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Finite element analysis of the limiting dome height (LDH) test has been conducted in order to determine the operant (or effective) friction coefficient pertaining to prephosphated galvanneal under punch stretching conditions. Simulation results indicate that an effective friction coefficient (μ) of about 0.05, which is substantially lower than the experimentally measured drawbead friction coefficient of 0.11, would be necessary to obtain the experimentally observed LDH formability enhancement with prephosphated galvanneal coating over a drawing quality steel substrate. The work also demonstrates that the effect of friction on dome height and minor strain at failure depends on specimen width and substrate strength level, with the relationship being quite nonlinear.

A two-stage finite element method has been used to evaluate the relative denting characteristics of four commercial grades of sheet steel (DQ, BABH, Rephos. IF, and HSLA-50) in a generic laboratory panel geometry. The first stage forming analysis is performed using LS-DYNA3D, a dynamic, nonlinear, explicit finite element analysis code, while the second stage denting analysis is performed using LS-NIKE3D, a large deformation implicit finite element code, in order to avoid dynamic effects. Material thinning and strain hardening during the first stage (forming) as well as bake hardening effects are explicitly accounted for in the subsequent denting analysis. Simulation results indicate that at a 400 N applied load level and a nominal sheet thickness of 0.76 mm, DQ exhibits the highest dent depth (0.86 mm), BABH and Rephos. IF exhibit similar behavior with a dent depth of approximately 0.5 mm, while no visible dent forms in the HSLA-50 grade for the specific panel geometry evaluated.

Galvanneal powdering was examined on a stabilized ultra low carbon steel substrate as a function of strain state using cup drawing and in-plane stretching experiments to simulate deformations encountered in production stampings. Significant powdering was encountered in drawing while minimal powdering occurred in in-plane stretching. Powdering was measured at specific locations and correlated with strains in those locations. A powdering map was generated in strain space using the experimental data. A few measurements of powdering on selected regions of an automotive stamped part are reported.

Surface friction is an important characteristic which influences the formability of sheet steel products. Numerous friction tests have been developed, and many previous investigations have reported effects of surface characteristics, coatings, lubrication, etc., on formability. Recently, increased attention has been focussed on reducing friction via the application of solid film lubricants or special surface post-treatments such as phosphates, metallics/intermetallics, etc. This paper presents the results of selected laboratory evaluations conducted using a variety of steels and surface treatments. Formability was measured using Limiting Dome Height and Drawbead Simulator friction testing, along with Limiting Draw Ratio testing in one instance. The examples highlight some potential opportunities which may be considered for improving formability in industrial stamping operations.