Tài liệu 06 metal forming processes 2 b

ADVANCED MANUFACTURING ENGINEERING 6: Metal Forming Processes 2 December 2016 2 Rolling • Rolling is a forming process which reduces the cross-sectional area of the incoming metal stock or produces a new cross-section of the material at the exit while improving its mechanical properties through the use of rotating rolls. • The process can be carried out hot, warm, or cold, depending on the application and the material involved. • Hot rolling is done at temperatures above the recrystallization temperature. http://youtu.be/AuuP8L-WppI 3 Rolling • The most usable shapes produced by the primary rolling process are :- slabs (thick, flat plate), blooms (large rectangular bar), billets (large square bar), structural sections & rail sections. • The gap between the rotating rolls is less than the thickness of the entering bar H0 therefore a friction force is necessary in order to bite the bar and to pull it through the rolls. 4 Rolling • Sheet and strip often are rolled cold in order to maintain close thickness tolerances. – Flat Rolling – Thread Rolling – Seamless Tubing 5 Extrusion • Extrusion is very similar to squeezing toothpaste from a tube. • Material is forced through a shaped hollow die in such a way that it is plastically deformed and takes up the shape of the die. http://youtu.be/MU3PJb2Ngqs 6 Extrusion • Practically, extrusion is limited to more ductile non-ferrous metals such as aluminum, zinc and copper alloys. • Possible to produce hollow sections using extrusion: In this case, the die contains a short piece (or mandrel) in the shape of the hole. This mandrel is attached to the die by one or more ‘bridges’. As the extruded material encounters the bridges it is forced to separate, but it flows around the bridges and joins up again. https://www.youtube.com/watch?v=OsdZ6cj3y_g 7 Extrusion • The hole in the die can have almost any shape, so if the die is circular, for example, a wire or rod is produced. Extruded shapes 8 Extruded products • The body of a food mixer. • Copper pipe for a central heating system. • The body of a pen. 9 Extruded products • The body of the food mixer has a complex 3D shape, so it certainly could not be extruded. • Copper pipe is ideal for manufacturing by extrusion, using a bridge die to extrude the hollow shape. • • The ink tube in the pen has probably been extruded. But the bodies, even the cheaper ones, vary in diameter along their length and are typically closed off at one end. This would suggest that the pen body is not extruded. 10 Deep drawing • Deep drawing is a process of cold forming a flat pre-cut metal blank into a hollow vessel without excessive wrinkling, thinning, or fracturing. • The parts may vary from 6mm diameter parts to aircraft or automotive parts large enough to require mechanical handling equipment. http://youtu.be/vAeP8q9Icqg 11 Deep drawing • The various forms produced may be cylindrical or box shaped, with straight or tapered sides or a combination of straight, tapered, and curved sides. • The maximum amount of deformation occurs in the cup walls, deformation of the flange is lower, bottom region undergoes no deformation. • The following alloys are used in deep drawing process (with high ductility): pure copper, some aluminum alloys and pure aluminum, 70-30 brass, mild steels. 12 Wire drawing • Used to reduce the cross-section of a wire by pulling the wire through a single, or series of, drawing die(s). • Applications: electrical wiring, cables, springs, tension- loaded structural components, paper clips, spokes for wheels, and stringed musical instruments. 13 Wire drawing • Although similar in process, drawing is different from extrusion, because in drawing the wire is pulled, rather than pushed, through the die. • Drawing is usually performed at room temperature, thus classified as a cold working process, but it may be performed at elevated temperatures for large wires to reduce forces. • The drawing process employs a series of dies with opening of similar shape and size which is slightly reduced in each consequent die. Manufacturing examples • Automotive wheel manufacture (7 mins) • • • • • http://youtu.be/kpi1LSvX1wA Making a pair of scissors http://youtu.be/PqFt3OLkb38 ( 5 mins) How to make a spring http://www.youtube.com/watch?v=omLKbKakDoY&feature =share&list=PLEF21E2A982519B68 (5 mins) How to make a engine block http://www.youtube.com/watch?v=lsDoT2oVQUQ&feature =share&list=PLEF21E2A982519B68 (5 mins) 14 Manufacturing Technology Rolling, Extrusion, Drawing Processes Rolling Operations • Rolling is the process of reducing the cross section of a long workpiece by compressive forces applied through a set of rolls Grain Structure in Hot Rolling Changes in the grain structure of metals during hot rolling. This is an effective method to reduce grain size and refine the microstructure in metals, resulting in improved strength and good ductility. In this process cast structures of ingots or continuous castings are converted to a wrought structure. Mechanics of Rolling Forward slip: Schematic illustration of the flatrolling process. (Note that the top roll has been removed for clarity.) Relative velocity distribution between roll and strip surfaces. The arrows represent the frictional forces acting along the strip-roll interfaces. Note the difference in their direction in the left and right regions. The Flat-rolling Process • • • • As the surface speed of the rigid roll is constant, there is relative sliding between the roll and the strip along the arc of contact in the roll gap, L At neutral point or no-slip point, the velocity of the strip is the same as that of the roll The maximum possible draft is defined as the difference between the initial and final strip thicknesses From the relationship, higher the friction and the larger the roll radius, the greater the maximum possible draft becomes ho  h f   R 2 Copyright © 2010 Pearson Education South Asia Pte Ltd Slab Method for Rolling Calculating Average Pressure: Case 1 Rough Estimate (Friction neglected), Pav=(Y1 +Y2)/2 (Y1 :yield strength of material if not given explicitly, Y2:flow stress Case 2:An estimate for low frictional conditions pav  Y  K n 2 Y  ,Y   Y n 1 3 Case 3 :Higher frictional conditions  L  pav  Y  1   2 h av   Force: F  LwPav where L is the length of contact: L  Rh Roll Torque & Power FL 2 Power  T roll Torque 