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Determination of Weld Line Characteristics in Tailored Blanks
Abstract
The use of tailored blanks by the automotive industry is a fact. New applications that use tailored blanks and improvements on existing uses are continuously being created. In order to get the most out of all the advantages of tailored blanks in the quickest and most cost effective manner an understanding of the weld and its behaviour under forming conditions is required. The purpose of this paper is to review existing formability tests for tailored blanks and to summarize current research at Queen's University toward the development of the methods that focus on the characteristics of the weld line.
A new test for sheet metal formability was designed, constructed, and used to evaluate several coated and uncoated sheet materials.
Results from the OSU Formability Test were also compared with standard limiting dome height (LDH) tests and with finite element
simulations. These results show that the new test is more reproducible, even using relatively uncontrolled equipment, more
closely follows the desirable plane-strain path, and takes roughly one fifth of the time to perform relative to LDH. Moreover,
there is good correlation between formability evaluated using the two tests. Strain measurements and finite element simulations
revealed that the improvements are a result of the new test geometry, which produces more stable and repeatable plane-strain
states near the fracture location.
The use of laser-welded blanks results in low stamping costs, assembly costs and material costs for automotive companies. It also makes automobile structures lighter and stronger. NKK studied the formability of laser-welded blanks made from cold-rolled steel sheets of various thicknesses, strengths and carbon equivalents. The formability of laser-welded blanks was correlated to parameters derived from base steel data such as the chemical composition, mechanical properties and thickness. The parameters described allow the formability of a blank to be predicted from base steel data.
The introduction of tailored welded blanks offers many potential benefits to both the steel production and automotive industries. However, there are difficulties associated with the pressing of such blanks, stemming from the industry's lack of experience concerning the forming characteristics of the weld and its effect on the surrounding material. This research programme has focused on laser welded steel samples of thicknesses typically used in the automotive industry. A modified Erichsen cupping test was used to deform the laser welded samples to fracture, and subsequent analysis of the surface strains around the weld and the fracture site was used to compare the formability of the samples. An assessment was also made of the effect of experimental accuracy in the cupping tests on the observed strain conditions. The most prominent influence on formability was found to be the sheet rolling direction and not, as expected, the presence of the laser weld or the weld route through the sample. Furthermore, an unpredicted result was obtained when the orientation of the laser weld was changed relative to the movement of the punch.
The drive towards reduced car body weights and improved safety could be met by the introduction of tailored welded blanks at the design stage. A tailored welded blank is a single panel formed by welding two or more different materials together to form a single composite sheet, which is then subsequently pressed. The materials within a single panel can have different properties, such as high strength or high corrosion resistance, and may even be of different thicknesses. The problems associated with the production of such panels come mainly at the stamping stage, in the form of weld or parent material failure. Work has begun with small scale cupping tests of laser welded samples to establish the forming behaviour of the weld. Forming limit diagrams have been constructed for several welded and unwelded material combinations to establish the limiting strain conditions during a forming operation
A simple simulative test was developed to evaluate the stamping formability of steel sheets in plane strain stretching deformation.
The stamping formability was evaluated by the limiting punch height (LPH) value in the plane strain punch stretching (PSS)
test compared to the minimum of the limiting dome height (LDHo) value in the hemispherical punch stretching test, the standard
LDH test. The PSS test shows a stable plane strain deformation and a good reproducibility with less scattering data. Moreover,
the LPH value in the PSS test ranks well the stamping formability of various sheet materials and shows good correlations with
press performance.
Beginning in 1992, tailor-welded blanks (TWBs) were used in the United States automotive industry to consolidate parts, reduce
tolerances, save weight, and increase stiffness. This business is expanding rapidly; more than $500 million of annual TWB
sales are expected by 1997. Welds in steel are generally stronger than the base material, such that weld failure by preferential
localization is not a critical issue. However, the forming characteristics of TWBs must be understood in order to design and
produce high-quality parts with reasonable production and tooling costs. Three formability issues were addressed in this study:
the intrinsic ductility and relative formability of three weld types (CO2 and Nd:YAG laser welds and mash-seam welds with and without mechanical postweld processing); the value and correspondence
of mechanical tests to each other and to press performance; and the prediction of the forming behavior using the finite element
method (FEM). Two failure modes for TWBs were identified. While the local ductility of welds can differ greatly, little difference
in press formability was measured among the weld types. More important than weld ductility are the changed deformation patterns
which depend on the differential strength but depend little on local weld prop-erties. Finite element method (FEM) simulations
of dome tests and scale fender-forming operations show good agreement with measurements, as long as boundary conditions are
known accurately. The importance of weld-line displacement is discussed and several simulations are compared with ex-periments.
A number of tests have been proposed as predictors of press shop formability. The most common of these are the Olsen cup,
tensile, and hardness tests. However, none of these tests are very successful. Recently, A.K. Ghosh proposed the Limiting
Dome Height test (LDH) to predict formability since this test modifies specimens to produce the same strain state at failure
as found in a particular stamping. This report describes a correlation study in which specimens were collected from lots of
steel and aluminum which exhibited either good or bad press performance in automotive stampings. The failure mode, in all
cases, was splitting which initiated within ± 2 pct of plane strain. These specimens were evaluated by the LDH, Olsen, and
tensile tests and compared to the actual press performance. Only the LDH test adequately separated the acceptable sheets from
the bad.
A simple technique to generate in-plane forming limit curves has been developed. This technique is based on the Marciniak
biaxial stretch test using a single punch/die configuration, but the specimen and washer geometries have been modified in
order to achieve failure in both drawing and stretching deformation modes. The experimental technique is described, and the
advantages of using this inplane method over the conventional out-of-plane dome method are discussed. It is shown that (a)
sheet thickness has an intrinsic influence on forming limits that is not related to small bending strain variations with thickness
or to deformation in the presence of friction and curvature, (b) plastic anisotropy (
[`(r)]\bar r
value) does not substantially affect forming limits, and (c) in-plane forming limits are slightly lower (5 to 6 pct) than
out-of-plane forming limits near plane strain; these differences are smaller than previously reported values (12 to 15 pct)
in the literature.
Two tests for sheet forming applications have recently been developed at the Ohio State University (OSU). The developments involved numerous experiments with several materials, comparison with results from previous tests, and finite element simulation to optimize and verify the geometries. The OSU Formability Test is more reproducible than the Limiting Dome Height (LDH) test, it more closely follows the desirable plane-strain path, and it takes roughly to of the time to perform. There is good correlation between formability evaluated using the two tests. The OSU Friction Test is a new technique for measuring the friction coefficient over the punch during sheet forming operations. It more closely simulates punch friction conditions in terms of rates and increasing wrap angle. Results show that punch friction depends on the angle of wrap, which varies with punch stroke, and on strain rate, which depends on punch velocity. The two tests will be described and typical results will be presented for several sheet materials.