Due to their FCC structure and their relatively slow rate of work hardening, aluminum alloys are highly formable at room temperature. The 2XXX and 7XXX high strength alloys can be readily formed at room temperature, provided
Fig. 2.14. Distortion of Machined Aluminum Forging13
the alloy is in either the O or W temper. The choice of the temper for forming depends on the severity of the forming operation and the alloy being formed. Although the annealed or O condition is the most formable condition, it is not always the best choice because of the potential for warping during subsequent heat treatment. The solution treated and quenched condition (W temper) is nearly as formable as the O condition and requires only aging after forming to obtain peak strengths, without the potential of warping after forming during the quenching operation. Both the 2XXX and 7XXX alloys must be formed immediately after quenching or be refrigerated after heat treating to the W temper prior to forming. Since aluminum has a relatively low rate of work hardening, a fair number of forming operations are possible before intermediate anneals are required. Tools for forming aluminum alloys require good surface finishes to minimize surface marking. The oxide film on aluminum is highly abrasive and many forming tools are therefore made of hardened tool steels.
As shown in Fig. 2.15, blanking is a process in which a shape is sheared from a larger piece of sheet, while piercing produces a hole in the sheet by punching out a slug of metal. Both blanking and piercing operations are usually preformed in a punch press. The clearance between the punch and die must be controlled to obtain a uniform shearing action. Clearance is the distance between the mating surfaces of the punch and die, usually expressed as a percentage of sheet thickness. The recommended clearance for the 2XXX and
7XXX alloys in the O condition is 6.5%. The walls of the die opening are tapered to minimize sticking, and the use of lubricants such as mineral oil mixed with small quantities of fatty oils also reduces sticking tendencies. A tolerance of 0.005 in. is normal in blanking and piercing of aluminum; however, wider tolerances, when permissible, will help in reducing costs.
Dull cutting edges on punches and dies have effects similar to excessive clearance with burrs becoming excessive. With sharp tools and proper clearance, the fractures are clean without evidence of secondary shearing or excessive burring. When the clearance is too small, secondary shearing can occur, and if the clearance is too large, the sheared edge will have a large radius and a stringy burr. Cast zinc tools, which are much less expensive than steel tools, are often used for runs of up to about 2000 parts.
In brake forming (Fig. 2.16), the sheet is placed over a die and pressed down by a punch that is actuated by the hydraulic ram of a press brake. Springback is the partial return of the part to its original shape after forming. The amount of springback is a function of the yield strength of the material being formed, the bend radius, and the sheet thickness. Springback is compensated by over bending the material beyond the final angle so that it springbacks to the desired angle. The springback allowance (i.e., the amount of over bend) increases with increasing yield strength and bend radius, but varies inversely with sheet thickness. The spring back allowance ranges from 1 to 12° for 2024-0 and 7075-0 and from 7.25 to 33.5° for 2024-T3. Since aluminum sheet tends to develop anisotropy during rolling operations, there is less tendency for cracking during forming if the bend is made perpendicular to the rolling or extrusion direction. The smallest angle that can be safely bent, called the minimum bend radius, depends on the yield strength and on the design, dimensions, and conditions of the tooling. The most severe bends can be made across the rolling direction. If similar bends are
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