Metal deforming – With temperature modification of tool or of specified... – Of tool
Reexamination Certificate
2000-07-25
2003-04-22
Tolan, Ed (Department: 3725)
Metal deforming
With temperature modification of tool or of specified...
Of tool
C072S342700, C072S347000
Reexamination Certificate
active
06550302
ABSTRACT:
BACKGROUND OF THE INVENTION
The field of the invention pertains to sheet metal stamping and in particular to an apparatus and method to facilitate forming of metal. Material stretches more at a deformation or corner and becomes thinner thereat.
Aluminum is a brittle material, that is, alumninum is less ductile than other materials. In the past, a die was entirely heated or the sheet of material was entirely heated to facilitate flow during the stamping/molding process. An individual punch could also be heated.
Heating of certain metals to modest temperatures above room temperature can increase their strain to failure, and simultaneously increase their strain rate sensitivity. These characteristics produce favorable conditions for forming sheet metals, but strain localization at elevated temperatures can be intense due to a loss in their work hardening capacity thus minimizing strain uniformity in the part. Maintaining of spatial variation in temperature on mated die surfaces can allow flow of softened material from certain sections of the part to other regions to enhance the overall formability of sheets. It is however not clear as how to provide appropriate control of differential temperature in different regions of the die or how to construct these dies to avoid excessive heat loss, support of internally imbedded heating elements without heat equilibrium between different regions and provide the most desirable extent of metal flow.
Recently there has been a remarkable increase in the use of aluminum alloys in automotive industry, e.g. the shipment of aluminum to automotive market increased from 1.6 billion pounds in 1987 to 4.04 billion pounds in 1997. This increase is attributed not only to issues of energy-saving, but also to those of safety, resource conservation and environment friendliness. However, structural and body parts that rely on the formability of sheet metals, aluminum alloys are ranked far behind low carbon steels in automotive applications, despite their higher strength-to-weight ratio and excellent corrosion resistance. The limited use of aluminum alloys in the automotive industry is partly due to their poor formability at room temperature and thus, if warm forming at a rapid forming rate can be implemented in production, many of the goals related to lightweighting, energy and environmental friendliness can be realized.
Warm forming by deep drawing both rectangular and circular cups from annealed and hardened aluminum sheet alloys has been investigated in the past. Studies showed significant improvement in the drawability (in terms of cup height) at a relatively moderate temperature of about 150 degrees C. even for the precipitation hardened alloys (like 2024-T4 and 7075-T6). The drawability of these hardened alloys are better than the annealed alloys at room temperature, suggesting the possibility of drawing high strength aluminum alloys for structural parts at moderate elevated temperatures rather than drawing them in the annealed state and heat-treating after forming.
Forming speed (strain rate) effect in addition to temperature effect was observed with cup height increased with increasing forming temperature and/or decreasing punch speed for an Al—2Mg alloy. Punch stretching alloy 5182-O, required similar temperature and forming speed. Strains near the neck of the stretched part were more uniformly distributed at higher temperatures and slower punch speeds, implying increased strain rate sensitivity. By punch stretching the same alloy at a typical automotive strain rate of 1 sec-1, forming temperature had to exceed about 250 degrees C. to make improvements over the room temperature value, or, the punch speed had to be slow enough (about 10% of a typical automotive strain rate) to exhibit improved warm forming performance over that of AKDQ steel at room temperature. Moreover, plant trials of warm forming were conducted, forming alloy 5182-O at 120 degrees C. in General Motors proved successful in producing inner door panels and a V-6 oil pan at commercial press speeds, by heating both the die and the blank and using a mica lubricant and a MoSi2/graphite release agent. Cooperative investigations between Alcan and Chrysler tested various alloys, precipitation hardenable bumper alloys 7046-T6 and 7029-T6 to the strain hardenable alloys 5182-H14 and 5083-H14, were tested at elevated temperatures using heated blanks but unheated dies. It was found that some precipitation hardened alloys could also be warm formed successfully to produce components at 250 degrees C. at a cycling rate (~5 parts/min.). The optimum forming temperatures were found to be 200 degrees C. and 250 degrees C. for the precipitation hardened and the strain hardened alloys, respectively. These early trials act as an important database for today's advanced manufacturing and/or further exploration of warm forming potential of existing and new aluminum alloys.
The need for Fuel Savings and Structural Weight Reduction in vehicles is driving the replacement of Steel by Aluminum. But formability of Al alloys is half that of steels. This poses a major economic barrier to its application Goal: Formability of Al alloys must be improved under rapid manufacturing conditions (strain rate ~1-10 s−1). Technical Issues: Most Aluminum Alloys have the lowest formability at or near room temperature. At temperatures below room temperature, Strain Hardening Rate of Al Alloys is improved somewhat, but not enough. At Modestly Elevated Temperatures (200-350° C.), the Strain Rate Sensitivity and Forming Limit of Al alloys are improved significantly. L(derÕs Band and Surface Defects are eliminated by Warm Forming. Critical Questions:Warm formability drops with increasing Forming Rate. Can sufficient formability be achieved at high strain rate? Which alloys and micro structure will maximize warm formability and yet not degrade room temperature strength?
In uniaxial tension, total elongation generally increases with increasing temperature but decreases with increasing strain rate. Strain rate sensitivity increases with increasing temperature. Strain hardening index decreases with increasing temperature, indicating a softening effect. However, the warm forming as described above has been directed to warming of the blank and/or the entire die and not selective warming of certain segments of a die.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide selective heating to a die facilitate warm forming.
It is also an object of this invention to provide such selective heating to enable material flow into a end region from a flat region of a die.
It is a further object of this invention to provide such selective heating by using a heating block with a heating element positioned to heat the flat region of the die.
By taking advantage of improved material flow by selectively warming the die, flat sections of the die can contribute to the flow of material throughout the workpiece. Distribution of heating at the flat lower strain central regions outside of the bend region allows a softer flow at a lower stress to enable material flow into the thinner, higher strain areas at the bend/s.
Often die geometry poses restrictions on the easy flow of metal from one region of the part to another, thus leaving relatively unstretched regions of the part bounded by heavily stretched areas. The formability of the metal is poorly utilized due to the strain non-uniformity, and the propensity for fracture increases. This occurs because it is difficult to transmit stresses into certain regions of the sheet metal workpiece due to high frictional resistance or larger cross-sectional area in these regions, (as in the flange of a die). To encourage more plastic stretching in these regions, the local area needs to be softened, such as by raising the local temperature.
The formability of a sheet metal is a complex measure of its ability to accommodate the strains experienced in a forming process and to produce a part satisfying specific requirements of dimension, appearance and mechanics. Formability
Burns Barbara M.
The Regents of the University of Michigan
Tolan Ed
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