Page 27 - MetalForming July 2014
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     Non-Heattreatable Aluminum Alloys
Heattreatable Aluminum Alloys
1XXX – Pure aluminum
2XXX – Aluminum Copper
3XXX – Aluminum Manganese
6XXX – Aluminum Magnesium + Silicon
4XXX – Aluminum Silicon
7XXX – Aluminum + other elements
5XXX – Aluminum Magnesium
         Fig. 1
metal alloys, researchers use the uni- versal tensile test. Elongated strips of material are machined to a specific geometry, clamped in the jaws of the tensile tester, and elongated while recording pulling force and sample elongation. The resulting data are con- verted to stress-strain curves that describe the capabilities of the material being evaluated.
The test reveals several material properties, including elastic modulus —a measure of the forces holding the atoms stable within the atomic cell. The initial tensile force applied to the specimen causes the inter-atom spac- ing to increase. The subsequent onset of plastic (permanent deformation) continues to increase this atomic spac- ing as forming forces increase. When the forces are removed, the atomic cell shrinks in an attempt to return to its neutral state—springback. Applying compressive forces results in a similar but opposite reaction.
Nearly all stamped parts exhibit springback. Unlike the tensile test, stamping contours created by plastic deformation act as barriers that prevent complete return of the atomic cell to the stable state. This creates dangerous residual or trapped stresses. Any sub- sequent forming, metal trimming, hole punching, bracket welding, heattreat- ment or other disruption of the bal- anced stresses will cause more spring- back and variance from dimensional specifications.
Springback poses a significant chal- lenge when forming nonferrous mate- rials such as aluminum and copper alloys. Springback relates to the elastic
modulus; a low modulus means high springback. Most nonferrous alloys have a modulus of 10 million, com- pared to 30 million for steel. There- fore, aluminum stampings exhibit three times the springback of steel stampings formed with the same design and thick- ness. Springback also is proportional to material yield strength—higher yield strength means more springback.
The workhardening exponent (n- value) is measured over a strain range of 10 percent to ultimate tensile strength (UTS), or load maximum, dur- ing the tensile test. Alloys with higher n- values allow more overall stretchabili- ty and increased ability to minimize the localization of strain gradients. Unfortunately, the metallurgy devel- oped to strengthen the material also causes a reduction in formability. For a given alloy, n-value decreases as strength increases. Most conventional steels have a constant n-value obtained from the power-law equation: σ = K€n. Some aluminum products have an n- value decreasing with increasing strain. For these alloys, the first published number is the maximum initial n-value and the number in parenthesis is the average n-value. An example is 0.35 (0.26).
The strain-rate hardening exponent (m-value) can present some challenges for aluminum. As forming speed increases, some alloys increase in strength (m=+), some remain unchanged (m=0) and some decrease in strength (m=─). The m-value becomes important when a deformation (strain) gradient begins to form in one loca- tion. Some local feature in the stamp-
ing, such as a character line, sharp embossment, tight bend, etc., starts to locally form. The material over the fea- ture is straining faster than the sur- rounding material. Ferrous alloys have positive m-values and become stronger when the rapid forming attempts to begin, thereby restraining the growth of the gradient.
Some aluminum alloys have nega- tive m-values, which lose local strength, accelerate the gradient growth and ini- tiate early failure. A sheared blank edge, punched hole extrusion or other heav- ily cold-worked areas of the stamping can sustain long early cracks with neg- ative m-values. Researchers continue to develop methods to change aluminum m-values from negative to positive through warm forming.
Stamping Aluminum
Most press-shop equipment used to form steel will suit aluminum. How- ever, die-design rules primarily devel- oped for steel must be modified for forming aluminum alloys. Critical process variables include:
• The formability characteristics of the aluminum blank;
• The geometry of the part and tool- ing, and
• Press setup, including lubrication and accurate control of blankholder pressure.
Aluminum forms a thin natural layer of aluminum oxide on the sheet surface that can break down during forming and abrade cutting and forming tools. This oxide film, very hard, brittle and tightly adhering, tends to break. This allows highly adhesive metal-to-metal contact, often requiring surface treat- ments (PVD, TD or other) to protect the tooling.
Due to the lower formability of alu- minum compared to steel, product design features such as radii, draw depths, wall angles, steps and transi-
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