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Research Papers

Effect of Laser Welding on Formability of DP980 steel

[+] Author and Article Information
N. Sreenivasan, S. Lawson, Y. Zhou

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.

J. Eng. Mater. Technol 130(4), 041004 (Sep 09, 2008) (9 pages) doi:10.1115/1.2969246 History: Received April 03, 2007; Revised October 17, 2007; Published September 09, 2008

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 centerline than in the location of the minimum hardness.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic representation of the limiting dome height testing (23)

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Figure 2

Typical hardness profile: (a) diode at 1 m/min and (b) Nd:YAG welds at 3 m/min

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Figure 3

(a) Optical photo; cross section of a Nd:YAG weld at 6 m/min, (b) SEM photos of the base metal, and (c) tempered zone (M, martensite; F, ferrite; and TM; tempered martensite)

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Figure 4

(a) Optical photo; cross section of a diode weld at 0.7 m/min and (b) SEM photo of (a) tempered zone (M, martensite; F, ferrite; and TM, tempered martensite)

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Figure 5

Relationship between the soft zone width and the degree of softening to various diode laser welding speeds

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Figure 6

Tensile test specimen of the diode weld at 1.45 m/min: (a) before test, (b) after test, and (c) close up view of the necking and fracture region

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Figure 7

Representative results of engineering stress versus engineering strain of the uniaxial tensile tests

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Figure 8

Typical limiting dome height results of the base metal and diode welded specimens at 1.6 m/min welding speed

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Figure 9

Relation between formability ratio and welding speed for welds with both lasers

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Figure 10

Top view of the dome tested specimens: (a) base metal, (b) closer view of the fracture, (c) diode welded specimen at 1.3 m/min, (d) closer view of the fracture in (c), (e) Nd:YAG welded specimen at 6.0 m/min, and (f) closer view of the fracture in (e)

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Figure 11

Softened zone and fracture distance from the weld center for welds with both lasers

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Figure 12

Relationship between reduction in hardness and limiting dome height

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Figure 13

Relationship between formability ratio and base metal elongation of the same grade of steel with laser weld of same welding parameters (3 kW, 4 m/min)

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Figure 14

Relationship between biaxial and uniaxial strains and weld speed

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