Manufacturing Chapter 17

Disadvantages of hot working

1. Lower dimensional accuracy 2. Higher total energy required 3. Work surface oxidation (scale) 4. Poorer surface finish 5. Shorter tool life

Advantages of warm working over cold working

1. Lower forces and power are required 2. More intricate geometries are possible 3. The need for annealing can be eliminated

Why friction is undesirable in metal forming processes

1. Metal flow in the work is retarded, causing residual stresses and sometimes defects in the product 2. Forces and power to perform the operation are increased 3. Tool wear can lead to loss of dimensional accuracy, resulting in defective parts and requiring replacement of tooling

Lubricants used in cold work

1. Mineral oils 2. Fats and fatty oils 3. Water-based emulsions 4. Soaps 5. Other coatings

Lubricants used in hot work

1. Mineral oils 2. Graphite 3. Glass

Metalworking lubricant benefits

1. Reduced sticking 2. Reduced forces 3. Reduced power 4. Reduced tool wear 5. Better surface finish 6. Removes heat from the tool

Molten glass becomes an effective lubricant for hot extrusion of

steel alloys

Disadvantages of cold working

1. Higher forces and power are required in the operation 2. Care must be taken to ensure the surface is free of scale and dirt 3. Ductility and strain hardening of the work metal limit the amount of forming that can be done to the part. In some cases, the metal must be annealed to allow for further deformation. In other cases, the metal is just not ductile enough to be worked

Advantages of hot working

1. Able to produce substantial plastic deformation of the material 2. Lower forces and power are required to deform the metal 3. Metals that usually fracture in cold work can be hot-formed 4. Strength properties are generally isotropic because of the absence of the oriented grain structure typically created in cold working 5. No strengthening of the part occurs from hot working

Sheet metalworking categories

1. Bending operations 2. Deep or cup drawing 3. Shearing processes 4. Miscellaneous processes

Metal forming processes categories

1. Bulk deformation 2. Sheet metalworking

Increasing workpiece temperature has which two of the following effects on the strain rate sensitivity equation?

1. Decreases the strength constant 2. Increases the strain rate sensitivity exponent

Bulk deformation categories

1. Rolling processes 2. Forging processes 3. Extrusion processes 4. Wire and bar drawing

Warm working Strain rate sensitivity categories

1. Temperature range - .3Tm-.5Tm 2. Exponent - .05<=m<=.1 3. Coefficient of Friction - .2

Hotworking Strain rate sensitivity categories

1. Temperature range - .5Tm - .75Tm 2. Exponent - .05<=m<=.4 3. Coefficient of Friction - .4 - .5

Cold working Strain rate sensitivity categories

1. Temperature range - <.3Tm 2. Exponent - 0<=m<=.05 3. Coefficient of Friction - .1

Considerations in choosing an appropriate metalworking lubricant

1. Type of forming process (rolling, forging, sheet metal drawing, etc.) 2. Whether used in hot working or cold working 3. Work material 4. Chemical reactivity with the tool and work metals 5. Ease of application 6. Toxicity 7. Flammability 8. Cost

The average flow stress is the flow stress multiplied by which of the following factors, where n is the strain-hardening exponent?

1/(1 + n).

Rolling

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

Forging

A deformation process in which two halves of a die squeeze the work part, causing it to assume the shape of the cavity

Metal forming

A large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces

Stamping

A part produced in a sheet metal operation

Shearing

A sheet metalworking operation that involves cutting the work using a punch and die

Drawing 2

A sheet metalworking process in which a flat metal sheet is formed into a hollow or concave shape, such as a cup, by stretching the metal.

Pressworking

A term applied to sheet metal operations because the machines used to perform them are presses

Warm working dividing line

Around .3Tm-.5Tm, where Tm is the melting point for the metal

Hot working dividing line

Around .6Tm, where the Tm is the melting point of the metal

Which of the following describes the typical starting work geometry in sheet metal processes?

Low volume-to-area ratio

Drawing 1

Deformation process in which the diameter of a round wire or bar is reduced by pulling it through a die opening

Extrusion

Compression process in which a ductile or semifluid work material is forced through a die orifice to provide long continuous product whose cross-sectional shape is determined by the shape of the orifice

Stresses applied in most metal forming operations are which one of the following?

Compressive

Sticking

Condition in which the hot work surface adheres to the rolls over the contact arc

Sheet metalworking processes

Forming and cutting operations performed on metal sheets, strips, and coils

Hot working

Forming metals at temperatures above the recrystallization temperature

Isothermal Forming

Forming operations that are carried out in such a way as to eliminate surface cooling and the resulting thermal gradients in the work part

Which two of the following are desirable properties of the work material in metal forming?

High ductility and low yield strength

Advantages of cold working compared to hot working

Important for mass production operations 1. Greater accuracy 2. Closer tolerances 3. Higher strength and hardness 4. Grain flow during deformation provides the opportunity for desirable directional properties in the resulting product 5. No heating of the work is required 6. Saves on cost and allows for higher production rates

Increasing strain rate tends to have which one of the following effects on flow stress during hot forming of metal?

Increases flow stress

The applied force in a forging operation is based on which of the following flow stress values?

Instantaneous flow stress.

The coefficient of friction between the part and the tool in cold working is which one of the following relative to its value in hot working?

Lower

Cold working (cold forming)

Metal forging performed at room temperature or slightly above

Warm working

Metal forming performed at temperatures somewhat above room temperature but below the recrystallization temperature

Bulk deformation processes

Metal forming processes generally characterized by significant deformations and massive slope changes, and the surface area-to-volume of the work is relatively small

The flow curve expresses the behavior of a metal in which region of the stress-strain curve?

Plastic region

Strain rate

Rate at which metal is strained in a forming process

Bending

Sheet metalworking operation that involves straining of a metal sheet or plate to take an angle along a (usually) straight axis

Average flow stress

The average value of stress over the stress-strain curve from the beginning of the strain to the final (maximum) value that occurs during deformation

Flow stress

The instantaneous value of stress required to continue deforming the material

Punch

The positive portion of a tool used in a sheet metal operation

Sticking friction

The tendency for the two surfaces in the relative motion to adhere to each order rather than slide. Also means friction stress between the surfaces exceeds the shear flow stress of the work metal, causing the metal to deform by shear processes beneath the surface rather than slip at the surface

Drawing is a term applied to both bulk deformation and sheet metalworking although the process is different for the two categories.

True

Isothermal forming refers to forming operations that are carried out by preheating the tools in contact with the part to the same temperature as the work metal, thus eliminating surface cooling and the resulting thermal gradients in the part.

True

Cup drawing

forming of a flat metal sheet into a hollow or concave shape, such as a cup, by stretching the metal.

Strain rate sensitivity

the effect of strain rate on strength properties