Fundamentals of Metal Forming

Question 1

What is metal forming?

Answer 1

Metal forming includes a large group of manufacturing processes in which plastic deformation is used to change the shape of metal work pieces

Question 2

What are the two main categories of metal forming?

Answer 2

Bulk deformation processes and sheet metal working processes

Question 3

What is the defining feature of bulk deformation processes?

Answer 3

Bulk deformation processes are generally characterized by significant deformations and massive shape changes, and the surface area-to-volume of the work is relatively small.

Question 4

What are the four most common bulk deformation processes?

Answer 4

Rolling
Forging
Extrusion
Drawing

Question 5

What is sheet metal working?

Answer 5

Sheet metalworking processes are forming and cutting operations performed on metal sheets, strips, and coils. The surface area-to-volume ratio of the starting metal is high

Question 6

What is a part produced in a sheet metal operation often called?

Answer 6

A stamping

Question 7

What tools are used for sheet metal operations.

Answer 7

Punch and die. The punch is the positive portion and the die is the negative portion of the tool set.

Question 8

What are the three basic sheet metal operations?

Answer 8

Bending
Drawing
Shearing

Question 9

Define flow stress

Answer 9

Flow stress is defined as the instantaneous value of stress required to continue deforming the material-to keep the metal “flowing”. It is the yield strength of the metal as a function of strain

Question 10

Define average flow stress

Answer 10

The average flow stress (also called the mean flow stress) is the average value of stress over the stress-strain curve from the beginning of strain to the final (maximum) value that occurs during deformation.

Question 11

What are advantages of cold working?

Answer 11

(1) greater accuracy, meaning closer tolerances can be achieved; (2) better surface finish; (3) higher strength and hardness of the part due to strain hardening; (4) grain flow during deformation provides the opportunity for desirable directional properties to be obtained in the resulting product; and (5) no heating of the work is required, which saves on furnace and fuel costs and permits higher production rates.

Question 12

What are disadvantages of cold working?

Answer 12

(1) higher forces and power are required to perform the operation; (2) care must be taken to ensure that the surfaces of the starting workpiece are free of scale and dirt; and (3) ductility and strain hardening of the work metal limit the amount of forming that can be done to the part.

Question 13

What temperature does warm working refer to.

Answer 13

Temperatures somewhat above room temperature but below the recrystallization temperature. The dividing line between cold working and warm working is often expressed in terms of the melting point for the metal. The dividing line is usually taken to be 0.3 x Melting
point

Question 14

What are advantages of warm working?

Answer 14

The lower strength and strain hardening at the intermediate temperatures, as well as higher ductility, provide warm working with the following advantages over cold working: (1) lower forces and power, (2) more intricate work geometries possible, and (3) need for annealing may be reduced or eliminated.

Question 15

What temperature does hot working occur at?

Answer 15

Temperatures above the recrystallization temperature. The recrystallization temperature for a given metal is about one-half of its melting point.

Question 16

Why is it possible for greater plastic deformation of metal to occur during hot working?

Answer 16

The flow curve of the hot-worked metal has a strength coefficient that is substantially less than at room temperature, the strain-hardening exponent is zero (at least theoretically), and the ductility of the metal is significantly increased

Question 17

What are advantages of hot working?

Answer 17

(1) the shape of the work part can be significantly altered, (2) lower forces and power are required to deform the metal, (3) metals that usually fracture in cold working can be hot formed, (4) strength properties are generally isotropic because of the absence of the oriented grain structure typically created in cold working, and (5) no strengthening of the part occurs from work hardening

Question 18

What are disadvantages of hot working?

Answer 18

(1) lower dimensional accuracy, (2) higher total energy required (due to the thermal energy to heat the workpiece), (3) work surface oxidation (scale), (4) poorer surface finish, and (5) shorter tool life.

Question 19

What is isothermal forming?

Answer 19

Isothermal forming refers to forming operations that are carried out in such a way as to eliminate surface cooling and the resulting thermal gradients in the work part. It is accomplished by preheating the tools that come in contact with the part to the same temperature as the work metal.

Question 20

Name and describe the four main bulk deformation processes

Answer 20

Rolling. This is a compressive deformation process in which the thickness of a slab
or plate is reduced by two opposing cylindrical tools called rolls. The rolls rotate so
as to draw the work into the gap between them and squeeze it.

Forging. In forging, a workpiece is compressed between two opposing dies, so that
the die shapes are imparted to the work. Forging is traditionally a hot working
process, but many types of forging are performed cold.

Extrusion. This is a compression process in which the work metal is forced to flow
through a die opening, thereby taking the shape of the opening as its own cross
section.

Drawing. In this forming process, the diameter of a round wire or bar is reduced by
pulling it through a die opening.

Question 21

Why is the orthogonal cutting model useful in the analysis of metal machining?

Answer 21

Orthogonal cutting is useful in the analysis of metal machining because it simplifies
the rather complex three-dimensional machining situation to two dimensions.