METAL FORMING PROCESS
The Materails to be used in places where the component is subjected to very high Impact load, Shock Load, intermittant load and in Power transmission lines, Need to be produced with dense grain structure. This requirement can be acheived by manufacturing process such as Forging, Rolling,Extrusion and Drawing.
COMPARISION BETWEEN HOT WORKING AND COLD WORKING
1. Hot working is done above recrystallisation temperature
2. Refinement of grains takes place
3. Impurities and porosity are removed from metals after hot working.
4. Residual stresses are eliminated.
5. Rapid oxidation or scaling of surfaces occurs which results in poor surface finish.
6. Close dimensional tolerances cannot be maintained.
7. Toolling and handling costs are more.
8. Mecanical properties such as Toughness,ductility,elongation are improved.
1. Cold working isdone below recrystallisation temperature.
2. Grain structure is distorted.
3. Impurities and porosities exist in metal after cold working.
4. Residual stresses are not eliminated.
5. No oxidation and hence good surface finish is obtained.
6. Close dimensional tolerances can be obtained.
7. Tooling and handling costs are less
8. Cold w2orking decreases elongation,reduction in area , hardness, tensile strength. Fatigue strength are improved.
Rolling is the most rapid method of forming metals into desired shapes by plastic deformation in between rolls.
The forming of bars, plates , sheets, rails and other steel sections are produced by rolling.
1.Classification of Rolling mill based on number of rolls.
1.Two High Rolling Mill.
2.Three high Rolling Mill
3.Four high Rolling Mill
4. Multi Rolling Mill.
5.Universal Rolling Mill
6.Planetary Rolling Mill.
1.Two High Rolling Mill.
2. DEFECTS IN ROLLED PARTS
There are two types of major defects on the rolled products.
(a) SURFACE DEFECTS
(b) INTERNAL SURFACE DETECTS.
(a) SURFACE DEFECTS Major surface defects on rolled products are scales, rust, scratches,cracks, and pits. These defects occurs on the rolled products due to the impurities and inclusions present in the original cast materials.,
(b).INTERNAL SURFACE DEFECTS
i. WAVINESS OR WAVY EDGES.
It occurs due to the bending of rolls. The rolls acts as a straight beam. If the material flow is continuous and to maintain this continuity, strains with in the material should adjust with itself. There are compressive strain on the edges and tensile strain at the centre. The edges are restrained from expanding freely in the longitudinal direction because of which wavy edges on the sheet will be produced.
ii. Zipper Cracks
It occurs due to poor material ductility, at the rolling temperatureCamber is provided to avoid this defect., Camber is providing slightly large diameter at the center of rolls than that at the edges.
Folds occur if the reduction per pass is very less.
It is the splitting of work piece along the horizontal plane on exit, with top and bottom part following the rotation of their respective rolls.
These are small cracks which may develop when reduction in thickness is excessive.
1. COMPARISON BETWEEN PRESS FORGING AND DROP FORGING.
SR NO Press forging : Drop Forging
1. It is a Faster process : Slow Process
2. Die alignment is easier : Die alignment is difficult
3. Operation is quite : Noisy operation
4. Quality of product is good : Quality of product is fair.
5. Stroke and ram speed is high : It is low.
6. It is one stroke operation : Multiple stroke operation.
7. Range is 20 tons to 1500 tons. : Range upto 10 tons
8. Shapes formed are dense and homogeneous in structure. : Coarse and not homogenous in structure.
2. Forging operations
Forging is a process in which the work piece is shaped by compressive forces applied through various dies and tools. It is one of the oldest metalworking operations. Most forgings require a set of dies and a press or a forging hammer. A Forged metal can result in the following: -
Decrease in height, increase in section - open die forging Increase length, decrease cross-section, called drawing out.
Decrease length, increase in cross-section on a portion of the length - upsetting
Change length, change cross-section, by squeezing in closed impression dies - closed die forging. This results in favorable grain flow for strong parts
3. Types of forging
Closed/impression die forging
Commonly used materials include
• Ferrous materials: low carbon steels
• Nonferrous materials: copper, aluminum and their alloys
Open-die forging is a hot forging process in which metal is shaped by hammering or pressing between flat or simple contoured dies.
Equipment. Hydraulic presses, hammers.
Materials. Carbon and alloy steels, aluminum alloys, copper alloys, titanium alloys, all forgeable materials.
Process Variations. Slab forging, shaft forging, mandrel forging, ring forging, upsetting between flat or curved dies, drawing out.
Application. Forging ingots, large and bulky forgings, preforms for finished forgings.
Closed Die Forging
In this process, a billet is formed (hot) in dies (usually with two halves) such that the flow of metal from the die cavity is restricted. The excess material is extruded through a restrictive narrow gap and appears as flash around the forging at the die parting line.
Equipment. Anvil and counterblow hammers, hydraulic, mechanical, and screw presses.
Materials. Carbon and alloy steels, aluminum alloys, copper alloys, magnesium alloys, beryllium, stainless steels, nickel alloys, titanium and titanium alloys, iron and nickel and cobalt super alloys.
Process Variations. Closed-die forging with lateral flash, closed-die forging with longitudinal flash, closed-die forging without flash.
Application. Production of forgings for automobiles, trucks, tractors, off-highway equipment, aircraft, railroad and mining equipment, general mechanical industry, and energy-related engineering production.
4.DEFECTS IN FORGED PARTS
In this some of the die cavity are not completely filled by the flowing metal.
This appears as small cracks at the corners of the forging. Caused due to improper design of die.
This is seen as irregular depositions on the surface of forging. This is primarily caused because of improper cleaning of the stock used for forging.
The oxides and scales gets embedded into the finish forging surface.
When the forging is cleaned by pickling, these are seen as deputations on the forging surfaces.
This is caused by the mis-alignment of the half dies, making the two halves of te forging to be of improper shape. It is also called as mismatch.
These are basically ruptures caused by the improper cooling of the large forging. Rapid cooling causes the exterior to cool quickly causing internal fractures.
6.Improper Grain Flow.
Due to improper design of the die, which makes the flow of the metal to be not in the final intended direction?
Laps are formed by webbuckling during forging. To avoid laps web thickness should be increased and properly edesigned.