When it comes to very thick or high-tensile-strength aluminum, traditional rules for determining minimum bend radii, minimum punch nose radii, die openings, bending force calculations, and tooling requirements may no longer apply.
Because the aluminum workpiece can be extremely thick and strong, you need to understand the variables and learn how to work with them. First, consider the minimum material’s chemical composition, its surface, and edge condition, as well as its thickness, and determine whether the bend is with or across the aluminum’s grain direction.
All forming, regardless of scale, involves some kind of plastic deformation. Material expansion occurs on the outside surface of the bend, and compression on the inside, and you need to know how to deal with both. The limits of material ductility will be the controlling factor for the minimum bend radius.
The strains associated with the plastic deformation when cold bending can cause the aluminum to strain-harden. This can change the material’s mechanical properties in the area of the bend, where plastic deformation is occurring. At this point, ductility and resistance to fracture will need to be considered in aluminum material.
2 Advice on Cold Bending Aluminum
Advice one: Choosing the Right Bend Radius
When bending aluminum, know that the smaller your inside bend radius, the larger the chance that cracking will occur in the part. For the best results and fewer cracks on the outside of the bend, the bend line should go across or diagonal to the material grain when and where possible.
Forming with the grain requires less bending force because the aluminum’s ductility is readily stretched. But this stretching causes the grains to spread, which manifests as cracking on the outside bend radius. To prevent or at least reduce this cracking when bending longitudinal to the grain direction, you may need to use a larger bend radius. When bending transverse to the grain direction, the reduced ductility will increase the required forming tonnage, but it will be capable of accepting a much tighter inside bend radius without destroying the outside surface of the bend.
For cold bending aluminum, you will find a variety of minimum bend radii-to-thickness ratios, and you will need to research these values in data provided by your material supplier.
As the thickness increases, so does the minimum radius. For 0.25-in.-thick 6061 in an “O” condition, the material supplier may specify a 1-to-1 inside radius-to-plate-thickness ratio. In 0.375-in.-thick aluminum, the minimum radius is 1.5 times the thickness; for 0.5-in.-thick, it’s 2 times the thickness.
The minimum radius also increases with harder material. For 0.25-in.-thick 6061 in a “T4” condition, the material supplier may specify the minimum radius to be 3 times the thickness; a 0.375-in.-thick plate may have a minimum radius of 3.5 times the thickness; for 0.5-in.-thick plate, it can be 4 times the thickness.
The trend is obvious: The harder and thicker the plate is, the greater the minimum bend radius. For 0.5-in.-thick 7050 aluminum, the minimum bend radius may be specified as much as 9.5 times the material thickness.
Again, the minimum inside bend radius is even larger when bending with the grain.
Aluminum Minimum Bend Radii for 90 Degree Cold Forming of Sheet and Plate
|Alloy||Temper||RADII FOR VARIOUS THICKNESSES EXPRESSED IN TERMS OF THICKNESS “t”|
|1/64 in.||1/32 in.||1/16 in.||1/8 in.||3/16 in.||1/4 in.||3/8 in.||1/2 in.|
Advice Two: Choosing the Right Die Angle
When forming aluminum, air forming any other material, you choose an appropriate die width based on the material thickness and the radius-to-thickness relationship.
Springback is the release of elastic strain and is related directly to the material yield strength. It’s the reason you need a greater bend angle to achieve the required angle, especially for most aluminum.
A certain sheet metal workpiece may have, say, 2 degrees of spring back, so you need a punch with a minimum included angle that’s at least 2 degrees less than the included die angle to provide the needed angular clearance. But as the radius increases, so will spring back, and the amount of spring back can be significant when the radius is large in relationship to the sheet or plate thickness.
The right die width and angle can help compensate for this excessive spring back. This includes relieved dies, with included angles of 78 or 73 degrees. Channel dies have included die angles that are perpendicular, straight up, and down. Both allow for the necessary penetration of the tool without interference between the die faces, punch, and material.
No matter the material, its gauge, or thickness, soft aluminum is much more ductile than high-strength materials and, therefore, can be bent to a sharper radius. When bending thick or high-tensile metals, you need to abide by a minimum inside bend radius. This will minimize the effects of strain hardening and cracking at the bend.
Generally, soft aluminum is necessary for good formability and a tight inside radius; but as the level of the hardness of the aluminum increases, its ductility and formability are limited, increasing the minimum radius that can be produced.