Metal stamping of work hardenable aluminum alloy sheets
Work hardenable aluminum alloy sheet material is progressively stamped to a final stamped shape in a succession of metal stamping operations. A blank sheet of annealed starting material is stamped into a least a first preform shape between heated metal stamping tools to form a preform shape and subsequently anneal the worked preform shape. After one or more annealed preform shapes have been progressively attained, the last preform shape is cooled, if necessary, and stamped to its final stamped shape between unheated metal stamping tools.
Automotive body panels and other sheet metal products have been made of suitable steel alloys by metal stamping processes at ambient temperatures. The edges of a steel sheet blank are gripped by a binder mechanism and a punch pushes and draws the metal against a generally concave forming surface. Often a steel alloy is available that is suitably formable and the metal is stretched into a complex shape, such as a body panel, without tearing, wrinkling or otherwise marring the sheet. A progressive sequence of metal stamping and, for example, piercing and trimming operations may be successively performed on a single steel sheet to make a panel with each step being completed in a matter of seconds.
Aluminum sheet alloys would be substituted for steel in many applications to save weight. For example, some Aluminum Association alloys of the 5xxx and 6xxx series have been used in sheet metal stamping operations. But such aluminum metal stamping alloys are not as ductile and formable as steel alloys and the aluminum sheet often tears if it is stamped to the same shape. The aluminum alloy work-hardens at metal stamping strain rates and some portion of the sheet yields and tears. This property of aluminum alloys has limited the product shapes to which they can be formed by high production rate metal stamping. Complex panel shapes often have to be made in multiple pieces and welded together. This usually results in higher manufacturing cost and may complicate dimensional control of the composite product.
It is an object of this invention to provide a method for using work-hardenable, magnesium containing aluminum sheet alloys in relatively high production rate metal stamping and annealing operations to form one-piece products whose stamped shape includes areas of deformation that exceed the strain limits of the starting sheet material.
A method is provided for forming a stamped configuration in a sheet of work hardenable aluminum alloy material where the sheet is drawn and/or stretched by a punch tool closing into conformance with a complementary metal stamping tool surface. The punch tool used in sheet metal metal stamping typically has convex or protruding surface regions and the opposing metal stamping tool surface has complementary concave or valley-like regions into which metal must be stretched to achieve the desired shape. The method is useful where the desired stamped configuration contains regions of elongated and deformed metal that would be torn or otherwise damaged if the sheet metal were stretched to form the local shape in a single closure of unheated metal stamping tools.
An aluminum sheet metal alloy is selected for the metal stamping of a particular part, the material having known tensile-strain properties in its cold rolled and tempered condition preparatory for metal stamping. A sheet metal blank of suitable area profile and thickness is specified for the physical requirements of the part. The capability of the blank to be stamped into the desired product shape using punch/concave cavity type cooling is assessed. Such assessment may be conducted experimentally and/or by a suitable computational model using, for example, a finite element analysis method. If it is found that the proposed part cannot be made in a metal stamping operation without tearing or wrinkling the sheet material, the following metal stamping/anneal process may be used.
The strategy of the practice of this invention is to stamp a work hardenable aluminum alloy sheet blank into a preform shape that embodies a substantial portion of the deformation required to acquire the final stamped shape of the part without marring the aluminum sheet and without excessive thinning of the sheet. In some instances a second, more severely formed preform shape may be required before a final stamped shape may be attained.
In accordance with this invention, a first set of metal stamping tools are prepared, comprising a punch tool and an opposing complementary concave metal stamping surface tool, for stretching and/or drawing the aluminum sheet blank between and against the tools into a first predetermined preform shape. The opposing tools, supported and actuated by a metal stamping press, open and close along a common axis. They open for placement of a sheet metal workpiece and close on the interposed sheet stretching and/or drawing the sheet metal between their facing surfaces to deform the sheet material into conformance. The tools are preferably made of unitary metal blocks (steel or like tool material) with provision for internal heating. The internal heating system is constructed and arranged to heat the opposing forming surfaces of the metal stamping tools to temperatures at which they can heat, by thermal conduction, a stamped sheet metal workpiece to its annealing temperature. The annealing takes place just as the metal has been deformed and is still confined between the opposing metal stamping tools.
The function of the metal stamping tools for a first predetermined preform shape is to close on a blank of the sheet metal and, thus, immediately stretch the metal into the preform shape. Typically, the preform shape is formed in a second or two. Some regions of the initially soft sheet material will be extensively deformed (elongated). Such regions will be work hardened and resist further easy elongation. But the preform shape is retained for a brief annealing period (a period, for example, of five to ten seconds after the metal stamping tools are closed) between the hot faces of the metal stamping tools.
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