Formability in Sheet metal design

What is the meaning of the term formability in the context of sheet metal design ? Why is it important while designing sheet metal components ?

When designing sheet metal components there are two major types of processes

Bending
Forming

While Bending is like folding paper, forming is a lot more complex but also opens up a lot more design possibilities for design engineers.

Forming cannot be done on all type of metal sheets . There are some material grades which are more formable than others.

This formability is a vital factor in design of stamping parts like Automotive doors, roofs, chassis brackets etc. Good design with a formable material results in a durable design of a sheet metal assembly maintaining structural integrity .

Formability is basically how intricately can we form or shape the sheet metal into desired shapes without the material fracturing .

Key factors which affect formability

Material Composition: Different metals and alloys exhibit varying degrees of formability. For instance, aluminum and soft steels are generally more formable than harder steels or titanium alloys.
Thickness: The thickness of the sheet metal plays a significant role. Thinner sheets are typically more formable than thicker ones.
Temperature: The formability of some metals can be enhanced by increasing the temperature during the forming process, a technique known as hot forming.
Grain Structure: The microstructure of the metal, including the size and orientation of its grains, can impact its formability.

Measuring Formability: Formability is often quantified using tests like the tensile test, the bend test, and the Erichsen cupping test. These tests help in understanding how the metal behaves under different types of strain and stress.

Importance in Design and Manufacturing: Understanding formability is crucial for designing sheet metal parts. It helps in selecting the right material and forming processes, which in turn affects the tooling design, the production speed, and the cost-effectiveness of the manufacturing process. Additionally, good formability reduces the risk of defects such as cracks, wrinkles, or tears, ensuring high-quality end products.

Forming tests are carried out as shown

The strains are then measured and plotted .

Due to deformation and stretching the thickness of the sheet reduces locally . While in some areas material accumulates and there is thickening . Both extremes of excessive thinning and thickening are not desirable . Thinning being more critical in terms of load bearing applications.

Forming limit diagram showing Minor strains and Major strains.

Forming limit diagrams are characterized for each sheet metal material grade and the design engineer selects materials which are suitable for good formability . The decision of good formability or bad formability is based on the Forming limit diagrams.

Using software tools like “ One step formability” we can get a basic idea of the thinning of material in forming

Forming simulation of a simple cup showing the thinned area in red.

With available forming Limit diagram from the manufacturer , the major and minor strains obtained from a forming simulation are plotted on the graph

Whether they lie within the safe forming zone? Will there be tearing?
Whether there is no excessive thinning?
Whether there will be wrinkling tendency ?

Categories: Sheet metal