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Peen forming
Source:Dalian Yuquan Metal Products Co., Ltd.    Hits:503 Times    Posted:2015-7-3    

Peen forming is a die-less cold forming process performed at room temperature with procedure of shot peening. The peen forming process is ideal for forming large panel shapes where the bend radii are reasonably large without abrupt changes in contour. Peen forming is best suited for forming curvatures where the radii are within the metal's elastic range.

Peen forming is effective on all metals!

Peen forming is the preferred method of forming aerodynamic contours into aircraft wing skins and has the following advantages:

·                     No forming dies are required.

·                     Process is performed at room temperature.

·                     Wing skin design changes are easily accomplished by altering the peen forming procedure. There is no expensive modification of dies required.

·                     All forming is accomplished using residual compressive stress. Peen formed parts exhibit increased resistance to flexural bending fatigue and stress corrosion cracking as a result.

·                     Peen formed skins exhibit compressive stress on top and bottom surfaces.

Peen forming is most often performed on a feed through, gantry type machine.

Slika: Feed through peen forming machine.jpg

CONTOUR CORRECTION

Shot peening utilizing peen forming techniques can be used to correct unfavourable geometry conditions. This is accomplished by shot peening selective locations of parts to utilize the surface loading from the induced compressive stress to restore the components to drawing requirements. Some examples are:

·                     Driveshaft/crankshaft straightening

·                     Roundness correction of ring shaped geometry

·                     Aircraft wing stiffener adjustment

·                     Welding distortion correction

The peen forming process avoids the unfavourable tensile residual stresses produced by other straightening methods by inducing beneficial compressive residual stresses.

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Peen forming is most widely used for forming aeroplane wings, fuselages and segments for space rockets. Peen forming can be utilized for forming of thin sheets of metal in many other industries.

The technology

Peen forming is a forming process derived from the shot peening surface treatment. The overall forming of the component is reached through a great number of statistically distributed shot impacts. Media for peen forming are usually steel balls which are accelerated to a specific velocity and directed in a desired pattern. During impact of the shot, a certain amount of its kinetic energy is transformed into plastification of the component. When shot velocity is fairly low only a thin layer of the component's material is elongated resulting in a convex curvature. If shot velocity is increased, concave curvatures are produced.


Due to the phenomenon shown above, peen forming allows for a greater accuracy in forming materials than traditional forming methods. All essential parameters of peen forming must be measured and controlled when shot peening is in progress. Parameters include:

·                     Shot Velocity

·                     Mass Flow

·                     Traverse Rate

·                     Mass per Area

·                     Shot coverage

The above must be closely monitored to achieve small tolerances given the variations and fluctuations in material characteristics and the peen forming process itself. When peening convex curved component parts, tolerances down to 0.2 mm are possible.
Parts formed by peen forming exhibit increased resistance to flexural-bending fatigue. Another distinct advantage with peen forming, unlike most other forming methods is that all surface stresses generated are of a compressive nature. Although peen-formed pieces usually require shot peening on one side only, the final result causes both sides to have compressive stress. These compressive stresses serve to inhibit stress corrosion cracking and to improve fatigue resistance. Some work pieces should be shot peened all over prior to or after peen forming to further improve fatigue and stress corrosion characteristics. Parts which have been cold formed by other processes often undergo shot peening to overcome the harmful surface tensile stresses set up by these other forming processes.

Peen forming is often more effective than rolling, stretching or twisting of metal to develop the required curvatures. Saddle-back shapes are possible and by being a die-less process, material allowance for trimming is reduced, and costly development and manufacturing time required to fabricate hard dies is eliminated. The process is flexible to design changes, which may occur after initial design. By only adjusting the air pressure, it is possible to incorporate curvature changes. Sheet thickness range in aluminum is 1.27 mm to 51.0 mm and alloy steel is 0.40 mm to 25.4mm.

 

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