Bulk deformation processes

Question 1

How do you define two different extrusion processes by direction?

Answer 1

direct extrusion (ram pushes material through die)
indirect extrussion (container with billet moves over ram, through die)

Question 2

Describe the stress curve for direct extrusion vs that for indirect and explain why this difference is caused

Answer 2

direct stres increases and peaks and then curvs off, indirect increases to flow stress and remains aprox constant. difference caused from friction between container wall and billet

Question 3

What is the result of changing cone angle in extrusion/wire drawing processes

Answer 3

When the cone angle is decreased the redundant work goes down but the frictional loss at the die surface increases
Increase in cone angle results in a subsequent increase in redundant work and decrease in frictinoal loss

Question 4

Name 5 extrusion defects

Answer 4

Inhomogenous deformation
surface cracking
Centre burst or chevron cracking
Variation in structure and properties
Hot shortness

Question 5

Describe inhomogenous deformation, and state what two conditions will increase its effect

Answer 5

the result of the movement in the center of the billet being faster than of the periphery. when the outer surface moves towards the centre and extrudes through the die, an internal oxide stringer is produced.
increase in wall friction
decrease in wall temp

Question 6

Why does surface cracking occur as an extrusion defect (include what it is caused by in hot and cold cases)

Answer 6

due to longtiduinal tensile stresses generated, most common cause is too high ram speed
HOt: usually associated with hot shortness
cold: from sticking in the die land and sudden build up of pressure

Question 7

Describe centre burst or chevron cracking

Answer 7

Can occur at low extrusion ratio due to loe frictional conditions on the zone of deformation at the extrusion die

Question 8

During extrusion what can cause variations in structure and properties?

Answer 8

nonuniform deformation in both longitudinal and transverse directions

Question 9

What condition causes Hot chortness in extrussion and what is it?

Answer 9

High temperatures generated cause incipient melting, which then causes cracking

Question 10

What is a rotary swage and what is its purpose?

Answer 10

A rotorary swage is a tool used to reduce the diameter of the ends of the wires or rods before they are fed through the wire drawing die