A large number of tests have been used in an effort to measure or predict the formability of sheet materials. Many of these have been criticized because of cost,
complexity, difficulty in the analysis of data, lack of correlation between laboratory results and field forming performance, etc. Some of these problems may be
overcome when test procedures have been standardized and a better understanding of the mechanics of the tests is achieved.
There will always be a need for formability tests. The effect of composition and processing modifications on formability must be determined during the alloy
development, preferably without resorting to expensive field forming trials in the initial stages. Tests are often needed in the analysis of field forming
problems requiring the comparison of problem lots to a data base. Tests are also needed for quality assurance, especially since it appears that many sheet
users are working toward the use of test results as acceptance criteria.
Part of the lack of correlation between laboratory test results and field forming performance is due to a misuse of the test results. This lack of correlation
leads to a lack of faith in the test procedures. If such a correlation is to be expected, the strain state of the test must match the strain state in which
the failure occurred in the field. One should not expect the results of a drawability test to correlate with field failures which occurred in plane strain tension.
This is because microstructural features respond differently to different states of stress, which has been demonstrated for both precipitation and dispersion
strengthened aluminum sheet alloys.
The standard material for the manufacture of automobile outer body panels has been, for many years, low-carbon steel. Throughout these years, aluminum alloys
have also been used for limited production applications.
In the current weight savings effort, interest in aluminum has increased, especially for overhang members beyond the wheel base area, such as bumpers,
engine-compartment hoods, and luggage-compartment deck lids. The direct substitution of aluminum sheets for low-carbon steel sheets in high-production
press operations, however, presents problems in maintaining shape to the exact standards required by the manufacturer and his customer, the automobile purchaser.
A trial production run of a 2036-T4 aluminum alloy in forming a rear deck lid resulted in three lots furnished by separate suppliers, being classified as
good, marginal, and poor. Although such variable performance is not unusual, these three materials were chosen as subjects for an evaluative study of the
forming properties and test parameters considered relevant to quality control in the production of aluminum for exposed automotive sheet metal applications.
This was the subject of a cooperative program carried out by members of the American Deep Drawing Research Group (ADDRG). As an adjunct to this study,
sheet metal formability tests, using the same equipment and operators, were simultaneously conducted on standard production lots of low-carbon steels.
This was done to minimize the scatter of results when comparisons are made after introducing variables of material, equipment, test procedures, operator,
and interpretation of results, as occurs in more general cooperative programs.
Aluminum alloy 2036-T4 is a member of the 2000 series, in which copper is the principal alloying element. These alloys require solution heat treatment to
attain optimum properties (T4 designation) comparable with mild steel. They are not as corrosion resistant as other aluminum alloys, being rated C in a
scale of A (best) to D (poor). The T4 designation indicates the coils have been given a solution heat treatment and then naturally aged to a substantially
stable condition. Surface strain (Luders lines) are less a problem with the 2036 alloy than the 2024 aluminum widely used as an aircraft alloy: Weldability of
2036-T4 is rated B in a scale of A (best) to D (poor). The maximum tensile strength of the sheet product is 345 MPa (49 ksi).
As part of the ADDRG cooperative program, 18 sheets of 1-mm (0.040-in.)-thick 2036-T4 aluminum was cut from 1283-mm (50.5-in.)-wide coils of the three lots
in lengths of 572 mm (22.5 in.). Six sheets of each lot were supplied to each testing laboratory, to test:
- Chemical analysis
- Standard mechanical properties (Yield Strength, Ultimate Tensile Strength, Elongation, Hardness)
- Metallographic microstructure.
Plastic deformation of sheet metals occurs in the load-deformation range between the yield and ultimate, the plastic flow range. Within this range there is a
balance between strengthening and flowing such that first an area flows and in so doing is cold worked and strengthened. Adjacent areas may be weaker and subject
to flow at lower loads than those necessary to continue deformation in the strained area. Subsequent strain is transferred to those adjacent weaker areas.
This balance, or distribution of strain, continues until the part form is completed, or the material breaks under the applied stress.
The strain hardening capacity for deformation loads below the ultimate strength, within the area of uniform elongation, is constant for most low-carbon steels.
Beyond the uniform elongation region, diffuse necking occurs, followed by localized neck-down and tearing apart of the metal when the cross section strength
relationship cannot support further loading.
In numerical modeling of sheet metal forming operations, material hardening data, as commonly represented by the so-called effective stress-effective strain relation,
are required. Such data are traditionally obtained via the uniaxial tension test. The useful range of strain attainable in this test, however, is limited because
of the occurrence of localized necking in the test specimen which terminates the state of uniform deformation.
To represent hardening behavior at higher strains, the conventional engineering approach is to fit the uniaxial tensile data by a preselected empirical equation
and then extrapolate the equation to high strain values. A number of such equations have been proposed in the literature. Among them, the one most often used is
the classical power law expression proposed by Hollomon. Since, for a given set of tensile data, more than one empirical equation can be made to fit, a natural
question to ask is which one is appropriate for forming calculations. While the physical nature of the hardening behavior of real materials with respect to strain
state, strain rate, grain size, texture, and temperature is a subject of continuing research, this series of tests investigates the effects of different hardening
representations in several standard sheet metal forming calculations. It is hoped that the quantitative information obtained in this way will be useful to both
analysts and experimentalists in their search for appropriate hardening representations for real materials.
As mentioned above, the material investigated is 2036-T4 aluminum, which is being used in the fabrication of automotive sheet metal parts. This material has little
rate sensitivity at room temperature and is almost isotropic in the plane of the sheet. Two hardening models, namely a power law model and a saturation stress model,
were used to represent its hardening behavior. The saturation stress model initially due to Voce was chosen as it appears to be in better accord with recently obtained
hydraulic bulge data.
The forming calculations carried out in this work were:
(a) a uniaxial tension test,
(b) a hydraulic bulge test,
(c) hemispherical punch stretching, and
(d) cup drawing with a hemispherical headed punch.
For the uniaxial tension test, a finite strain, large displacement elastic-plastic finite element procedure, derived from a general procedure for sheet metal forming
operations, was employed. This procedure has been applied to the calculation of in-plane deformation of 2036-T4 aluminum.
For the other three cases, an axisymmetric version of the general finite element procedure was employed.