Manufacturing Processes phần 2 pptx

PLASTIC WORKING TECHNIQUES 13-11

press forging) decreases slightly up to 500C (932F), rises until 750C

(1,382F), drops rapidly at 800C (1,472F) (often called blue brittleness),

and beyond 850C (1,562F) increases rapidly to hot forging temperature

of 1,100C (2,012F). Therefore, substantial advantages of low material

resistance (low tool pressures and press loads) and excellent workability

(large flow without material failure) can be realized in the hot-working

range. Hot-working temperatures, however, also mean poor dimensional

tolerance (total dimensional error), poor surface finish, and material loss

due to scale buildup. Forging temperatures above 1,300C (2,372F) can

lead to hot shortness manifested by melting at the grain boundaries.

MATERIAL RESPONSE IN METAL FORMING

The deformation conditions in metalworking processes span a range of

deformation parameters, including strain and strain rates (Fig. 13.2.4) that

are much higher than those encountered in conventional testing methods

(Fig. 13.2.5). In machining, the strains are high and the strain rates can

reach 105

/s, while in explosive forming, strains are small at high strain

rates providing extremely small response times. Forging and extrusion

cover a wide range of strains and strain rates. Sheet forming carried out

as small strains and strain rates differs from superplastic forming at

extremely low strain rates but high strains. Consequently, different meth￾ods have been developed to test material response for different ranges of

deformation parameters, i.e., strain and strain rate (Fig. 13.2.5).

PLASTIC WORKING TECHNIQUES

In the metalworking operations, as distinguished from metal cutting,

material is forced to move into new shapes by plastic flow. Hot-working

is carried on above the recovery temperature, and spontaneous recovery,

or annealing, occurs about as fast as the properties of the material are

altered by the deformation. This process is limited by the chilling of the

material in the tools, scaling of the material, and the life of the tools at

the required temperatures. Cold-working is carried on at room tempera￾ture and may be applied to most of the common metals. Since, in most

cases, no recovery occurs at this temperature, the properties of the metal

are altered in the direction of increasing strength and brittleness

throughout the working process, and there is consequently a limit to

which cold-working may be carried without danger of fracture.

A convenient way of representing the action of the common metals

when cold-worked consists of plotting the actual stress in the material

against the percentage reduction in thickness. Within the accuracy

required for shop use, the relationship is linear, as in Fig. 13.2.6. The

lower limit of stress shown is the yield point at the softest temper, or

anneal, commercially available, and the upper limit is the limit of ten￾sile action, or the stress at which fracture, rather than flow, occurs. This

latter value does not correspond to the commercially quoted “tensile

strength” of the metal, but rather to the “true tensile strength,” which is

the stress that exists at the reduced section of a tensile specimen at frac￾ture and which is higher than the nominal value in inverse proportion to

the reduction of area of the material.

As an example of the construction and use of the cold-working plots

shown in Fig. 13.2.6, the action of a very-low-carbon deep-drawing

steel has been shown in Fig. 13.2.7. Starting with the annealed material

with a yield point of 35,000 lb/in2 (240 MN/m2), the steel was drawn

to successive reductions of thickness up to about 58 percent, and the

Fig. 13.2.3 Effect of forging temperature on forgeability and material properties. Material: AISI 1015 steel. f

strain

rate; f*

limiting strain; sf

flow stress; Stot

dimensional error; FeL

scale loss. (K. Lange, “Handbook of Metal

Forming,” McGraw-Hill, 1985.)

0.01 0.1 1.0 10

Strain

10−1

101

103

105

Sheet metal

forming

Explosive

forming

Forging

Strain rate, s−1

Extrusion

Machining

Fig. 13.2.4 Range of deformation parameters for various metalworking

processes. (Source: P. F. Bariani, S. Bruschi, and T. Dal Negro, Enhancing

Performances of SHPB for Determination of Flow Curves, Annals of the CIRP, 50,

no. 1, pp. 153–156.)

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