Sheet Metal Forming Technology

Transcription

1 Sheet Metal Forming Technology Dr. N S Mahesh 1

2 Session Objectives At the end of the session the delegates should have understood Sheet metal cutting and bending Significance of Forming Limit Diagram (FLD) The different technologies of tube and sheet hydroforming Other forming processes 2

3 Introduction Products made by sheet metal forming processes are all around us; they include metal desks, file cabinets, appliances, car bodies, aircraft fuselages, and beverage cans. Sheet forming dates back to 5000 B.C. when household utensils and jewelry were made by hammering and stamping gold, silver, and copper. Sheet metal = 0.4 (1/64) to 6 mm (1/4in) thick Plate stock > 6 mm thick 3

4 Sheet Metal Forming PEMP Sheet metal forming is a process that 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. High rate of production and formability is determined by its mechanical properties 4

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6 Outline of Sheet-Metal Forming Processes 6

7 Classification of sheet metal parts (based on contour) 1) Singly curved parts 2) Contoured flanged parts, i.e., parts with stretch flanges and shrink flanges. 3) Curved sections. 4) Deep-recessed parts, i.e., cups and boxes with either vertical or sloping walls. 5) Shallow-recessed parts, i.e., dish shaped, beaded, embossed and corrugated parts. 7

8 Classification of sheet metal forming (based on operations) 8

9 Sheet-metal Characteristics PEMP Elongation the capability of the sheet metal to stretch without necking and failure. Yield-point elongation Lüeder s bands on Low-carbon steels and Al-Mg alloys.lüder s bands can be eliminated by cold-rolling the thickness by %. Anisotropy Crystallographic and mechanical fibering anisotropy Grain Size effect on mechanical properties Residual Stress, Springback and Wrinkling Testing method Cupping test Forming Limit Diagram 9

10 Bending and contouring PEMP (a) Three-roll bender: sometimes does not provide uniform deformation in thin-gauge sheet due to the midpoint of the span localisation of the strain. 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. (c) Wrap forming: 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. 10

11 Bending and Contouring Machines PEMP 11

12 Spinning Processes Deep parts of circular symmetry stainless steel, such as tank heads, television cones. Materials: aluminum and alloys, high strength - low alloy steels, copper, brass and alloys, 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. 12

13 Shearing The separation of metal by the movement of two blades operated based on shearing forces. Shearing and blanking PEMP 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. Clearance (normally 2-10% thickness) Proper -- clean fracture surface. Insufficient -- ragged fracture surface. Excessive -- greater distortion, greater energy required to separate metal. Thickness Clearance 13

14 Shearing action in metal 14

15 Clearance Figure (a) Effect of the clearance, c, between punch and die on the deformation zone in shearing. As the clearance increases, the material tends to be pulled into the die rather than be sheared. In practice, clearances usually range between 2% and 10% of the thickness of the sheet. (b) Microhardness (HV) contours for a 6.4-mm (0.25-in) thick AISI 1020 hot-rolled steel in the sheared region. 15

16 Shearing Figure (a) Schematic illustration of shearing with a punch and die, indicating some of the process variables. Characteristic features of (b) a punched hole and (c) the slug. 16

17 Punch force The force required to punch is, basically, the product of the shear strength of the sheet metal by the area sheared. Friction between the punch and the work piece can, however, increase this force substantially. The medium punch force, F, can be estimated from the equation: F = 0.7TL(UTS) where T is sheet thickness, L is the total length sheared (such as the perimeter of a hole ), and UTS is the ultimate tensile strength of the material. 17

18 Shearing Operations Die cutting is a shearing process that consists of the following operations Perforating: punching a number of holes in a sheet. Parting: shearing the sheet into two or more pieces Notching: removing pieces (or various shapes) from the edges; and Lancing: leaving a tab without removing any material. PEMP Figure (a) Punching (piercing) and blanking. (b) Examples of various shearing operations on sheet metal. 18

19 Fine blanking Very smooth and square edges can be produced by fine blanking PEMP (a) (b) Figure (a) Comparison of sheared edges produced by conventional (left) and by fine-blanking (right) techniques. (b) Schematic illustration of one setup for fine blanking. 19

20 Fine blanked parts used in the automobile industry Fine blanked components in the automatic transmission of a passenger car 20

21 Processes and Equipment In mass production, punches and dies made of tool steel or sintered tungsten carbide (WC) are used. Complex geometries can be created in compound dies, while several sheet forming operations are sequentially performed in progressive dies. 21

22 Forming Equipments PEMP 1) Forming presses 2) Dies 3) Tools Forming machines 1) Mechanical presses - energy stored in a flywheel is transferred to the movable slide on the down stroke of the press. -quick - acting, short stroke.. 2) Hydraulic presses - slower - acting, longer stroke 22

23 Actions of presses PEMP (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 operate the hold-down, which prevents wrinkling in deep drawing. 3) Triple - action press two actions above the die, one action below the die. 23

24 Example: Press brake single action PEMP A single action press with a very long narrow bed. Used to form long, straight bends in pieces such as channels and corrugated sheets. 24

25 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. Punch and die in stamping Punches and dies Die materials: High alloy steels heat treated for the punches and dies. 25

26 Compound dies 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 of thin sheets. Transfer dies Transfer dies are also called compounding type dies. The part is moved from station to station within the press for each operation. PEMP 26

27 Compound and Progressive Die PEMP (a) (b) (c) (d) Schematic illustrations: (a) before and (b) after blanking a common washer in a compound die. Note the separate movements of the die (for blanking) and the punch (for punching the hole in the washer). (c) Schematic illustration of making a washer in a progressive die. (d) Forming of the top piece of an aerosol spray can in a progressive die. Note that the part is attached to the strip until the last operation is completed. 27

28 Progressive forming Punches and dies are designed so that successive stages in the forming of the part are carried out in the same die on each stroke of the press. Progressive dies are also known as multistage dies. Example: progressive blanking and piercing of flat washer. washers PEMP The strip is fed from left to right. The first punch is to make the hole of the washer. The washer is then blanked from the strip. The punch A is piercing the hole for the next washer. 28

29 Metal sheet used in blanking process Progressive die Optimize the material usage. Determining factors are 1) volume of production 2) the complexity of the shape 29

30 Sheet metal Bending Bending is a manufacturing process by which metal can be deformed by plastically deforming the material and changing its shape. The material is stressed beyond its yield strength but below its ultimate tensile strength. A process by which a straight length is transformed into a curved length. produce channels, drums, tanks Principle Types of Bending Air bending Bottoming Coining 30

31 Springback PEMP Dimensional change of the formed part after releasing the pressure of the forming tool due to the changes in strain produced by elastic recovery. Springback is encountered in all forming operations, but most easily occurs in bending 31

32 Air Bending 32

33 Air bending 33

34 Bottoming 34

35 Coining 35

36 Other common types of bending V-bending 36

37 U-die bending 37

38 Wiping die 38

39 Rotary bending 39

40 Hydraulic flexible forming process 40

41 Hydroforming Hydroforming is a manufacturing process where liquid pressure is used to form complex shapes Rolled sheet steels and extruded parts are used to form complex parts using hydroforming Advantages include weight reduction due to part consolidation, reduced investment in tooling, and reduced number and complexity in production steps 41

42 a b PEMP c d e F axial P F axial f a. Raw tube is loaded into hydroforming dies b. Hydroforming press closes c. The sealing rods engage the part, seal the ends and fill it with water d. Pressure inside the part increases e. The sealing rods push the tube into the die (endfeed) and the internal pressure is ramped to its maximum value. The hydroformed part takes on the shape of the die. f. Final hydroformed part is removed 42

43 Hydroforming -- exhaust manifold 43

44 HYDROFORMED ENGINE CRADLE Rover 75 A. Roof Headers B. Instrument Panel Supports C. Radiator Supports D. Engine Cradles E. Roof Rails F. Frame Rails. In addition: Cross Car Beams Headers A,B,C Pillars Seat Frames Roll Bars Control Arms 44

45 Automotive Hydroformed Parts Automotive radiator supports, Instrument panel beams, Catalytic converter cones and exhaust components, cross members and engine cradles are among the parts currently being manufactured with hydroform technology. 45

46 Hole punching, notching and bending during hydroforming 46

47 Stretch Forming Stretch forming is widely used in producing automotive body panels. Unlike deep drawing, the sheet is gripped by a blank holder to prevent it from being drawn into the die. It is important that the sheet can deform by elongation and uniform thinning. During stretch-forming, the steel sheet is subjected to a mixture of elastic and plastic deformation. When the sheet is unloaded, the plastic component of deformation is retained. The elastic component however attempts to neutralize itself; this recovery of deformation is known as springback. 47

48 Stretch forming PEMP 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 because the stress gradient is relatively uniform. Stretch forming feasible for aluminum, stainless steel, titanium 48

49 Stretch forming equipment PEMP 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. 49

50 Deep drawing PEMP The metalworking process used for shaping flat sheets into cupshaped articles. Pressing the metal blank of appropriate size into a shaped die with a punch Examples: bathtubs, shell cases, automobile panels. 50

51 Deep drawing It is best done with double-action press. Using a blank holder or a 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. 51

52 As the metal being drawn, Change in radius Increase in cup wall 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. 52

53 Redrawing Use successive drawing operations by reducing a cup or drawn part to a smaller diameter and increased height known as redrawing. PEMP Examples: slender cups such as cartridge case and closed end tubes. 1) Direct or regular redrawing : 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 Fig (a). Tapered die allows lower punch load, Fig (b). 2) Reverse or indirect redrawing : the cup is turned inside out the outside surface becomes the inside surface, Fig (c). Better control of wrinkling and no geometrical limitations to the use of a hold down ring. 53

54 Formability Formability is an important property for strip steels used for beverage cans, automotive panels, domestic appliance casings, etc. Unfortunately, "formability" cannot be simply defined using a single material property, but is dependent on the particular forming process (e.g. deep-drawing, stretch forming, etc.) and on the sheet thickness. 58

55 The most appropriate measure of formability for stretch forming is the strain hardening exponent, or n value. σ = kε n where k is a constant. A high value of n is desired if the strip is to show good stretch formability. Hot rolled strip steels have values of n ~ 0.1, whereas cold-reduced Al treated low carbon strip steels have n values of

56 Circle Grid Analysis The plastic deformation during forming can be measured by circle grid analysis (CGA). A sheet of metal is prepared for CGA by etching a circle grid onto the surface. Plastic deformation in the steel during the forming operation causes the circles to deform into ellipses. The amount of plastic strain at each circle can be calculated from the major and minor diameters of the ellipses. The major strain is always positive, the minor strain can be positive or negative (or zero). 60

57 Nakazima Tests and Forming Limit Diagrams CGA is used in the Nakazima test to construct forming limit curves. These give us useful information about which combinations of major and minor strains are likely to result in failure for a material of a given thickness 61

58 Nakazima Tests and Forming Limit Diagrams (cont) PEMP A series of blank test pieces of different width are etched with a grid of circle The sheets are then deformed in a hemispherical shell test until splitting occurs 62

59 Nakazima Tests and Forming Limit Diagrams (cont) PEMP The narrowest strip (A) has deformed uniaxially whilst the wide strip (C) has deformed biaxially. Strips of intermediate width will deform under a range of stress states 63

60 Nakazima Tests and Forming Limit Diagrams (cont) PEMP The dimensions of the unbroken ellipses closet to the splits are measured and the major and minor strains calculated 64

61 These values are then plotted on a graph of major strain on the y-axis, against minor strain on the x-axis Values from samples of intermediate widths are also added A curve is then drawn through the data points FLC At strain values above this curve, the material for this thickness will fail 65

62 A parallel curve drawn 10% below the FLC is usually given. For strains below this, the material will be safe from failure Strain levels within the marginal zone should normally be avoided as small variations in steel stock, lubrication conditions etc could lead to the possibility of splitting 66

63 Factors Affecting Stretch-Formability All forming limit curves for the grades of steel used in automotive bodies have essentially the same shape. The difference is their vertical position on the diagram, which is determined by the strain hardening exponent, n, and thickness of the steel. Steels with a high FLD 0 have a better formability than those with a low FLD 0. PEMP What would be the consequence on formability by using thinner, higher strength steels with a lower n value? 67

64 Forming Limit Diagram for a Car Door Panel A car door panel is shaped by stretch forming. Major and minor strains at different points across the surface is calculated using CGA 68

65 Forming Limit Diagram for a Car Door Panel (contd) PEMP The strains are plotted on the FLD at different points across the surface using CGA. All the points on the FLD fall within the safe area, and no failure (splitting or necking) is expected 69

66 Forming Limit Diagram for a Car Door Panel (contd) PEMP The material (thickness, n value) and process variables (lubrication, die geometry etc) are changed, and some of the points now fall above the FLD. In this case splitting would expect to occur 70

67 Forming limit diagram for different materials PEMP 71

68 Defects in formed parts PEMP Springback problem Crack near punch region 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. 72

69 Defects in formed parts Radial cracks in the flanges and edge of the cup due to not sufficient ductility to withstand large circumferential shrinking. Wrinkling 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 use finer grained metals. 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 deep drawn cup due to anisotropic properties. PEMP Earing in drawn can 73

70 Defects in formed parts PEMP Stretcher strain in low-carbon steel Stretcher strains or worms (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 until the when the entire part is covered it has a strain equal to B. Solution: give the steel sheet a small cold reduction (usually 0.5-2% reduction in thickness). Ex: temper-rolling, skin-rolling to eliminate Relation of stretcher strain to stress yield point. Relation of stretcher strain to stress strain curve. 74

71 Summary The session covered following aspects of sheet metal forming Sheet metal cutting and bending- types and methods Dies for sheet metal punching, blanking etc., The different technologies of tube and sheet hydroforming Significance of Forming Limit Diagram (FLD) Case studies on forming limit of metals Defects in formed parts Thank you 75