ME 330 Exam 2

Which of the following statements is true for flat rolling?

 

None of the statements is true.

Ideally, when applying back tension in flat rolling,

rolling torque decreases

The presence of workpiece inclusions is likely to result in one of these defects during deformation:

center cracking in extrusion

In flat rolling, which pair of the following problems have opposite effects on the workpiece?

 

Rolls deflection and rolls thermal cambering

In flat rolling, edge cracks are usually associated with:

 

Rolls deflection

In flat rolling, which of the following statements is TRUE about the neutral point?

 

None of the options is correct

Chevron cracks in extrusion process can be reduced by:

 

Multiple options are correct

Hydrostatic extrusion is commonly used for:

 

brittle materials

Which of the following statements is TRUE in wire drawing process?

 

Low carbon steel (K= 530MPa, n=0.26) has better drawability (i.e., maximum reduction in diameter) than 6061-T6 Aluminum (K=410 MPa, n=0.05).

In wire drawing, which of the following statements is true for a rigid-perfectly plastic material?

 

Average flow stress is constant

Which of the following statements is FALSE in forging process?

 

Cold shut defect in closed die forging is mainly due to high porosity in the workpiece.

Which of the following statements is FALSE in rolling process?

 

Applying equal front and back tensions simultaneously moves the neutral point towards the center of the rolls

An advantage of cold forming over hot forming:

 

slower tool wear in cold forming

Which of the following statements is TRUE in rolling process?

 

None of the statements is true.

Flash in closed die forging is often used for:

 

complex shapes

The average flow stress in rolling processes increases with:

 

increasing draft

In closed die forging, flash can be desired since it:

 

Increases workpiece strength

In closed die forging, cold shut defect is mainly attributed to:

 

Tooling design

In flat rolling, increasing friction between rolls and workpiece results in:

 

None of the options

Plane strain conditions are often assumed in the case of:

 

None of the options is correct

For a strain hardening materials (n=0.3), the maximum possible strain in a wire drawing process is:

ε_max = 1 + n = 1 + 0.3 = 1.3

The maximum ratio of diameters (D₀/D_f) for wire drawing of a perfectly plastic material is approximately:

ε = 2ln(D₀/D_f) → (D₀/D_f) = e^0.5

For a perfectly plastic material, the maximum true strain is approximately:

ε_max ​≈ 1

In an open-die forging of a 2-in diameter cylindrical workpiece (K=30 kpsi, n=0.2) from an initial height of 4-inch to a final height of 2-inch, using a press with 100 hp capacity.

1. Calculate the deformation strain.

ε = ln(h₀/h_f) = ln(4/2) = ln(2) = 0.693

In an open-die forging of a 2-in diameter cylindrical workpiece (K=30 kpsi, n=0.2) from an initial height of 4-inch to a final height of 2-inch, using a press with 100 hp capacity.

2. Calculate the average flow stress in psi.

Y_f = (Kεⁿ) / (n + 1) = 30000(0.693)^0.2 / 0.2 + 1 = 23325

In a rolling operation of a 24-inch wide Aluminum plate (K=28,500 psi, n = 0.23) from an initial thickness (h₀ = 1.5") to a final thickness (h_f = 0.5") using a roll radius (R = 3") at N=100 rpm, and a coefficient of friction (μ = 0.4).

1. Calculate the roll gap in inches.

L = SQRT(R(h₀ - h_f)) = SQRT(3(1.5 - 1)) = 1.732

In a rolling operation of a 24-inch wide Aluminum plate (K=28,500 psi, n = 0.23) from an initial thickness (h₀ = 1.5") to a final thickness (h_f = 0.5") using a roll radius (R = 3") at N=100 rpm, and a coefficient of friction (μ = 0.4).   

2. Calculate the deformation strain.

ε = ln(h₀/h_f) = ln(1.5/0.5) = 1.099

In a rolling operation of a 24-inch wide Aluminum plate (K=28,500 psi, n = 0.23) from an initial thickness (h₀ = 1.5") to a final thickness (h_f = 0.5") using a roll radius (R = 3") at N=100 rpm, and a coefficient of friction (μ = 0.4).   

4. Calculate the rolling force in lb_f.

Y_f = Kεⁿ/(1 + n) = (28500(1.099)^0.23)/1.23

In a rolling operation of a 24-inch wide Aluminum plate (K=28,500 psi, n = 0.23) from an initial thickness (h₀ = 1.5") to a final thickness (h_f = 0.5") using a roll radius (R = 3" = 0.25 ft) at N=100 rpm, and a coefficient of friction (μ = 0.4).   

5. Calculate the total rolling power in HP.

v = 2pi()rN = 2pi()(0.25)(100) = 157.1 P = Fv/33000 = (655000(157.1))/33000 = 3117 hp

Explain the effect of hot working in metal forming on the dimensional accuracy of the workpiece

Poor → Due to non-uniform contraction during cooling

Explain the effect of hot working in metal forming on the surface finish of the workpiece

Poor → Due to surface oxidation

What is meant by plane-strain conditions in the forging process?

Deformation occurs in two dimensions only

How do we compensate for plane-strain conditions in the forging process

Multiply/correct Y by 1.15

What is the effect of flash in closed die forging on the mechanical properties of the part?

Flash improves mechanical properties due to excessive strain hardening

List two approaches for moving the neutral point in rolling process closer to the center of the rolls

1- Applying back tension on workpiece

2- Reducing friction between rolls & workpiece

What is the effect of rolls deflection on the edge condition of the workpiece in flat rolling?

If workpiece is ductile it’ll end up with wavy edges

If workpiece is rigid the edges crack

What is the main cause for chevron cracking in the extrusion process?

Poor pressure distribution inside the deformation zone (non-overlapping over center line)

What is the effect of strain hardening of metals on the maximum drawing ratio?

n increases → Max D.R. increases

Cold Forming pros include:

Better/improved mechanical properties (e. strength) and dimensional accuracy (no shrinkage), better corner details and surface finish (no oxidation)

Cold forming cons include:

Requires higher forces and energy consumption as well as higher strength (and cost) tooling than hot forming

What are the key factors which determine the type of lubricants used in a rolling operation? Why?

The friction coefficient (μ) between the rolls and the workpiece. Higher μ allows for higher draft (reduction in thickness) but increases the forces and energy consumption and may result in damage to the surface of rolls and work piece

What is the relationship between wavy edges, crown defect, rolls deflection, and rolling forces?

High rolling forces result in rolls deflection (if the rolls are too small and/or the supporting rolls shafts are not rigid enough). This results in a crown defect in the workpiece (thick centers) combined with wavy edges due to faster material flow at the sides.

What is the effect of die angle on the amount of energy consumed in an extrusion process?

There is an optimum die angle at which the amount of consumer energy is a minimum.

What is the effect of the strain hardening behavior of a metal rod on the maximum stress allowed in a wire drawing process?

A higher strain hardening coefficient (n) of the workpiece material for a higher drawing stress (i.e higher max stress)

What would be the benefits of the cogging process

Improve mechanical properties