Powder Forging

The design issues in P/F are similar to the requirement of any precision, closed-die forging. The difference is the starting preform; in the case of P/F, the preform is a sintered powder metal part, typically 80 to 85% of theoretical density, with a shape similar to the final part configuration. By contrast, in a precision closed-die forging the preform is a wrought steel blank with very little shape detail. Preform design for P/F fabrication determines the extent of product shape detail required to meet the performance requirements of the finished P/F part. Preform design is a complex, iterative process currently modeled by computer simulation software programs to help reduce design time and development costs.

In the forging step, the P/M preform is removed from the reheat (or sintering) furnace, coated with a die lube, and forged in a heated, closed die operation. The forging process reduces the preform height and forces metal into the recesses of the closed die. This step also brings all features to their final tolerances and densities.

Configuration guidelines, typical of precision closed-die forged parts, also apply to P/F parts as follows:

• Radii on inside corners of the forging as large as possible to promote metal flow around corners in the tool and promote complete fill of all details.

• Radii of at least 1 mm (0.040 in.) on all outside corners of the forging to aid in material flow to define features.

• Shape of the forging should be such that, when placed in the die, the lateral forces will be balanced. Shapes that are symmetrical along a vertical plane, such as connecting rods and shapes that are axisymmetric (or nearly so), are preferred.

• Zero draft is possible on surfaces formed by the die and core rod, but not by the upper punch.

• Re-entrant angles (undercuts) cannot be forged.

• Axial tolerances--in the direction of forging--are driven by variations in the mass of metal in the preform. Lateral tolerances are driven by metal flow as the cavity fills. Typical axial tolerance of 0.25 to 0.5 mm (0.010 to 0.020 in.) are encountered, with diametric tolerances of 0.003 to 0.005 mm/mm of diameter.

• Concentricity of a P/F part is determined by the quality and density distribution in the preform. Concentricity is normally double that of the preform.

0 0

Post a comment