Page 13 - MetalForming-Dec-2018-issue

Alternative Forming Methods for Aluminum Alloys
Formability of material at room temperature and the speed at which deformation occurs determine the degree of complexity at which aluminum alloys become
elaborate shapes using traditional stamping dies.
To help overcome the limitations of temperature and speed, there are alternative forming methods such as stretch forming, super-plastic forming, warm forming and high- energy-rate explosive forming. The characteristics of each
are presented here.
pulled by pneumatic or hydraulic force to stretch the sheet (Fig. 1). Tooling consists of a solid contoured piece, known as a stretch-form block, against which the sheetmetal presses. As the form die drives into the sheet, the tensile forces increase until the sheet plastically deforms into its new shape.
The two basic types of stretch-forming equipment—lon- gitudinal and transverse—stretch the workpiece along its length (longitudinal) and width (transverse). The presses ensure accuracy and efficiency, while CNC controls help provide part-to-part repeatability.
Superplastic Forming
Some alloys slowly stretch well beyond their normal lim- itations at elevated temperatures. For these, there’s super- plastic forming (SPF), a process conducted at high temper- ature and under controlled strain rate to give a tenfold increase in elongation compared to conventional room- temperature processes.
The process begins by placing the sheetmetal in an appro- priate SPF die, representative of the final part to be produced (Fig. 2). The sheet and tooling are heated and gas pressure (usually argon) applied to plastically deform the sheet into the shape of the die cavity. The pressure profiles must be closely controlled during the process, since alloys of interest exhibit SPF behavior only within specific temperatures ranges and strain rates. SPF processes produce parts that are impossible to form using conventional techniques. How- ever, only specific alloys with the fine-grained microstructure and the strain rate sensitivity (m-value) necessary for SPF processes can be used.
Warm Forming
Some studies show a significant increase in formability of 5XXX and 6XXX alloys with warm forming (T. Altan, G. Daehn, Ohio State University). The process makes possible the forming of complex sheet products that cannot be manufactured at room temperature, and at a lower cost
Fig. 2—Super-plastic forming
Fig. 1—Stretch forming
Stretch Forming
Used by aircraft manufacturers to build fuselage skin sections, stretch forming stretches and bends a piece of sheetmetal simultaneously to form a large contoured shape. Benefits include the absence of surface marring, distortions and ripples, and the accurate alignment of complex profiles.
Performed on a stretch press, gripping jaws attached to a carriage secure the sheetmetal along its edges and are
Peter Ulintz has worked in the metal stamping and tool and die industry since 1978. His background includes tool and die making, tool engineering, process design, engineering management and advanced product development. As an educator and technical presenter, Peter speaks at PMA national seminars, regional roundtables, international confer- ences, and college and university programs. He also provides onsite training and consultations to the met- alforming industry.
Peter Ulintz
Technical Director, PMA pulintz@pma.org
Tooling by Design
By Peter Ulintz
Gripping jaws
Form die
Sheetmetal
Carriage
Press table Ram
Press platen
Sheet- metal blank
Tool cavity
Heater tubes
Clamping force
Cell is slowly pressurized
Gas
Cell heated to superplastic temperature of material
Sheetmetal conforms accurately to tool cavity
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