May 2021 Volume 3

FORGING RESEARCH economical, potential savings from the material (i.e., high-cost metals) and machining (i.e., hard-to- work metals) and the increased equipment and process costs must be carefully assessed and balanced. The warm forming process, an alternative that falls between hot and cold forming, can also be adopted. It benefits from a smaller forming force and lower costs than that of cold forming and much higher achievable precision than that of hot forming. In warm forming, however, the temperature must be carefully selected. In lower temperature regions, warm forming is limited by blue-brittleness, the tonnage of the equipment and the achievable deformation. In higher temperature regions, it is limited by red-brittleness and scale formation. Table 1 compares the major features of the three forming temperatures. Figure 5 illustrates the relationships of temperature, flow stress, formability and scale formation in some steels.

3.2 Hot, warm and cold forming Plastic deformation can be conducted at different temperature zones. Hot forming is the most commonly used method in the forming and forging industries because of its technical and economic advantages. Large plastic deformation, in combination with heat from a working temperature above recrystallization, is very effective for refining microstructure and grain size, breaking up macro-segregation patterns, collapsing and sealing porosity. Hot forming requires less force to deform a metal and is able to stretch the material significantly from its initial shape to the final shape. Very complex features can therefore be made. When forming at high temperatures, workpieces always experience inevitable problems such as scale, decarburization, surface defects, size variations and shape distortions.Therefore, adequate machining stocks must be added, which can lower the material utilization. Most hot formed components require subsequent rough machining to remove the imperfections and to meet the specified dimensions before undergoing the final finishing procedures. Cold forming is usually applied at room temperature to make near net-shaped or net-shaped components that require high precision. The cold formed components usually require less or no subsequent machining to achieve high material utilization and low machining costs. They can also gain considerable strength through work hardening. In cold forging, particularly, the flow stress of a metal at room temperature can be several times higher than that at hot forming temperatures. As a result, the forming force is substantially higher, requiring a much bigger piece of forging equipment. With increasing forming force, the elastic deformation in the equipment is proportionally increased, making the required precision in the products more difficult to achieve. Because of the hindered metal flow in the workpiece at room temperature, filling a complex shape without defects is a challenge. All these issues limit cold forging applications primarily to relatively small-sized components. Another limitation of cold forging is its overall high cost. To make the cold forging process economical, potential savings from the material (i.e., high-cost metals) and machining (i.e., hard-to-work metals) and the increased equipment and process costs must be carefully assessed and balanced. The warm forming process, an alternative that falls between hot and cold forming, can also be adopted. It benefits from a smaller forming force and lower costs than that of cold forming and much higher achievable precision than that of hot forming. In warm forming, however, the temperature must be carefully selected. In lower temperature regions, warm forming is limited by blue-brittleness, the tonnage of the equipment and the achievable deformation. In higher temperature regions, it is limited by red-brittleness and scale formation. Table 1 compares the major features of the three forming temperatures. Figure 5 illustrates the relationships of temperature, flow stress, formability and scale formation in some steels.

Table 1. Forming temperature and its influence

Table 1. Forming temperature and its influ nce Hot

Warm

Cold

Above recrystallization temp: 950°C–1050°C

Below recrystallization temp: 750°C–950°C

Forming Temperature

Room temperature

Workpiece Weight

Up to many tons

Up to 50kg

Up to 30kg

Material Utilization and Requirement to Blank

Low

High

Highest

Tool Design

Easy

Medium

Challenging

Stock Condition and Subsequent Machining Dimensional Precision and Surface Finish

Large: scale, defects, distortion

Small

Minimal

Highest: near net- shape or net-shape

Low

High: near net-shape

Formability

Best

Good

Fair

Low: ~ 20%-30% flow stress Low: ~ 20%-30% flo stres

Medium: ~30%-50% flow stress Medium: ~30%-50% flow stress

High: 100% flow stress

High: 100% flow stress

Deformation Force

Deformation Force

5

Tool Design

Easy

Medium

Challenging

Tool Design

Easy

Medium

Challenging

Cold forming zone Warm forming zone Hot forming zone Cold forming zone Warm forming zone Hot forming zone

Figure 5. Flow stress, formability and scale formation in various temperature zones [4]

Figure 5. Flow stress, formability and scale formation in various temperature zones [4] 3.3 Selection of a metal forming process Metal forming processes have many advantages over other manufacturing methods. They have high productivity and low material waste, and can improve the products’ mechanical properties. One of the significant benefits of adopting forming processes is that the grain flow closely and continuously follows the contour of the formed body (Figure 6a) in contrast to the interrupted grain flow in a machined component (Figure 6b), thus enhancing its strength, damage resistance and service life. Selection of a metal forming process Metal forming processes have many advantages over other manufacturing methods. They have high productivity and low material waste, and can improve the products’ mechanical properties. One of the significant benefits of adopting forming processes is that the grain flow closely and continuously follows the contour of the formed body (Figure 6a) in contrast to the interrupted grain flow in a machined compon nt (Figure 6b), thus enhancing its strength, damage resistance and service life. a b Figure 6. Grain flow patterns resulting from forging and machining methods (courtesy alanhawk.com) Selection of a particular forming process is largely dependent on the product to be formed. Once the specifications of the product (for example, the material grade and formability, dimensions, geometries and precisions, as well as the property requirements) are thoroughly evaluated, the type of metal Figure 5. Flow stress, formability and scale formation in various temperature zones [4] Selection of a metal forming process Metal forming processes have many advantages over other manufacturing methods. They have high productivity nd l w material waste, and can improve the produc s’ mechanical prop ties. One of the significant benefits of adopting forming processes is that the grain flow closely and continuously follows the contour of the formed body (Figure 6a) in contrast to the interrupted grain flow in a machined component (Figure 6b), thus enhancing its strength, damage resistance and service life. b Figure 6. Grain flow patterns resulting from forging and machining methods (courtesy alanhawk.com) Selection of a particular forming process is largely dependent on the product to be formed. Once the specifications of the product (for example, the material grade and formability, dimensions, geometries and precisions, as well as the property requirements) are thoroughly evaluated, the type of metal a b Figure 6. Grain flow patterns resulting from forging and machining methods (courtesy alanhawk.com) 6 6 a

FIA MAGAZINE | MAY 2021 69