Hot Forming of Al-Mg Alloy Sheet

At this time, when fossil-fuel conservation is all-important, it is appropriate to consider the increased use of alloy sheet for the construction of lighter car bodies.

Highly ductile pure aluminum is not suitable for the mass-produced car because of its low strength and proneness to handling damage. Some years ago the stronger, aluminum-magnesium alloys were used for the press forming of panels at ambient temperature, but these alloys were less formable than extra deep-drawing quality steel due to their low r-values and reduced ductility.

Press forming was something of a problem. More recently it has been shown that the formability of such aluminum/magnesium alloys can be greatly improved by modest increases in forming temperature and the idea that difficult pressings could be made by warm-forming has emerged.

It has been tested warm to hot forming and investigated the uniaxial tensile behavior of a range of aluminum/magnesium alloys containing 2 to 6 percent magnesium over the temperature range 20 to 500°C (68 to 932°F) at strain rates from 1x10^-3 to 5x10^-2 s^-1.

It is shown that tensile elongations between 150 and 200 percent can be obtained with all alloys under the appropriate conditions of temperature and strain rate. Unfortunately, under some conditions, intergranular cavitation is observed after deformation, the degree of cavitation increasing with increasing temperature, higher magnesium content, and decreasing strain rate, while the ductility of the alloy with the lowest magnesium is limited by grain growth.

Metallurgically, up to ~300°C (572°F), dynamic recovery appears to be the major softening mechanism and this is substantiated by a calculated activation energy of ~146 kJ/mole (~35 kcal/mole). Above 300°C (572°F), the activation energy increases to ~167 kJ/mole (~40 kcal/mole) at 932°F, which suggests a growing contribution from dynamic recrystallization.

The need to reduce fuel consumption in motor vehicles by reducing their weight has forced manufacturers to consider using metals other than low-strength deep drawing quality steels for press-formed components. Strong aluminum alloys are obvious candidates, but their reduced press-formability compared with that of an EDD steel, with which the industry is familiar, must lead to forming problems.

An improvement in formability could possibly be achieved by forming at elevated temperatures, for in general metals are more ductile hot than cold. An extreme example of ductility at elevated temperatures occurs with superplastic alloys, where, generally under rather special conditions of alloy composition, temperature, and strain rate, large tensile elongations (800 to 2500 percent) can be achieved. Obviously it would be useful if more modest increases in ductility could be obtained in conventional alloys under less critical conditions.

Following this approach the some authors and co workers at General Motors have examined the warm-formability of a number of aluminum alloys and the aluminum/magnesium series was found to be the most responsive. Other work has suggested that elongations greater than 400 percent can be achieved in aluminum-magnesium alloys deformed at higher temperatures.

Matsuki, using a noncommercial, aluminum-magnesium-zirconium alloy, achieved elongations of 885 percent at 520°C at a strain rate of 0.83x10^-3 s^-1, and Taplin and Smith obtained elongations between 300 and 400 percent in Al-4.5 Mg alloy at temperatures between 500 and 550°C (932 and 1022°F) and strain rates between 1x10^-3 and 1x10^-4 s^-1.

In view of the obvious forming potential of these alloys, the work reported here continues that previously presented and extends warm to hot forming by investigating the uniaxial tensile behavior of a range of aluminum/magnesium alloys containing 2 to 6 percent Mg over the temperature range 300 to 500°C (572 to 932°F).

Tension specimens, gage length 25 by 10 mm, were cut along the rolling direction from the following 1.2 mm sheet aluminum alloys:

• Aluminum 6 percent magnesium alloy in the annealed condition (Al-6Mg).
• Aluminum 3 percent magnesium alloy in the annealed condition (Al-3Mg).
• Aluminum 2 percent magnesium alloy in the annealed condition (AI-2Mg).

Tension specimens were deformed to failure in a split furnace mounted on a Nene 50 KN tension testing machine operated at constant cross heat speeds between 1.5 and 75 mm/min (1/16 and 3 in./min) and at temperatures between 20 and 500°C (68 and 932°F). True-stress, true-strain data were determined from the load versus elongation data, assuming that the initial deformation was uniform. Then the coefficient of strain-rate sensitivity (m) was calculated from a series of these stress- strain curves all determined at different strain rates.

Optical microscopy was used to examine the deformed structures. Strain gradients were measured along the central axis of the broken tension specimens and samples were taken at particular strain levels (e₃ = -0.25, -0.50, and -0.75) together with a sample from the undeformed grip-end of the tension specimen. These were mounted parallel to the plane of the sheet and metallographically prepared in the conventional way.

Microstructures for AI-6Mg, which can be summarized as follows:

• The grain size (~20 mm) is very stable.
• Cavitation occurs at 500°C (932°F).
• Cavitation occurs at the slower strain rate.
• Grains become elongated during deformation, but do not reflect the total elongation of the specimen.

Taking these two figures together, it can be seen that the volume fraction and number of cavities increases with increasing strain and decreasing strain rate, and so the increase in cavitation is a consequence of initiation as well as cavity growth. The results obtained for Al-2Mg and Al-3Mg show these alloys exhibit only a small amount of cavitation, and at the highest temperatures and slowest strain rates.

The cause of cavitation is not clear. It does not appear to be due to inclusions formed from trace elements, since Al-6Mg is purer than both Al-2Mg and Al-3Mg, in which there is far less cavitation. It appears that an increase in magnesium content increases the susceptibility to cavitation, even though at the test temperature all alloys should be single phase.

Conclusions

1. The pattern of warm forming behavior previously established for aluminum-magnesium alloys deformed at temperatures below 300°C (572°F); namely, that increased magnesium content, increased temperature, and decreased strain rate give increased formability-breaks down at temperatures between 300 and 500°C (572 and 932°F).

2. Intergranular cavitation occurred at temperatures above ~300°C (572°F) at strain rates less than 1 X 10^-2 s^-1. The degree of cavitation in- creased with:

• increased magnesium content,
• higher temperatures,
• slower strain rates, and
• increased tensile strain.

With these alloys, the phenomenon was significant only in AI-6Mg.

3. Grain growth occurs under certain conditions with Al-2Mg and this limits its ductility.

4. Activation energy determinations suggest dynamic recovery is operational at the lower temperatures, with the possibility of some contribution from dynamic recrystallization at higher temperatures.