Bài giảng Công nghệ tạo hình vật liệu - Lê Thái Hùng

changing the workpiece geometry. • reducing ’ o by increasing the temperature. 211 +1  In the case of sticky friction, if we replace the force y with k (the average shear stress of the material) in Eq.14 d y − 2 .Eq. 14= dx  y h _ 2kthen we have 2 dx dx .Eq. 29'd y = − o − odx = − = h h h3 x h Integrating .Eq. 30 − '= +C y o Since y = then ’ o at x = a, a = ' + 'C .Eq. 31 o o h Replacing C in Eq. 30 we then have x + ' a − xa  Or Eq. 27 = − ' + ' = ' +1o y o o o h h h  212 211 212 14-Feb-20 107 Example: A block of lead 25x25x150 mm3 is pressed between flat dies to a size 6.25x100x150 mm3. If the uniaxial flow stress o = 6.9 MPa and = 0.25, determine the pressure distribution over the 100 mm dimension (at x = 0, 25 and 50 mm and the total forging load in the sticky friction condition. Since 150 mm dimension does not change, the deformation is plane strain. From Eq.19. 2 exp 2 (a − x) 2  ' =  = where o o  3y o h3   At the centreline of the slab (x = 0) 2(6.9) exp2(0.25) (50 −0) = = 435MPa max 6.253 Likewise, at 25 and 50 mm, respectively. the stress distribution will be 58.9 and 8.0 MPa 213 The mean forging load (in the sticky friction condition) from Eq.28 is _ p = 2 a   + 1 o   2h3   8MPa 2(6.9)  50 +1 _ p = = 39.   12.53 We calculate the forging load on the assumption that is based on 100 percent sticky friction. Then the stress distribution The forging load is P = = = = stress x area (39.8x106)(100x10-3)(150x10-3) 597 kN 61 tonnes. 214 213 214 14-Feb-20 108 Effect of forging on microstructure grain structure resulting from (a) forging, (b) machining and (c) casting. • The formation of a grain structure in forged parts is elongated in the direction of the deformation. • The metal flow during forging provides fibrousmicrostructure (revealed by etching). This structure gives better mechanical properties in the plane of maximum strain but (perhaps) lower across the thickness. • The workpiece often undergo recrystallisation, therefore, provide finer grains compared to the cast dendritic structure resulting in improved mechanical properties. 215 Forming textures Redistribution of metal structures occurring during forming process involves two principle components; 1)redistribution of inclusions and 2)crystallographic orientation of the grains 1) The redistribution of inclusions Redistribution during forming of (a) soft inclusions (b) hard inclusions 216 215 216 14-Feb-20 109 Forming textures 2) Crystallographic orientation of the grains Castings Forgings Cast iron structure Fibre structure in forged steels Redistribution of grains in the working directions Mainly epitaxial, dendritic or equiaxed grains 217 Residual stresses in forging •The residual stress produced in forgings as a results of inhomogeneous deformation are generally small because the deformation is normally carried out well into the hot-working region. • However, appreciable residual stresses and warping can occur on the quenching of steel forgings in heat treatment. • Large forgings are subjected to the formation of small cracks, or flakes at the centre of the cross section. This is associated with the high hydrogen content usually present in steel ingots of large size, coupled with the presence of residual stresses. • Large forgings therefore have to be slowly cooled from the working temperature. Examples: burying the forging in ashes for a period of time or using a controlled cooling furnace. • Finite element analysis is used to predict residual stresses in forgings. 218 217 218 14-Feb-20 110 Typical forging defects files.bnpmedia.com •Incomplete die filling. •Die misalignment. •Forging laps. •Incomplete forging penetration- should forge on the press. •Microstructural differences resulting in pronounced property variation. •Hot shortness, due to high sulphur concentration in steel and nickel. Fluorescence penetrant reveals Forging laps www.komatsusanki.co.jp See simulation 219 Typical forging defects •Pitted surface, due to oxide scales occurring at high temperature stick on the dies. •Buckling, in upsetting forging. Subject to high compressive stress. •Surface cracking, due to temperature differential between surface and centre, or excessive working of the surface at too low temperature. •Microcracking, due to residual stress. Buckling 220 219 220 14-Feb-20 111 Typical forging defects Cracking at the flash Cold shut or fold Internal cracking • Flash line crack, after trimming-occurs more often in thin workpieces. Therefore should increase the thickness of the flash. • Cold shut orfold , due to flash or fin from prior forging steps is forced into the workpiece. • Internal cracking, due to secondary tensile stress. 221 Summary • Mainly hot forging – Blacksmith, now using water power, steam, electricity, hydraulic machines. • Heavyforging - - - - Hydraulic press = slow, high force squeeze. Pieces up to 200 tonnes with forces up to 25,000 tonnes. Simple tools squeeze metal into shape (open-die forging). Sufficient deformation must be given to break up the ‘ascast’ structure. - Reheating is often needed to maintain sufficient temperature for hot working. - Forging is costly but eliminates some as-cast defects - Continuous ‘grain flow’ in the direction of metal flow is revealed by etching. - Impurities (inclusions and segregation) have become elongated and (unlike casting) gives superior properties in the direction of elongation. 222 221 222 14-Feb-20 112 References • Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN 0-07-100406-8. • Edwards, L. and Endean, M., Manufacturingwith materials, 1990, Butterworth Heinemann, ISBN 0-7506-2754-9. • Beddoes, J. and Bibbly M.J., Principlesofmetalmanufacturing process, 1999,Arnold, ISBN 0-470-35241-8. • Lange, K., Handbook ofmetal forming, 1919, McGraw-Hill Book • Metal forming processes, Prof Manus. 223 Chapter 5 Drawing of rods, wires and tubes Subjects of interest • • • • • • Introduction/objectives Rod and wiredrawing Analysis of wiredrawing Tube drawing processes Analysis of tube drawing Residual stress in rod, wire and tubes 224 223 224 14-Feb-20 113 Objectives •This chapter provides fundamental background on processes of drawing of rods, wires and tubes. •Mathematical approaches for the calculation load will be introduced. • Finally drawing defects occurring during the of drawing process will be highlighted and its solutions will be included. 225 Introduction : • Wire drawing involves reducing the diameter of a rod or wire by passing through a series of drawing dies or plates. wire drawing • The subsequent drawing die must have smaller bore diameter previous drawing die. than the Drawing die Undrawn Drawn wire wire 226 225 226 14-Feb-20 114 Introduction : Tube drawing • Tube drawing involves reducing the cross section and wall thickness through a draw die. • The cross section can be circular, square hexagonal or in any shapes. Brass tubes for heat exchanger cheap, strong, good corrosion resistant – 227 Introduction • Drawing operations involve pulling metal through a die by means of a tensile force applied to the exit side of the die. •The plastic flow is caused by compression force, arising from the reaction of the metal with the die. •Starting materials: hot rolled stock (ferrous) and extruded (non- ferrous). • Material should have high ductility and good tensile strength. • Bar wire and tube drawing are usually carried out at room temperature, except for large deformation, which leads to considerable rise in temperature during drawing. • The metal usually has a circularsymmetry (but not always, depending on requirements). 228 227 228 14-Feb-20 115 Rod and wiredrawing •Reducing the diameter through plastic volume remains the same. deformation while the •Same principals for drawing bars, rods, and wire but equipment is different in sizes depending on products. Metal wires Metal rods Rods → Wires → relatively larger diameter products. small diameter products < 5 mm diameter. 229 Rod drawing Draw head Metal •Rods which can not be coiled, are produced on drawbenches. stock Rod is swaged Machine capacity : • • • 1 MN drawbench 30 m of runout 150-1500 mm.s-1 Insert though the die draw speedClamped to the jaws of the drawhead The drawhead is moved by a hydraulic mechanism 230 229 230 14-Feb-20 116 Wire drawing die Drawing die Conical drawing die • Shape of the bell causes hydrostatic pressure to increase and promotes the of lubricant into the die. flow • The approach angle – where the actual reduction in diameter occurs, giving the half die angle  • The bearing region produces a • The die nib made from cemented frictional drag on the wire and also carbide or diamond is encased for protection in a thick steel casing. remove surface damage due to die wear, without changing dimensions. • The back relief allows the metal to expand slightly as the wire leaves the die and also minimises abrasion if out of alignment. the drawing stops or the die is 231 Example of wiredrawing dies Drawing die Undrawn Drawn wire wire A drawing of wire drawing die 232 231 232 14-Feb-20 117 Example of wiredrawing dies Wire drawing die made from cemented tungsten carbide with polycrystalline diamond core. 233 Drawing diematerials • Cemented carbides are the most widely used for drawing dies due to their superior strength, toughness, and wear resistance. img.tradekey.com • Most drawing dies are cemented carbide or industrial diamond (for fine wires). • Polycrystalline Diamond (PCD) used for wire drawing dies – for fine wires. Longer die life, high resistance to wear, cracking or bearing. • Cemented carbide is composed of carbides of Ti, W, Ni, Mo, Ta, Hf. 234 233 234 14-Feb-20 118 Wiredrawing equipment Bull block drawing machines Multiple bull block machines - common •The wire is first passed through the overhead loop and pulley, brought down and then inserted through the die of the second drum and drawn through this die for further reduction. •Thus, the wire is drawn through all the wire drawing drums of the set in a continuous manner to get the required finished diameter of the wire. Speed of each draw block has to be synchronised to avoid slippage between the wire and the block. • The drawing speed ~ ~ up to 10 m.s-1 for ferrous drawing up to 30 m.s-1 for nonferrous drawing. 235 Wiredrawing process Hot rolled rod Remove scale -causing surface defects. • Cu and Sn are used as lubricants for high strength materials. Or conversion coating such as sulphates or oxalates. Pickling, descaling Lubricating • • • Oils and greases for wire drawing Mulsifiable oils for wet wire drawing Soap for dry drawing.drawing •Bull block drawing allows the generation of long lengths •Area reduction per drawing pass is rarely greater than 30-35%. die wire 2 `   DOutletcoil %RA= 1−    Bull block  D  100 InletSide view of bull blockTop v iew 236 235 236 14-Feb-20 119 Example: Drawing of stainless wire www.metalwire-mesh.com •Stainless steels: 304, 304L, 316, 316L •Applications: redrawing, mesh weaving, soft pipe, steel rope, filter elements, making of spring. www.metalwiremesh.com Stainless steel rope • Larger diameter stainless wire is first surface examined, tensile and hardness tested, diameter size measured. •Surface preparation by pickling in acid (ferrictic and martensitic steels) and basic solutions (austenitic steels). The prepared skin is then coated with lubricant. •Cold drawing is carried out through diamond dies or tungsten carbide dies till the desired diameter is obtained. • Cleaning off oil/lubricant is then carried out and Stainless steel meshes the wire is heat-treated (annealing at about 1100oC or plus skin pass). 237 Stepped-conemultiple-passwiredrawing • More economical design. • Use a single electrical motor to drive a series of stepped cones. The diameter of each cone is designed to produce a peripheral• speed equivalent to a certain size reduction. 238 237 238 14-Feb-20 120 Heat treatments •Nonferrous wire / low carbon steel wire € Tempering (ranging from dead soft to full hard). This also depends on the metal and the reduction involved. •Steels (C content > 0.25%) normally 0.3-0.5% require Patenting heat treatment before being drawn. Patented wire have improved reduction of area up to 90% due to the formation of very fine pearlite. Heating above the upper critical temp T~970oC • Provide austenitic structure with rather large grain size. • Rapid cooling plus small cross section of wire change microstructure to very fine pearlite preferably with no separation of primary ferrite. Cooling in a lead bath at T~315oC Good combination of strength and ductility. 239 Defects in rod and wiredrawing Defects in the starting rod (seams, slivers and pipe). Defects from the deformation process, i.e., centre burst or chevron cracking (cupping). Centre burst or chevron cracks • This defect will occur for low die angles at low reductions. reduction to• For a given reduction and die angle, the critical the friction.prevent fracture increases with 240 239 240 14-Feb-20 121 Tube-drawing processes Tube drawing •Following the hot forming process, tubes are cold drawn using dies, plugs or mandrels to the required shape, size, tolerances and mechanical strength. •provides good surface finishes. •increase mechanical properties by strain hardening. •can produce tubes with thinner walls or smaller diameters than can be Seamless stainless tubes/pipes obtained from methods. other hot forming • can produce more irregular shapes. Copper and brass tubes 241 Classification of tube drawing processes There are three basic types of tube-drawing processes • • Sinking Plug drawing - Fixed plug - Floating plug Mandrel drawing.• Tube sinking Fixed plug Floating plug Moving mandrel 242 241 242 14-Feb-20 122 Tube sinking • The tube, while passing through the die, shrinks in outer radius from the original radius Ro to a final radius Rof. •No internal tooling (internal wall is not supported), the wall then thicken slightly. •Uneven internal surface. •The final thickness of the tube depends on original diameter of the tube, the die diameter and •Lower limiting deformation. friction between tube and die. 243 Fixed plug drawing www.scielo.br • Use cylindrical / diameter. conical plug to control size/shape of inside • • • Use higher drawing loads than floating plug drawing. Greater dimensional accuracy than tube sinking. Increased friction from the plug limit the reduction in area (seldom > 30%). • can draw and coil long lengths of tubing. 244 243 244 14-Feb-20 123 Floating plug drawing • A tapered plug is placed inside the tube. • As the tube is drawn the plug and the die act together to reduce both the outside/inside diameters of the tube. • • • • Improved reduction in area than tube sinking (~ 45%). Lower drawing load than fixed plug drawing. Long lengths of tubing is possible. Tool design and lubrication can be very critical. 245 Moving mandrel drawing • Draw force is transmitted to the metal by the pull on the exit section and by the friction forces acting along the tube -mandrel interface. • minim ised friction. • Vmandrel = Vtube • The mandrel also imparts a smooth inside finish surface of the tube. • mandrel removal disturbs dimensional tolerance. 246 245 246 14-Feb-20 124 Example: schematic alternate pass reduction schedule for tube making 247 Analysis of tube-drawing •The greatest part of deformation occurs as a reduction in wall thickness. •The inside diameter is reduced by a small amount equal to dimensions of the plug or mandrel inserted before drawing. •There is no hoop strain and the analysis can be based on plane- strain conditions. For tube drawing with a plug, the draw stress can be expressed by And  = friction coefficient between tube and die wall. = friction coefficient between tube and plug. = semi die angle of the die. 1 2   = semi cone angle of the plug. 248 247 248 14-Feb-20 125 In tube drawing with a moving mandrel, the friction forces at the mandrel-tube interface are directed toward the moving mandrel, B’ can be expressed as exit of the die. For a 1 − 2B ' = Eq.20 tan − tan If 1 = 2, which is often be B ’ = 0. The differentialthe case, then equation of equilibrium for this simple case is hd x + ( x + p)dh = 0 Eq.21 hd + ' dh = 0 x o Integration of this equation and by using Boundary = hb, the draw stress becomes condition xb = 0 and h hb 1Ideal homogeneous deformation Eq.22  =  ' ln =  ' lnxa o o 1− rha It is possible that 2 > 1,→ B by is negative, the draw stress is frictionless ideal deformation.there for less than required 249 The stresses in tube sinking have been analysed by Sachs and Baldwin. Assumption: the wall thickness of the tube remains constant. The draw stress at the die exit is similar to wiredrawing. The cross sectional area of the tube is related to the mid-radius r and the wall thickness h by A ~ 2rh. stresses Eq.23 Where’’ o ~ 1.1o to account for the complex in tube sinking. 250 249 250 14-Feb-20 126 Residual stresses in rod, wire and tubes Two distinct types of residual-stress patterns in cold-drawn rod and wire: Longitudinal residual stresses - Surface→ compressive tensile drop off to zero tensile compressive tensile Small reduction (<1% reduction per pass) - Axis → - Surface→ Radial stresses - Axis →Deformation is localised in the surface layers. - Surface→ Circumferential stresses - Axis → Longitudinal residual stresses - Surface→ tensile compressive Large reduction (Commercial significance) - Axis → Radial stresses - Axis → compressive - - Surface→ tensile compressive Circumferential stresses Axis → 251 Effects of semi die angleand reductionperpass on longitudinal residual (by Linicus and Sachs) stress in cold-drawn brass wire • At a given reduction,  longitudinal stress • Maximum values of longitudinal residual stress ~ 15-35% area. reduction in 252 251 252 14-Feb-20 127 Defects in cold drawn products • Longitudinalscratches (scored die, poor lubrication , or abrasive particles) • • • Slivers (swarf drawn into the surface). Long fissures (originating in ingot). Internal cracks (pre-existing defects in starting material or ruptures in the centre due to overdrawing). • Corrosioninduced cracking due to internal residual stresses. 253 References • Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN 0-07-100406-8. • Edwards, L. and Endean, M., Manufacturingwith materials, 1990, Butterworth Heinemann, ISBN 0-7506-2754-9. • Beddoes, J. and Bibbly M.J., Principlesofmetal manufacturing process, 1999, Arnold, ISBN 0-470-35241-8. 254 253 254 14-Feb-20 128 Chapter 6 Sheet-metal forming Subjects of interest • • • • • • • • • Introduction/objectives Deformation geometry Forming equipments Shearing and blanking Bending Stretch forming Deep drawing Forming limit criteria Defects in formed parts 255 Objectives •Methods of sheet metal processes such as stretching, shearing, blanking, bending, deep drawing, redrawing are introduced. •Variables in sheet forming process will be discussed together with formability and test methods. •Defects occurring during the forming process will be emphasised. The solutions be given. to such defect problems will also 256 255 256 14-Feb-20 129 • Sheetmetal forming is a process that Introduction materials undergo permanent deformation by cold forming to produce a variety of complex three dimensional shapes. • The process is carried out in the plane of sheet by tensile forces with high ratio of surface area to thickness. •Friction conditions at the tool-metal interface are very important and controlled by press conditions, lubrication, tool material and surface condition, and strip surface condition. •High rate of production and formability is determined by its mechanical properties. 257 Classification of sheet metal parts (based on contour) 1) 2) Singly curved parts Contoured flanged parts, i.e., parts with stretch flanges and shrink flanges. Curved sections. Deep-recessed parts, i.e., cups and boxes with either vertical or sloping walls. Shallow-recessed parts, i.e., dish- shaped, beaded, embossed and corrugated parts. (a) Singly curve (b) Stretch flange 3) 4) (c) Shrink flange (d) Curved section 5) (e) Deep drawn cup (f) Beaded section 258 257 258 14-Feb-20 130 Classificationofsheetmetalforming (based on operations) Blanking Deep drawingStretching StampingCoining Ironing Roll forming of sheet Wiping down a f langeFolding Bending 259 Stress state in deformation processes •The geometry of the workpiece can be essentially three dimensional (i.e., rod or bar stock) or two dimensional (i.e., thin sheets). •The state of stress is described by three principal stresses, which act along axes perpendicular to principal planes. σ1, σ2 and• The principal stresses are by convention called σ3 where σ1> σ2 > σ3 σ3 σ2 σ1 Principal stresses on an element in a three-dimensional stress state • Hydrostatic stress state is σ1 = σ2 = σ3 when 260 259 260 14-Feb-20 131 • Shear stresses provide driving force for plastic deformation.a) Uniaxial • Hydrostatic stresses cannot contribute to shape change but involve in failure processesb) Biaxial • Tensile c) Hydrostatic → crack growth or void formation • Compressive → hinder crack, close void. d) Triaxial 261 Stress system in (a) sheet processes and (b) bulk processes. • In sheet deformation processes (i.e., sheet metal forming, vacuum forming, blow moulding), the workpiece is subjected to two dimensional biaxial stresses. • In bulk deformation processes (i.e. forging, rolling and extrusion), the workpiece is subjected to triaxial stresses, which are normally compressive. (also depending on geometry) 262 261 262 14-Feb-20 132 Deformation Planestress geometry • Principalstresses σ1 and σ2 theirare set up together with associated strain in the x-y plane. • The sheet is free to contact (not constrained) in the σ3 (z) direction. There is strain in this direction but no stress, thus σ3 = 0., resulting in biaxial stress system. • Since the stress are effectively confined to one plane, this stress system is known as plane stress. Plane stress condition 263 Planestrain •Deformation (strain) often occurs in only two dimensions (parallel to 1 and 2). •3 is finite, preventing deformation (strain) in the z direction (constrained), which is known as plane strain. Example: the extrusion of a thin sheet where material in the centre is constrained in the z direction. Plane strain condition 264 263 264 14-Feb-20 133 Forming equipments Forming equipments include 1) 2) 3) Forming Dies Tools presses www.ptu.tu-darmstadt.de/.../ictmp/img00011.gif Equipments in sheet metal forming process 265 Forming machines • Using mechanical or hydraulic presses. 1) Mechanical presses Shearing machine (mechanical)- energy stored in a flywheel is transferred to the movable slide down stroke of the press. on the - quick - acting , short stroke. 2) Hydraulicpresses - slower - acting, longer stroke. Hydraulic deep drawing press 266 265 266 14-Feb-20 134 Actionsofpresses (according to number of slides, which can be operated independently of each other.) 1) Single -action press - one slide - vertical direction 2) Double -action press - two slides - the second action is used to operated the hold-down, which prevents wrinkling in deep drawing. 3) Triple -action press - two actions above the die, one action below the die. 267 Example: Press brake – single action • A single action press with a very long narrow bed. • Used to form long, straight bends in pieces such as channels and corrugated sheets. 268 267 268 14-Feb-20 135 Tooling Basic tools used with a metalworking press are the punch and the die. • Punch → A convex tool for making holes by shearing , or making surface or displacing metal with a hammer. • Die → A concave die, which is the female part as opposed to punch which is the male part. Punches and dies Die materials: • High alloy steels heat treated for the punches and dies. Punch and die in stamping 269 Compound dies www.lyons.com •Several operations can be performed on the same piece in one stroke of the press. •Combined processes and create a complex product in one shot. • Used in metal stamping processes thin sheets. of Compound die www.deltatooling.co.jp/ Transfer dies • Transfer dies are also called compounding type dies. • The part is moved from station to station within the press for each operation. Transfer die 270 269 270 14-Feb-20 136 www.bgprecision.com Schematic diagram of a die set A die set is composed of pilot 1) Punch holder which holds punch plate connected with blanking and piecing punches for cutting the metal sheet. Die block consists of die holder and die plate which was designed to give the desired shape of the product. Pilot is used to align metal sheet at the correct position before blanking at each step. Striper plate used for a) alignment of punch and die blocks b) navigate the punch into the die using harden striper inserts and c) remove the cut piece from the punch. 2) 3) 4) 271 Forming method There are a great variety of sheet metal forming methods, mainly using shear and tensile forces in the operation. • • • Shearing and blanking• • • • • Progressive forming Rubber hydroforming Stretch forming Bending and contouring Deep drawing Spinning processes Explosive forming 272 271 272 14-Feb-20 137 Progressiveforming • Punches and dies are designed so that successive stages in the forming of the part are carried out in the same die each stroke of the press. on • Progressive dies are also known as multi-stage dies. Example: progressive blanking and piercing of flat washer. washers Punch Stripper plate • The first punch is to hole of the washer. make the Die • The washer is then blanked from the strip. Strip washer the next washer. 273 Progressive die www.bestechtool.com Metal sheet used in blanking process www.hillengr.com Progressive die • Optimise the material usage. • Determining factors are 1) volume of production 2) the complexity of the shape 274 273 274 14-Feb-20 138 Rubber hydroforming •Using a pad of as a die. •A metal blank block, which is rubber or polyurethane is placed over the form fastened to the bed of a single - action hydraulic press. •During forming the rubber (placed in the retainer box on the upper platen of the press) transmits a nearly uniform hydrostatic pressure against the sheet. •Pressure ~ 10 MPa, and where higher Guerin process local pressure can be obtained auxiliary tooling. by using 275 Hydroforming www.egr.msu.edu Upper fluid chamber DrawLower fluid chamber blank material fluid Stamp hydroform ing machine setup with a fluid supplied from one side of the draw blank A drawing of hydroform ing setup with fluid supplied from to both sides of the materials. • Used for sheet forming thermoplastics. of aluminium alloys and reinforced 276 275 276 14-Feb-20 139 Bending and contouring Bendmachine (a)Three-roll bender: sometimes does not provide uniform deformation in thin-gauge sheet due to theWiper rolls midpoint of the span € localisation of the strain. Form block Often need the forth roll. (b)Wiper-type bender: The contour is formed by successive hammer blows on the sheet, which is clamped at one end against the form block. Wiper rolls must be pressed against the block with a uniform pressure supplied by a hydraulic cylinder. Clamp Clamp Tension (c)Wrapforming: The sheet is compressed against a form block, and at the same time a longitudinal stress is applied to prevent buckling and wrinkling. Ex: coiling of a spring around a mandrel 277 Bending and contouring machines Pipe bending machine www.rollfab.com.au www.diydata.com www.macri.it www.lathes.co.uk 278 277 278 14-Feb-20 140 Materials: aluminium and alloys, highSpinning processes strength - low alloy steels, copper, brass and alloys, stainless steel,• Deep parts of circular symmetry such as tank heads, television cones. (a) Manual spinning (b) Shear spinning • The metal blank is clamped against a form block, which is rotated at high speed. •The blank is progressively formed against the block, by a manual tool or by means of small-diameter work rolls. Note: (a) no change in thickness but diameter, (b) diameter equals to blank diameter but thickness stays the same. 279 Explosiveforming •Produce large parts with a relatively low production lot size. •The sheet metal blank is placed over a die cavity and an explosive charge is detonated in medium (water) at an appropriate standoff distance velocity. • The shockwave propagating from the blank at a very high from the explosion serves as a ‘friction-less punch’ 280 279 280 14-Feb-20 141 Shearing and blanking Shearing The separation of metal by the movement of two blades operated based on shearing forces. •A narrow strip of metal is severely plastically deformed to the point where it fractures at the surfaces in contact with the blades. •The fracture then propagates inward to provide complete separation. (a) Proper clearance Ragged surface (normally 2-10% thickness) Clearance Proper• • • → clean fracture surface. ragged fracture greater distortion, greater energy (b) Insufficient clearance Insufficientblurr required to separate metal. Thickness clearance(c) Excessive clearance 281 Excessive → → Maximum punch force •No friction condition. •The force required to shear a metal sheet ~ length cut, sheet thickness, shearing strength. • The maximumpunch force to produce shearing is given by  0.7uhLPmax u h L where = the ultimate tensile strength = sheet thickness = total length of the sheared edge The shearing force by making the edges of the cutting tool at an inclined angle 282 281 282 14-Feb-20 142 Blanking : The shearing of close contours, when the metal inside the contour is the desired part. Punching or piercing : The shearing of the material when the metal inside the contour is discarded. Notching : The punch removes material from the edge or corner of a strip or blank or part. 283 www.americanmachinist.com/ Parting : The simultaneous cutting along at least two lines which balance each other from the standpoint of side thrust on the parting tool. Slitting : Cutting or shearing along single lines to cut strips from a sheet or to cut along lines of a given length or contour in a sheet or workpiece. Trimming : Operation of cutting scrap off a partially fully shaped part to an established trim line. or 284 283 284 14-Feb-20 143 Shaving : A secondary shearing or cutting operation in which the surface of a previously cut edge is finished or smoothed by removing a minimal amount of stock. Ironing : A continuous thinning process and often accompanies deep drawing, i.e., thinning of the wall of cylindrical cup by passing though an ironing die. a it Fineblanking : Very smooth and square edges are produced in small parts such as gears, cams, and levers. 285 Bending • A process by which a straight length is transformed into a curved length. • produce channels, drums, tanks. 286 285 286 14-Feb-20 144 Bending The bend radius R = the radius of curvature on the concave, or inside surface of the bend. Fibres on the outer surface are strained more than fibres on the inner surface are contracted. Fibres at the mid thickness is stretched. Decrease in thickness (radius direction) at the bend to preserve the constancy of volume. R thickness bending on 287 Condition: - - - No change in thickness The neutral axis will remain at the centre fibre. Circumferential stretch on the top surface ea = shrink on the bottom surface, eb strainR 1 (2R/ h)+1 ea = −eb = Eq.1 R bend radius h thickness The minimum bend radius • For a given bending operation, the smallest bend radius can be made without cracking on the outer tensile surface. • Normally expressed in Example: a 3T bend without cracking though thickness T. multiples of sheet thickness. radius means the metal can to three times be bend the sheeta radius equal 288 287 288 14-Feb-20 145 Effect ofb/h ratio on ductility • • (2/1 ratio ratio)Stress state is biaxial Width / thickness b/h biaxialityb/h Strain, ductility Cracks occur near centre of the sheet the Effect of b/h on biaxiality and bend ductility 289 Spring back Dimensional change of the formed part after releasing the pressure of the forming tool due to the changes in strain produced by elastic recovery. Yield stress Elastic modulus Plastic Spring strain back Springback is encountered in all forming operations, but most easily occurs in bending. 290 289 290 14-Feb-20 146 For aluminium alloys and austenitic stainless steels in a number of cold-rolled tempers, approximate springback in bending can be expressed by 3 Ro Ro -3 Ro    = 4  +1 Eq.2 R f Eh Eh Where Ro Rf = the radius of curvature before release of load = the radius of curvature after release of lead and Ro < Rf Solutions: compensating the springback by bending to a smaller radius of curvature than is desired (overbending). By trial- and-error. The force Pb required to bend a length L about a radius R may be estimated from  Lh2 o Eq.3P = 2(R+ h / 2)tan 2b 291 Tube bending •Bending of tube and structural material for industry, architecture, medical, refinery. •Heat induction and hot slap bending require the heating of pipe, tube or structural shapes. •Heat Induction bending is typically a higher cost bending process and is primarily used in large diameter material. www.precision-tube-bending.com 292 291 292 14-Feb-20 147 www.dynabil.com Stretch forming •Forming by using tensile forces to stretch the material over a tool or form block. •used most extensively in the aircraft industry to produce parts of large radius of curvature. (normally for uniform cross section). •required materials with appreciable ductility. •Springback is largely eliminated Stretch forming feasible for aluminium, stainless steel, titanium. because the stress gradient relatively uniform. is 293 Stretch formingequipment Ram Ram • Using a hydraulic driven ram (normally vertical). • Sheet is gripped by two jaws at its edges. Form block is slowly raised by the ram to deform sheet• above its yield point. • The sheet is strained plastically to the required final shape. Examples: large thin panel, most complex automotive stamping involve a stretching component. 294 293 294 14-Feb-20 148 Diffuse necking (a limit to forming) In biaxial tension, the necking which occurs in uniaxial tension is inhibited if 2/1 > 1/2, and the materials then develops diffuse necking. (not visible) The limit of uniform deformation in strip loading occurs at a strain equals to the strain-hardening exponent n.  = nu Localised necking  ~ 55o for an isotropic material in pure tension • Plastic instability of a thin sheet will occur in the form of a narrow localised neck .€ sheet. followed by fracture of the • Normal strain along X’2 must be zero.  = 2nu Localised necking in a strip in tension 295 Deep drawing The metalworking process used for shaping flat sheets into cup-shaped articles. Examples: bathtubs, shell cases, automobile panels. Deep drawing of cylindrical cup a Pressing the metal blank of appropriate size into a shaped die with a punch. Before drawing After drawing 296 295 296 14-Feb-20 149 Punch • It is best done with double-action press. • Using a blank holder or a holddown ring Holddown ring •Complex interaction between metal and die depending on geometry. •No precise mathematical description can be used to represent the processes in simple terms. 297 Flange As the metal being A cup is subjected to three different types drawn, Triaxial of deformation. • Change in radius • Increase in cup wall Cup wall Punch region Biaxial Biaxial Thickness profile of drawn cup Clearance between the punch and the die > 10-20% thickness.Stresses and deformation in a section from a drawn cup • Metal in the punch region is thinned down € biaxial tensile stress. • Metal in the cup wall is subjected to a circumference strain, or hoop and a radial tensile strain. • Metal at the flange is bent and straightened as well as subjected to a tensile stress at the same time. 298 297 298 14-Feb-20 150 Redrawing •Use successive drawing operations by reducing a cup or drawn part to a smaller diameter and increased height – known as redrawing. Examples: slender cups such as cartridge case and closed- end tubes. 1) Direct or regular redrawing : 2)Reverse orindirectredrawing : smaller diameter is produced by means of a hold-down ring. The metal must be bent at the punch and unbent at the die radii see the cup is turned inside out € the outside surface becomes the inside surface, Fig (c). Better control of wrinkling and no geometrical lim itations to the use of a hold- down ring. Fig (a). Tapered die allows punch load, Fig (b). lower 299 Punch force vs.punch stroke Punch force = + +Fdeformation Ffrictional (Fironing) Fdeformation Ffrictional Fironing - varies with length of travel - mainly from hold down pressure - after the cup has reached the maximum thickness. 300 299 300 14-Feb-20 151 Drawability (deep drawing) Drawability is a ratio of the initial blank diameter (Do) to the diameter of diameter (DP) the cup drawn from the blank ~ punch Lim iting draw ratio (LDR)  LDR  Do  e   max Eq.4D p Where is an efficiency term accounting for frictional losses. Normally the averagemaximum reduction in deep drawing is ~ 50%. 301 Practical considerations affecting drawability • Die radius – should be about 10xsheetthickness. • Punch radius – a sharp radius leads to local thinning and tearing. Clearance between punch and die should be about 20- 40% > sheet thickness. • • • Hold-down pressure – about 2% of average o and u. Lubrication of die side - to reduce friction in drawing. Material properties - low yield stress, high work hardening rates, high values of strain ratio of width to thickness R. • Since the forming load is carried the side wall of the cup, failure by therefore occurs at the thinnest part. •In practice the materials always fails either at (a) the shoulder of the die and (b) the shoulder of the punch. 302 301 302 14-Feb-20 152 Practical considerations for round and rectangular shells • Different pressures (tension, compression, friction, bending) force the material into shape, perhaps with multiple successive operations. www.drawform.com Round shell •Different flow patterns at sides and corners. •Corners require similar flow as round shells while sides need simple bending. •The corner radii control the maximum draw depth. • Centre to center distance of corners  6 x • Bottom radius  corner radius corner radius Rectangular shell 303 To improve drawability •To avoid failures in the thin parts (at the punch or flange), metal in that part need to be strengthened, or weaken the metal in other parts (to correct the weakest link). •If sufficient friction is generated between punch and workpiece, more of the formingload is carried bythethicker parts. • Concerning about crystallographic texture (slip system), degree of anisotropy or strain ratio R. The dependence of limitingdraw ratio on R and work hardening rate, n 304 303 304 14-Feb-20 153 ln(w w R = o ln(ho h The plastic strain ratio R measures the normal anisotropy, which denotes high resistance to thinning in the thickness direction. ln( / )/ w) oR = Eq.5/ ) ln(ho / h) Where wo and w are the initial and final width ho and h are the initial and final thickness. But it is difficult to measure thickness on thin sheets, therefore we have ln(wo / w) R = Eq.6 ln(wL/ wo Lo ) 305 Example: A tension test on a special deep-drawing steel showed a 30% elongation in length and a 16% decrease in width. What limiting draw ratio would be expected for the steel? LL− Lo = 1.30= 0.30 Lo w Lo w− wo = 1− 0.16 = 0.84= −0.16 wowo ln(wo / w) ln(1/ 0.84) ln1.190 ln1.092 R = =1.98= = ln((w/ wo)(L/ Lo )) ln(0.84  1.30) From Fig. 20-16 Dieter page 673, the limiting draw ratio ~ 2.7 306 305 306 14-Feb-20 154 Forming limit criteria • Tensile test only provides ductility, work hardening, but it is in a uniaxial tension with frictionless, which cannot truly represent material behaviours obtained unequal biaxial stretching occurring in sheet metal forming. from • Sheetmetal formabilitytests are designed to measure the ductility of a materials under condition similar to those found in sheet metal forming. 307 Erichsen cupping test •Simple and easy. •symmetrical and equal biaxial stretching. •Allow effects of tool-workpiece interaction and lubrication on formability to be studied. •The sheet metal specimen is hydraulically punched with a 20mm diameter steel ball at a constant load of 1000 kg. •The distance d is measured in millimetres and known as Erichsen number. Results of cupping test on steel sheets. 308 307 308 14-Feb-20 155 The forming limit diagram • The sheet is marked with a close packed array of circles using chemical etching techniques. or photo printing Grid analysis (a) before (b) after deformation of sheet. •The blank is then stretched over a strain 1(%)punch, resulting in stretching of Major 120 circles into ellipses. 100 •The major and minor axes of an ellipse represent the two principal strain directions in the stamping. Failure 80 A B602 AK steel2•The percentage changes in these Safe40 strains are compared in the diagram. 1 1 20 • Comparison is done in thickness of the sheet. a given -40 -20 0 20 Minor 40 60 80 100 strain 2(%) Forming limit diagram 309 Example: A grid of 2.5 mm circles is electroetched on a blank of sheet steel. After forming into a complex shape the circle in the region of critical strain is distorted into and ellipse with major diameter 4.5 mm and minor diameter to failing in this critical region? 2.0 mm. How close is the part strain 1(%)Major 120Major strain 100 Failure 4.5−2.5 e =  0 10= 80% 80 1 2.5 A B602  AK steel2 Safe40 Minor strain 1 1 20 2.0 − 2.5 e = 10= −20% -20 0 40 60 100-40 20 Minor 802 2.5 strain 2(%)0 Forming limit diagram part is in imminent dangerThe coordinates indicate that the of failure. 310 309 310 14-Feb-20 156 Defects in formed parts www.bgprecision.com Springback problem • Edge conditions for blanking. •Local necking or thinning or buckling and wrinkling in regions of compressive stress. •Springback tolerance problems. •Cracks near the punch region in deep drawing € minimised by increasing punch radius, lowering punch load. Crack near punch region 311 •Radial cracks in the flanges and edge of the cup due to not sufficient ductility to withstand large circumferential shrinking. •W rinkling of the flanges or the edges of the cup resulting from buckling of the sheet (due to circumferential compressive stresses) € solved by using sufficient hold-down pressure to suppress the buckling. •Surface blemishes due to large surface area. EX: orange peeling especially in large grain sized metals because each grain tends to deform independently € metals. use finer grained •Mechanical fibering has little effect on formability. •Crystallographic fibering or preferred orientation may have a large effect. Ex: when bend line is parallel to the rolling direction, or earing in properties. deep drawn cup due to anisotropic Earing in drawn can aluminium.matter.org.uk 312 311 312 14-Feb-20 157 •Stretcher strains or ‘worm s’ (flamelike patterns of depressions). Associated with yield point elongation. •The metal in the stretcher strains has been strained an amount = B, while the remaining received essentially zero strain. •The elongation of the part is given by some intermediate strain A. •The number of stretcher strains increase during deformation. The strain will increase Stretcher strain in low-carbon steel. until the when the entire part is covered it has a strain equal to B. BA Solution: give the steel sheet a small cold reduction (usually 0.5-2% reduction in thickness). Ex: temper-rolling, skin-rolling to eliminate yield point.Relation of stretcher strain to stress strain curve. 313 References • Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN 0-07-100406-8. • Edwards, L. and Endean, M., Manufacturingwith materials, 1990, Butterworth Heinemann, ISBN 0-7506-2754-9. 314 313 314 14-Feb-20 158 Chapter 8 Advanced processes Subjects of interest metal forming • • • • Introduction/Objectives Superplastic forming Pressing and sintering Isostatic pressing 315 Objectives • This chapter aims to provide additional information on several techniques of metal forming processes other than those conventional process already mentioned in previous chapters. • The requirements for the process selection will be added, which are based on advantages and disadvantages of each type of non-conventional metal forming processes. 316 315 316 14-Feb-20 159 Introduction • Advanced techniques for metal forming are listed below; 1) 2) 3) Superplastic forming Pressing Isostatic and sintering pressing (hot and cold) 317 Superplastic forming • The term superplasticity describe materials that can is used to be formed to high strains without the formation of unstable tensile necks. •Require controlled conditions of appropriate temperature and strain rate, by using low force. •Produce complex shapes (3D) with essentially constant section thickness. • • • Good surface finishes. Poor creep due to small grain size. Superplastic forming Machines and dies are costly. 318 317 318 14-Feb-20 160 Femaledrape formingFemaleforming •Graphite-coated blank clamped over ‘tray’ containing heated male mould. •Air pressure forces metal into close contact with mould. •Graphite coated blank put into •Air into heated hydraulic press. pressure forces sheet close contact with mould. 319 Plug-assistedsnap back maleforming •Graphite-coated blank put into heated press. •Blank formed into a bubble. •Male mould pressed into bubble. •Air into pressure forces metal close contact with mould. 320 319 320 14-Feb-20 161 Pressing and sintering •Powder is pressed in closed dies to form a green compact which is then sintered at elevated temperature. • • • Produce 3D solid shapes for mainly metals and ceramics. Near net shape process € 100% material utilization. Automated machinery and dies are relatively costly. Sequence of operations for production of cylindrical bearing Sintering Operation 321 Sinteringofapowder compact Sintering is the "welding" together of separate powder particles into a single solid material, •Takes place below the melting point of the material, but at a temperature sufficiently high to Sintered products allow an acceptable rate of diffusion to occur. 322 321 322 14-Feb-20 162 Isostatic pressing •Powder is placed within a deformable container and subjected to hydrostatic pressure. •Produce 3D bulk solid shapes for metals and ceramics. •Allows simultaneous densification of metal powder € products have relatively low porosity. •Distortion is possible in high aspect ratio components Hot isostatic pressing (HIP). • Near net shape process € utilization. • High operating cost. 100% material process HIP products 323 HotIsostaticPressing (HIP) •Components are loaded into furnace, which is placed into pressure vessel. •Temperature and pressure are raise simultaneously and held. •Cooling is carried out as the gas is released (clean and recycled) and the furnace is removed from the pressure vessel. •Components are removed from the furnace. 324 323 324 14-Feb-20 163 Cold IsostaticPressing (CIP) • Powder is sealed in a flexible mould (or ‘bag’), of for example polyurethane and then subjected to a uniform hydrostatic pressure. CIP graphite blocks 325 References • Edwards, L. and Endean, M., Manufacturingwith materials, 1990, Butterworth Heinemann, ISBN 0-7506-2754-9. • www.designinsite.dk/ htmsider/pb0278.htm • www.twi.co.uk/j32k/ getFile/ceramics_hip.html • • www.thrive-metal.com/ product2.html • www.sv.vt.edu/.../ diffusion/apps/sinter.html • www.sti-us.net/ newdesign.htm 326 325 326