Forming Flashcards
(31 cards)
Compare hot and cold forming
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
What is the roll of friction in forming?
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
Describe reversing rolls
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
Define rolling load
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
What is the friction hill?
Extra energy needed to overcome friction
Q = μLp/average h
Friction = friction coefficient.length in contact with roller/average height
How is Lp related to roll radius and rolling draught
Rolling draught hd- height bar is reduced by
Lp = (r(hd))^0.5
r = radius
What products are made by plain rollers and which by shaped rollers?
Plain - plate, strip
Shaped - rods, rails, girders, beams
Describe how shaped rollers create I-beam
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
What microstructure changes occur due to hot rolling?
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
What are perceived problems of hot rolling? (Final material and energy)
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)
Describe modifications needed to change hot rolling to cold rolling
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
What are the microstructure changes of cold rolling?
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)
Name the two ways of producing tubes
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
Describe roll forming
Passing sheet/sheet through a series of rolls to create desired shape
_/ shape made from \ — / rolls
Describe pressing process
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
Describe deep drawing process
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
Describe spinning
Small production rates and low cost products
Rotating sheet is forced onto die by guide, sheet takes form of die
When is forging preferred to shaping?
When thick walled and greater mechanical properties are needed forging is the preferred route
Describe open die forging
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
Describe closed die forging
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)
What lubrication is used for Ti alloys and why?
Glass lubricant (as Ti reactive) Ti alloys are heated and rolled in glass = continuous layer -> dissolved oxygen = increased surface strength
What is a flash gutter?
Flash is material leaving gutter, flash gutter fills with liquid = increased friction which stops more liquid leaving die = more complete filling of die
How does forging affect heat treatment?
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
Describe direct extrusion
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)
