N. Sreenivasan
Centre for Advanced Materials Joining,
University of Waterloo,
Waterloo, ON, N2K 4K8, Canada
M. Xia
Centre for Advanced Materials Joining,
University of Waterloo,
Waterloo, ON, N2K 4K8, Canada;
Central Iron and Steel Research Institute,
Beijing 100081, P.R.C.
S. Lawson
Y. Zhou
Centre for Advanced Materials Joining,
University of Waterloo,
Waterloo, ON, N2K 4K8, Canada
Effect of Laser Welding on
Formability of DP980 steel
Limiting dome height (LDH) tests were used to evaluate the formability of laser butt
welded blanks of the dual phase 980 steel in comparison with the base metal. Two
different lasers were used: diode and Nd:YAG, giving a wide range of welding thermal
cycles. A sharp decrease in LDH was observed in the welded specimens due to the
formation of a softened zone in the outer heat affected zone. Softened zone characteristics
were correlated with the LDH. Larger softened zones led to a larger reduction in the
LDH. The welding orientation relative to the rolling direction or to the punch surface did
not influence the formability, as the softened zone dominated the formability behavior. It
was observed that in both uniaxial and biaxial strain tests, the fracture occurred in the
softened zone of the welded samples consistently slightly farther out from the weld cen-
terline than in the location of the minimum hardness. DOI: 10.1115/1.2969246
Keywords: diode laser, Nd:YAG laser, formability, DP980 steel, softened HAZ
1 Introduction
Currently, many new types of advanced high strength steels
AHSSs, including dual phase DP steels, are being produced by
steel manufacturers that have higher strength and ductility, which
contribute to weight reduction in the automotive industries.
Most of the early weldability research on DP steels was focused
on the resistance spot weld RSW technique 1,2. Recent re-
search has dealt with laser welding of DP steels with focus on the
process development and optimization of the laser welding pro-
cess 3–5. Most commonly used lasers for welding are CO
2
and
Nd:YAG 6–8; however, the high power diode laser HPDL has
also been confirmed to be well suited for welding of butt and
lap-fillet joint geometries in the aluminum sheet for automotive
applications as well as for butt welds in the steel 9–11.
Formability aspects of these steels and tailor welded blanks
TWBs are also of interest and have been studied. Numerous
studies have confirmed that in formability experiments of TWB,
strain is concentrated and fracture occurs in the thinner/weaker
side of the blank 12–15. Further research has also suggested that
the welding and rolling direction of the material may influence the
formability 8,16. On the other hand, it is known that laser welds
in AHSS are harder/stronger than the base metal 17, and the
surrounding heat affected zone HAZ contains regions that may
be harder or softer 17. Therefore, some significant effects of
welding on formability would be expected.
The softened HAZ phenomenon has been observed in DP steels
with various welding processes such as arc, RSW, and laser 5.
The softened HAZ in the arc and resistance welded DP steels had
a significant influence on the formability 5. Laser welding cre-
ated a narrower softened zone with higher laser power and weld-
ing speed 6 kW, 7 m/min, in contrast to a wider softened zone
created with lower power and speed 3 kW, 4 m/min18.
An introductory study on the influence of welding phenomena
on the formability of high strength low alloy HSLA and DP980
steels was conducted at the authors’ laboratory 19. It was found
that formability was not affected by the laser welding process on
the HSLA material, whereas a decrease in formability was ob-
served on the DP980 steel. Uniaxial transverse tensile testing of
DP980 also showed a sharp decrease in maximum strain after
welding. Furthermore, the influence of softened outer HAZs of the
welds of the DP980 steel on the formability was observed.
This study focuses on the formability effects of the welds made
with two different power density types of laser on the DP 980
steel, spanning a wide range of welding thermal cycles. The in-
fluence of the softened zone on formability is examined. The ef-
fect of the welding speed on fracture and softened zone distances
from the weld centerline is studied. Furthermore, the relationship
between uniaxial and biaxial strains is examined.
2 Experimental Procedures
2.1 Material. The DP 980 steel of 1.17 mm thickness with a
coating described as galvannealed GA 48–55 g / m
2
with about
10% Fe was used in this study. The chemical and mechanical
properties are summarized in Tables 1 and 2. Since the full chemi-
cal composition of the steel was considered proprietary, a sum-
mary of the alloying is provided including carbon equivalent us-
ing the well known Yurioka formula 20. This DP steel has a
ferrite matrix with a significant volume fraction of fine martensite
laths.
2.2 Laser Properties. Diode and Nd:YAG are the two lasers
used in this work and their characteristics are shown in Table 3.
The Nuvonyx 4 kW diode laser was mounted on the manipulator
arm of a welding robot. Its laser beam is rectangular in shape,
with dimensions 12 0.5 mm
2
at the focal point. The energy
density/irradiance of the diode laser, which is less than
10
6
W / cm
2
, generates only a conduction mode of laser welding
21,22. Ultrahigh purity argon gas was supplied at a flow rate of
14 l/min as a shielding gas at the leading edge of the laser beam
spot on the top surface side of the welds. Welds were conducted in
the bead on a plate mode, i.e., a butt weld with full penetration on
the blanks of uniform thickness and material. Welding was con-
ducted on the blanks within the speed range of 0.7–1.9 m/min.
The welding speed of less than 0.7 m/min led to excess weld size
and sag; the speed above 1.9 m/min resulted in partial penetration.
The Haas HL3006D Nd:YAG laser employed fiber optic beam
delivery from a remote laser system to the final delivery optics
without any inert gas shielding. The full power of 3 kW was used
for the welding process. The energy density/irradiance of the
Nd:YAG laser, which is higher than 10
6
W / cm
2
, generates a key-
hole mode of laser welding 21,22. Operating speeds were used
Contributed by the Materials Division of ASME for publication in the JOURNAL OF
ENGINERING MATERIALS AND TECHNOLOGY. Manuscript received April 3, 2007; final
manuscript received October 17, 2007; published online September 9, 2008. Assoc.
Editor: Hamid Garmestani.
Journal of Engineering Materials and Technology OCTOBER 2008, Vol. 130 / 041004-1
Copyright © 2008 by ASME
Downloaded 27 Mar 2009 to 129.97.27.206. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm