Eren BillurHeating Helps Forming, Part 2: Warm and Hot Forming of Aluminum
September 30, 2020Comments
In Part 1 of our two-part series on how heating helps forming (August 2020 issue) we focused on hot stamping of high-strength steels. Here we turn our attention to aluminum, which has approximately one-third the density of steel and long has been known for its lightweighting potential in automotive and aerospace applications. In the automotive industry, three series of aluminum typically find use: 5000-, 6000- and 7000-series alloys.
5000 Series
These grades contain magnesium as the main alloying element, with small amounts of manganese added in some cases. These sheet grades have low to medium strength, and their strength level depends on alloying content and the amount of cold work.
Automotive applications typically are limited to unexposed surfaces since they tend to form stretcher strains when cold formed (i.e., Lüders bands) and can cause poor appearance after painting (Fig. 1). The most common grades include 5182 (AlMg4.5Mn0.4), 5754 (AlMg3) and 5083 (AlMg4.5Mn0.7―used in superplastic and quick-plastic forming, as explained later).
6000 Series
These aluminum alloys contain magnesium and silicon and find use for exposed surfaces; they can be heat treated for strengthening via precipitation hardening. Strength levels of these alloys vary depending on the temper.
The most common automotive grades include 6014 (AlMg0.6Si0.6V), 6016 (AlSi1.2Mg0.4), 6082 (AlSi1MgMn) and 6111; stampers can cold form all of these grades to relatively complex geometries (A-pillar reinforcements, for example). They generally cannot be drawn as deep as 5000-series alloys, or mild steels. The yield strength of some 6000 grades may effectively be doubled during a typical automotive paint-bake cycle. A specially developed automotive alloy may have 120-MPa (17,500-psi) yield strength in the as-delivered T4 temper, and approximately 250-MPa (36,000-psi) yield strength after 2-percent deformation followed by a paint-bake cycle.
Another method used to form relatively complex and high-strength automotive components (yield strength exceeding 280 MPa/40,000 psi) with 6000-series aluminum alloys is cold forming the material in a relatively soft temper, and then applying post-form heat treatment to harden the parts. Jaguar-Land Rover began using this technique in production in 2012, and the 13th-generation Ford F-150 has 14 components produced using this technique.
In one hot forming example, blanks are heated in excess of 500 C (930 F) and then transferred to a press for forming and then quenching in a cooled and lubricated die―a process similar to press hardening of steel. This process leaves the part in W temper from forming/quenching. As a last step, the part undergoes an artificial aging process to restore its final desired properties (strength and elongation).
Referring to Fig. 4 showing stress-strain curves for Al 5083, at room temperature the alloy has a yield strength of approximately 150 MPa (22,000 psi) and more than 20-percent total elongation. Once the strain (deformation) rate hits the sweet spot of superplasticity, stress level may be as low as 5 to 10 MPa (7250 to 14,500 psi) and ductility may exceed 300 percent. In aerospace grades, total elongation can exceed 1000 percent. 