Peng Chen
Department of Mechanical Engineering,
University of Michigan,
Ann Arbor, MI 48109-1340
Muammer Koç
1
NSF I/UCR Center for Precision Forming (CPF),
and Department of Mechanical Engineering,
Virginia Commonwealth University (VCU),
Richmond, VA 23298-0565
e-mail: mkoc@vcu.edu
Michael L. Wenner
Manufacturing Systems Research Laboratory,
R&D Center,
General Motors,
Warren, MI 48089-2544
Experimental Investigation of
Springback Variation in Forming
of High Strength Steels
The use of high strength steels (HSSs) in automotive body structures is a prominent
method of reducing vehicle weight as an alternative to use of aluminum and magnesium
alloys. However, parts made of HSSs demonstrate more springback than parts made of
mild steels do. Moreover, variations in the incoming material, friction, and other process
conditions cause variations in the springback characteristics, which prevent the practical
applicability of the springback prediction and compensation techniques. Consequently, it
leads to amplified variations and quality issues during assembly of the stamped compo-
nents. The objective of this study is to investigate and gain an understanding of the
variation of springback in the forming of HSSs. Two sets of experiments were conducted
to analyze the influence of the material property (dual-phase steels from different suppli-
ers), lubrication, and blank holder pressure on the springback variation. The experimen-
tal results showed that the variation in the incoming blank material is the most important
factor. In summary, the thicker the blank is, the less the springback variation. On the
other hand, blanks without a coating show less springback variation. The application of
lubricant helps us to reduce springback variation, although it actually increases the
springback itself. The more uniform the friction condition, the less the springback varia-
tion. The influence of blank holder pressure on the springback variation is not distin-
guishable from the system-level noise in our experiment. DOI: 10.1115/1.2951941
Keywords: high strength steels, HSS, springback variation, variation analysis, noncon-
tact measurement
1 Introduction
The use of high strength steel HSS in automotive body and
structural applications offers unique opportunities for reducing the
vehicle weight, enhancing the crash performance, and decreasing
the cost when compared to other lightweight materials such as
aluminum and magnesium alloys. However, wide application of
HSS in many potential body and structural parts is still limited
due to challenges mainly in their limited formability, increased
springback, difficulties in joining, and reduced die life issues. The
concerns about springback and quality control grow among auto-
makers and steel industry as the use of high strength–low weight
materials increases 1.
Springback and its variation are major concerns with the use of
HSS as they lead to more springback compared to mild steel
grades. The degree of springback correlates with the yield strength
level after forming rather than the yield strength in the flat sheet
and the tensile strength. Furthermore, variation of springback pre-
vents the applicability and use of well-known and developed
springback prediction and compensation techniques. As a result, it
leads to amplified variations and problems during assembly of
components, and in turn, results in quality issues. Thus, in-depth
understanding, accurate characterization, prediction, control, and
reduction of springback variation present itself as one of the vital
research topics in this area to achieve methods of decreasing the
development times and reducing scrap rate in mass production to
achieve cost effective fabrication of HSS parts.
Springback behavior of materials has long been studied for over
40 years. An early study by Baba and Tozawa 2 investigated the
effect of stretching a sheet by a tensile force, during or after bend-
ing, in minimizing springback. Other studies investigated the role
of process variables on springback. Zhang and Zee 3 showed the
influence of blank holder force, elastic modulus, strain hardening
exponent, blank thickness, and yield strength on the magnitude of
the final springback strain in a part. Geng and Wagoner 4 studied
the effects of plastic anisotropy and its evolution in springback.
Some researchers investigated springback by computer simula-
tions 5–9. For more detailed background information, readers
are referred to Refs. 10–13.
Most of the research efforts on springback focused on the ac-
curate prediction and compensation of springback. The issue of
springback variation was seldom investigated. As lead times are
shortened and materials of high strength–low weight are used in
manufacturing, a fundamental understanding of the springback
variation has become essential for accurate and rapid design of
tooling and processes in the early design stage. As far as the
variation analysis is concerned, most available references are re-
lated to the assembly processes 14–18.
The objective of this study is to investigate the effects of varia-
tions in material mechanical properties and process blank
holder force and friction on the springback variation for an open-
channel shaped part made of dual-phase DP steel. Open channel
was chosen since many autobody components have a similar
shape. The variations in stamping process are explained in Sec. 2.
In Sec. 3, Phase I experiments with HSS material from different
suppliers and their results are discussed after describing the ex-
perimental methodology and measurement techniques used. In
Sec. 4, Phase II experiment and results are discussed. In this case,
HSS materials from a single supplier but different batches were
tested. Section 5 presents a discussion of the results and conclu-
sions.
2 Variations in Stamping Process
In this study, the objective is to investigate the variation of
springback in an open-channel drawing considering the variations
of material and process. As shown in Fig. 1, the variation of
1
Corresponding author.
Manuscript received February 6, 2007; final manuscript received December 3,
2007; published online July 10, 2008. Review conducted by Zhongqin Lin.
Journal of Manufacturing Science and Engineering AUGUST 2008, Vol. 130 / 041006-1
Copyright © 2008 by ASME
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