Laser Welding of Complex Phase and Dual Phase Advanced High Strength Steels - The Effects that Welding has on Microstructure and Formability
Abstract
To assist in the successful applications of tailor-welded assemblies made from advanced high
strength steel (AHSS), there needs to be a thorough understanding of how laser welding process
parameters influences the weld cross-section profile, mechanical properties, global formability and
local formability performance of the base metal. This study investigates microstructure
formability correlation of fiber laser welds of an un-coated complex phase (CP) AHSS and a hot
dipped galvannealed (HDGA) dual phase (DP) AHSS. Both steels were developed to have a
minimum tensile strength of 980 MPa, a minimum yield strength of 590 MPa and a minimum total
elongation of 12% in the material’s transverse direction - 90° to the material’s rolling direction.
Tensile tests, limiting dome height (LDH) tests, bi-axial stretch tests, forming-strain analyses, and
hole expansion tests (HET) were used to compare base metal (BM) samples to laser welded
samples.
The LDH and bi-axial stretch tests showed that, for both materials, the global formability of the
welded samples was lower than that of the base metal.
For the CP 980 steel, observations during global formability were correlated to the martensitic
dominant regions within the weld’s heat affected zone (HAZ). The welds resisted strain during
forming, forcing the surrounding material to accommodate for the restriction. The failure
propagated through the path which had the highest about of strain. For the CP steel, this path was
through the BM, perpendicular to the welds. Hole expansion tests (HET) showed that the welds
significantly decreased the local formability of the BM. Failure during HET initiated in, and
propagated along, the weld HAZ. This was correlated to the microstructures created in the HAZ
which were more sensitive to edge stretch failure as compared to the microstructure of the BM.
For the DP 980 steel, observations during global formability tests were correlated to the larger soft
region within the weld’s HAZ. During forming, strain localized in this zone and caused the
material to fail. The failure propagated along the length of the weld which was the path containing
the highest about of strain. HET showed that the welds only slightly decreased the local formability
of the BM. The failure during HET initiated, and propagated, in the BM rather than the weld. This
was correlated to the microstructures created in the HAZ which were less sensitive to edge stretch
failure as compared to the microstructure of the BM.
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Cite this version of the work
Christopher Kenneth Haines Martin-Root
(2020).
Laser Welding of Complex Phase and Dual Phase Advanced High Strength Steels - The Effects that Welding has on Microstructure and Formability. UWSpace.
http://hdl.handle.net/10012/15883
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