R&D Update: Warm-forming magnesium sheet, Part III

Warm-forming techniques for magnesium and aluminum are based on partial heating or cooling of the tools. Heating and cooling have dramatic effect on the material’s formability, which is quantified in terms of the limiting draw ratio (LDR), the ratio of the maximum blank diameter to the punch diameter (D0/ Dp).

LDR and Temperature

During warm deep drawing, the LDR increase because of higher sheet temperature in the flange area and deceased as the punch temperature increases.

Deep-drawing experiments at the University of Hannover, Germany, using round-cup tooling found that LDR decreases approximately 10 percent for magnesium—and aluminum— alloy cups when the punch is 200 degrees C compared to a punch at room temperature (see Figure 1 ).

Aluminum Research

Defects associated with warm forming of aluminum include fracture and wrinkling. Figure 2a shows early fracture because of insufficient flange temperature and higher blank holder force. The sheet temperature at the flange was not high enough to facilitate a draw into the die cavity. In deep-drawing applications, blank holder force must be high enough to prevent wrinkling (see Figure 2b ).

Excessive blank holder force and insufficient punch cooling cause early part fracture (see Figure 2c ) because the drawing stress the cup wall can withstand decreases with temperature. Therefore, a higher LDR can be obtained with warm deep drawing as compared to conventional deep drawing at room temperature (see Figure 2d ).

When Li and Ghosh warm-formed rectangular cups using various aluminum alloys, they found that A15754, A15182, and A16114-T4 exhibit significant improvements in formability at temperatures ranging from 200 degrees C to 350 degrees C. They also concluded that warm forming in a range of 200 degrees C to 350 degrees C may not cause a drastic loss in a part’s strength level. A part formed at 350 degrees C maintains its strength at a level comparable to that hot-rolled tempers.

Magnesium Results

Minimum bending radii for annealed and hardened magnesium sheet such as AZ31B-O and AZ31B-H24 are shown in Figure 3 . Bendability of an annealed (-O) magnesium alloy is higher (5.5 X t0) than a hardened alloy (-H) (8 x t0 ) at room temperature. Also, the minimum bending radius decreases with increasing temperature. Surprisingly, at elevated temperatures of approximately 260 degrees C, the minimum bending radius becomes 2 X t0 for annealed and hardened alloys. This shows that heat treatment is not important at elevated temperatures to achieve a minimum bending radius.

Lightweight materials such as aluminum and magnesium are more in demand today to save weight. However, forming of these lightweight alloys is quite limited at room temperature. Therefore, warm forming becomes important and has to be considered as a system that takes into account material properties, tool design, control, and saftey.