon or HSLA steels. Hardenability elements are usually
considered detrimental to resistance spot welding.(5) There is very little research available on
spot weldability of these newly developed AHSS. The present work was initiated to study the
weldability of DP600 and to increase the lobe width using various types of welding pulses.
Equipment
A 75-kVA AC, pedestal spot welder with a (Medar) Legend control unit was used to conduct the
spot welding trials. Power supply used was 60-Hz frequency. The secondary welding current
and time (cycles) were monitored using a Miyachi high-precision weld checker. It can measure
and monitor welding current and time at every half cycle. The electrode force was maintained
with a pneumatic force application system. A portable force gauge (piezo-electric type) was
used to measure the electrode pressure as and when necessary. It has an accuracy of ±2%.
Digital calipers were used for measuring the button diameter. The electrodes used were RWMA
Class II truncated cone 8.0-mm-diameter copper-chromium.
Figure1. 75-kVA AC Spot Welding Machine
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3. Sheet Metal Welding Conference XI Sterling Heights, MI May 11-14, 2004
Material
Spot welding trials were conducted on the following two material combinations commonly used
for production vehicles.
1. 2.0-mm DP600 welded to 2.0-mm DP600 (rocker welded to rocker).
2. 2.0-mm DP600 welded to 2.0-mm grade 350 HSLA (rocker welded to front rail).
Figure 2 shows the actual position of these combinations in a vehicle.
Front Rail
Rocker
Figure 2. Automobile Body Structure
All materials were hot-dipped galvanized (HDG) 60G/60G on either side, supplied by Dofasco
Inc., Hamilton, Canada. 60G has an average coating thickness of 0.025 mm.(7)
Table 1 shows the mechanical properties of the materials used in this study.
Table 1. Mechanical Properties of Materials
Thickness YS UTS TE(a) n(b)
Grade (mm) (MPa) (MPa) (%))
DP600 2.0 385 626 26.1 0.22
HSLA-350 2.0 350 454 33 0.18
(a) TE = total elongation
(b) n = strain-hardening coefficient.
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4. Sheet Metal Welding Conference XI Sterling Heights, MI May 11-14, 2004
Experimental Procedure
Sample preparation, electrode installation, electrode and weld stabilization, material
characterization, and selection of welding parameters were conducted according to A/SP
standard procedures.(8) Weld lobes were plotted for a given material combinations using A/SP-
recommended welding schedules. Peel test and panel coupons were prepared with rolling
direction considerations. All samples were prepared after coil edge removal and were
randomized with proper mixing. The following steps were followed to plot the lobe for each
material combination:
1. Electrodes were installed, aligned, and dressed.
2. Electrode tip imprints were recorded.
3. Electrodes were stabilized according to A/SP procedures.(8)
4. Tip imprints were taken after electrode stabilization.
5. Three different welding times were selected according to sheet thickness.(8)
6. Lobe was plotted for selected welding time.
7. Electrode tip imprints were taken.
8. Steps 6 and 7 were repeated twice more.
The final lobe diagram is thus the average of three lobes. Minimum and expulsion nugget
diameters were determined through peel tests.
Weld Button Criterion
• Minimum weld nugget size = 4 × t½(8)
o Where, t = average sheet thickness
• Maximum nugget diameter = nugget diameter at expulsion
Figure 3. Weld Button Criterion
Results and Discussion
Work from Milititsky et al.(5) confirmed that higher electrode force can result in wider current
ranges. In the present work, 20% higher electrode force than suggested by A/SP was used for
plotting the lobes. All other welding parameters were selected according to A/SP welding
schedules.(8) Three different welding lobes were plotted at welding times of 22, 26, and 30
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