Corso di Studi di Fabbricazione

Transcription

1 Corso di Studi di Fabbricazione 3a Richiami dei processi tecnologici di trasformazione FUNDAMENTAL OF METAL FORMING 1 METAL FORMING Large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces The tool, usually called die, applies stresses that exceed the yield strength of the metal The metal takes a shape determined by the geometry of the die 2

2 METAL FORMING Metal forming dominates the class of shaping operations identified as the deformation process 3 ROLLING This is a compressive deformation process in which the thickness of a slab or plate is reduced by two opposing cylindrical tools called rolls. The rolls rotate so as to draw the workpiece into the gap between them and squeeze it 4

3 ROLLED PRODUCTS MADE OF STEEL From soaking, the ingot is moved to the rolling mill, where it is rolled into one of three intermediate shapes called blooms, billets or slabs. 150 mm x150 mm 250 mm x40 mm 40 mm x40 mm 5 ROLLED PRODUCTS MADE OF STEEL Blooms are rolled into structural shapes and rails for railroad tracks. 6

4 ROLLED PRODUCTS MADE OF STEEL Slabs are rolled into plates, sheets and strips. Hot-rolled plates are used in shipbuilding, bridges, boilers, welded structures for various heavy machines, tubes pipes, and many other products. 7 ROLLED PRODUCTS MADE OF STEEL Billets are rolled into bars and rods. These shapes are the raw materials for machining, wire drawing, forging and other metalworking processes 8

5 TWO-HIGH ROLLING MILL (a) 2-high rolling mill. 9 THREE-HIGH ROLLING MILL (b) 3-high rolling mill. 10

6 FOUR-HIGH ROLLING MILL (c) four-high rolling mill. 11 CLUSTER MILL Multiple backing rolls allow even smaller roll diameters (d) cluster mill 12

7 A series of rolling stands in sequence TANDEM ROLLING MILL (e) tandem rolling mill. 13 A series of rolling stands in sequence TANDEM ROLLING MILL (e) tandem rolling mill. 14

8 FORGING In forging, a workpiece is compressed between two opposing dies, so that the die shapes are imparted to the work. Forging is traditionally a hot working process, but many types of forging are performed cold. 15 FORGING 16

9 OPEN-DIE FORGING In open die forging, the workpiece is compressed between two flat (or almost flat) dies, thus allowing the metal to flow without constraint in a lateral direction relative to the slide surface. 17 IMPRESSION-DIE FORGING In impression die forging, the die surfaces contain a shape or impression that is imparted to the workpiece during compression, thus constraining metal flow to a significant degree. 18

10 FLASHLESS FORGING In flashless forging, the workpiece is completely constrained within the die and no excess flash is produced. The volume of the starting workpiece must be controlled very closely so that it matches the volume of die cavity. 19 IMPRESSION-DIE FORGING Sequence in impression-die forging: (1) just prior to initial contact with raw workpiece, (2) partial compression, and (3) final die closure, causing flash to form in gap between die plates. 20

11 TRIMMING AFTER IMPRESSION-DIE FORGING Trimming operation (shearing process) to remove the flash after impression-die forging. 21 EXTRUSION This is a compression process in which the work metal is forced to flow through a die opening, thereby taking the shape of the opening as its own cross section 22

12 EXTRUSION Compression forming process in which work metal is forced to flow through a die opening to produce a desired cross-sectional shape similar to squeezing toothpaste out of a toothpaste tube In general, extrusion is used to produce long parts of uniform cross sections Two basic types: Direct extrusion Indirect extrusion 23 INDIRECT EXTRUSION In indirect extrusion process the die is mounted to the ram rather than at the opposite end of the container. As the ram penetrates into the workpiece, the metal is forced to flow through the clearance in a direction opposite to the motion of the ram. Since the billet is not forced to move relative to the container, there is no friction at the container walls, and the ram force is therefore lower than in direct extrusion 24

13 EXTRUSION 25 COMPLEX CROSS SECTION A complex extruded cross section for a heat sink (photo courtesy of Aluminum Company of America) 26

14 WIRE AND BAR DRAWING In this forming process, the diameter of a round wire or bar is reduced by pulling it through a die opening 27 WIRE AND BAR DRAWING Cross-section of a bar, rod, or wire is reduced by pulling it through a die opening Similar to extrusion except work is pulled through die in drawing (it is pushed through in extrusion) Although drawing applies tensile stress, compression also plays a significant role since metal is squeezed as it passes through die opening 28

15 CONTINUOUS WIRE DRAWING Continuous drawing machines consisting of multiple draw dies (typically 4 to 12) separated by accumulating drums Each drum (capstan) provides proper force to draw wire stock through upstream die Each die provides a small reduction, so desired total reduction is achieved by the series Annealing sometimes required between dies to relieve work hardening 29 CONTINUOUS TUBE DRAWING 30

16 SHEET METAL BENDING Bending involves straining of a metal sheet or plate to take an angle along (usually) straight axis 31 SPRINGBACK Increase in included angle of bent part relative to included angle of forming tool after tool is removed Reason for springback: When bending pressure is removed, elastic energy remains in bent part, causing it to recover partially toward its original shape 32

17 ROLL FORMING Continuous bending process in which opposing rolls produce long sections of formed shapes from coil or strip stock Roll forming of a continuous channel section: (1) straight rolls, (2) partial form, (3) final form. 33 ROLL BENDING Large metal sheets and plates are formed into curved sections using rolls 34

18 DRAWING In sheet metalworking, drawing refers to the forming of a flat metal sheet into a hollow or concave shape, such as a cup, by stretching the metal. A blank holder is used to hold down the blank while the punch pushes into the sheet metal. 35 DRAWING Drawing of a cup-shaped is the basic drawing operation. A blank of diameter D b is drawn into a die by means of a punch of diameter D p. The punch and die must have a corner radii given by R p and R d. If the punch and die were to have sharp corner a hole punching operation would be accomplished rather than a drawing operation. 36

19 DEFECTS IN DRAWING Wrinkling in the flange, that consists of a series of ridges that form radially in the undrawn flange of the workpart due to compressive buckling Wrinkling in the wall, if and when the flange is drawn into the cup, these ridges appear in the vertical wall. 37 DEFECTS IN DRAWING Tearing is an open crack in the vertical wall due to high stresses that cause thinning and failure of the metal in this location Earing that is caused by an anisotropy of the material Surface scratches that can occur if the punch and die are not smooth or if lubrication is insufficient 38

20 SHEARING OF SHEET METAL This process is somewhat out-of-place in our list of deformation processes, because it involves cutting rather than forming. A shearing operation cuts the workpiece using a punch and a die. Although it is not a forming process, it is included here because it is necessary and very common operation in sheet metalworking 39 QUESTIONS? 40