Forming

Question 1

Compare hot and cold forming

Answer 1

Hot > 0.6Tm, cold < 0.3Tm
Hot = greater deformation in a pass = no elastic spring back, material not work hardened (annealing occurs) = constant flow stress, CRSS reduced, width constant = one set of rolls, but oxidation occurs = low surface finish & reduced accuracy & inc higher cost

Question 2

What is the roll of friction in forming?

Answer 2

Needed to draw metal into die/rolls and prevents poisons effect (width constant) but want it to be minimised (wasted energy) - need to add lubricant (water to less reactive, glass to more reactive)
Friction coefficient cold = 0.1, hot = 0.2

Question 3

Describe reversing rolls

Answer 3

Because width is the same, one set of rolls can be used (only for hot rolling as Fr > poisons), roll separation is decreased and metal passed back through
Decreases cost as only one set of rolls is needed

Question 4

Define rolling load

Answer 4

Load applied by rolls to deform metal - want to minimise this in most systems, Pp = P/bLp
Specific roll load = minimum load/width.length of bar in contact with roll
As width is constant = constant load required (for hot only)
P increases with roll radius, μ and rolling draught

Question 5

What is the friction hill?

Answer 5

Extra energy needed to overcome friction
Q = μLp/average h
Friction = friction coefficient.length in contact with roller/average height

Question 6

How is Lp related to roll radius and rolling draught

Answer 6

Rolling draught hd- height bar is reduced by
Lp = (r(hd))^0.5
r = radius

Question 7

What products are made by plain rollers and which by shaped rollers?

Answer 7

Plain - plate, strip

Shaped - rods, rails, girders, beams

Question 8

Describe how shaped rollers create I-beam

Answer 8

Square strip - indent rolled top and bottom 3/4 passes, then roll end to elongate into I-beam shape
Or can be combined so 4 rollers work on one strip to create continuous shape (more expensive but much higher production run

Question 9

What microstructure changes occur due to hot rolling?

Answer 9

Recrystallisation occurs meaning big grains are broken into smaller ones, pores are closed, vacancies can be removed and material is annealed
mechanical properties are improved

Question 10

What are perceived problems of hot rolling? (Final material and energy)

Answer 10

Hot rolls are large = large rolling load = increased energy but offset by yield stress of material reduced, does require heating material to >0.6Tm, hot rolled also needs to be descaled to reduce oxide layer (removes material also - for small volume materials = big % material)

Question 11

Describe modifications needed to change hot rolling to cold rolling

Answer 11

Reversing rolls aren’t used, but smaller rolls are added down the production line to reduce thickness more, annealing furnaces and accumulators (stores extra material so at end of coil process doesn’t stop, bobbins move closer to continue insertion rate) can also be used between rolls

Question 12

What are the microstructure changes of cold rolling?

Answer 12

Reduced mechanical properties by aligning grains with rolling direction, can also break up inclusions into multiple inclusions (if brittle) or elongate inclusion (if plastically deforms)

Question 13

Name the two ways of producing tubes

Answer 13

Flat blanks are fed into conical die which forces sheet into die shape and edged welded together = tube
Strip is spiralled around a mandrel and continually welded together = tube

Question 14

Describe roll forming

Answer 14

Passing sheet/sheet through a series of rolls to create desired shape
_/ shape made from \ — / rolls

Question 15

Describe pressing process

Answer 15

A sheet is lubricated and clamped into a fixed die, moving die then presses sheet around fixed die shape

• If sheet gripped too tightly = elongation instead of pressing
• corners are most likely failure points
• elastic loading occurs = original over shaping needed

Question 16

Describe deep drawing process

Answer 16

Sheet is clamped and then a moving die pushes sheet upwards (no fixed die above = shape of moving die), sides are effectively cold rolled by die separation decreasing
How cans are made
Material needs to draw from clamps = not too tight

Question 17

Describe spinning

Answer 17

Small production rates and low cost products

Rotating sheet is forced onto die by guide, sheet takes form of die

Question 18

When is forging preferred to shaping?

Answer 18

When thick walled and greater mechanical properties are needed forging is the preferred route

Question 19

Describe open die forging

Answer 19

Slab is hit with punch and compressed several times, then rotated and repeated - must be done well in hot range (90% work = heat)
This breaks up cast structure (mechanical deformation), pores healed up, segregation reduced and annealed

Question 20

Describe closed die forging

Answer 20

Pre-heated does (to avoid quenching) are forced around hot slab to produce die shape 
Hot = no springback, high capital cost but high production rate 
Oxidation occurs unless in vacuum (needs to be removed after)
Lubrication needed (glass/water)

Question 21

What lubrication is used for Ti alloys and why?

Answer 21

Glass lubricant (as Ti reactive)
Ti alloys are heated and rolled in glass = continuous layer -> dissolved oxygen = increased surface strength

Question 22

What is a flash gutter?

Answer 22

Flash is material leaving gutter, flash gutter fills with liquid = increased friction which stops more liquid leaving die = more complete filling of die

Question 23

How does forging affect heat treatment?

Answer 23

Forging involves breaking up coarse precipitates but cooling rate doesn’t allow for fine particles to form (reduced mech prop)
Heavily alloyed materials have cooling rates that are high enough to prevent excess solute precipitating = heat testing at low temps = increased strength

Question 24

Describe direct extrusion

Answer 24

Piston forces bullet through chamber and through die =continuous seamless shape
Metal will get trapped in dead zone and shear from main billet = waste - occurs as middle moves faster than edges in contact with chamber (Fr occurs)

Question 25

Describe indirect extrusion

Answer 25

Rams act on die and move it into billet, only friction is on billet in contact with die = reduced forces required, due to moving rams into billet, length of extrudate possible is reduced

Question 26

Why is the extrusion rate and friction important?

Answer 26

Higher friction = more energy needed for process, friction reduces over time for direct (less billet and chamber contact) constant for indirect
If friction or extrusion rate too high = surface cracking as middle moves too quickly for outside

Question 27

Define extrusion load and draw indirect and direct time vs extrusion load graphs

Answer 27

Extrusion load = work needed to flow into die + friction between chamber and billet
Direct - like a stress strain curve (linear, peak, reducing over time)
Indirect - peak a lot lower and then constant over time

Question 28

Describe cold drawing

Answer 28

Essentially continuously extruding wire, material is pulled through die = high ductility needed (cold as ductility dec as T inc), wound on bull block to hold material then passed through another die (may be annealed between dies), 2 bull block rotated faster than 1st (longer wore), Good surface finish and small dimensions

Question 29

Describe how seamless tubes are made

Answer 29

If short billet hole may be pierced (not machines as work hardened)
Large dimension = hot extruding first to reduce work hardening, then dies rotate to draw wire in, plug placed inside and wire pulled through die
Die separation = outer diameter

Question 30

Why are seamless tubes useful? And what is the role of the plug?

Answer 30

Absence of joining = more uniform structure and properties
Plug decided inner diameter and prevents tube bending
Short distances = fixed plug, long distances = floating plug held by friction

Question 31

What is pilgering and when is it used?

Answer 31

Asymmetric rolls compressed tube onto die and causes friction to drag material into rolls
Good for non-ductile materials as tensile stresses required are lower