2. Powder Metallurgy, Forming, Machining, and Grinding
Chapter 26
Powder Metallurgy, Forming, Machining, and Grinding
Powder Metallurgy • Primary Forming Processes • Secondary Forming Processes • Machining • Grinding

3. Powder metallurgy (P/M) consists of several steps.
The process of making components by powder metallurgy consists of mixing, compacting, sintering, and secondary operations. Additional steps are employed in specialized cases. See Figure 26-1.

4. Compacting consolidates and densifies the component for transportation to the sintering furnace.
Compacting consists of automatically feeding a controlled amount of mixed powder into a precision die, after which it is compacted. See Figure Compacting is usually performed at room temperature. Pressures range from 10 tons per square inch (tons/in2) (138 MPa) to 60 tons/in2 (827 MPa), or more.

5. Cold isostatic pressing is performed at room temperature with liquid as the pressure medium. Hot isostatic pressing is performed at elevated temperature with gas as the pressure medium.
Special P/M operations include cold isostatic compacting (cold isostatic pressing) and hot isostatic compacting (hot isostatic pressing, or HIP). See Figure These special P/M operations apply high pressures simultaneously from all directions on the P/M compact. The metal powder is placed in a flexible mold or container and immersed in a fluid bath or gas within a pressure vessel. The fluid or gas is put under high pressure, which compacts the metal powder. Isostatically compacted components are characterized by their uniform, high density.

6. A macroetched section through a forging indicates that the grain flow follows the contour of the component, which often maximizes strength in the direction of greatest operating stress.
Forging is the process of working metal to the desired shape by impact or pressure. The process is performed using hammers, upsetters, presses, rolls, and related equipment. The principal advantage of forging is that it orients the grain flow to the contour of the component. See Figure This often provides the highest strength in the direction of greatest stress. The higher strength-to-weight ratio achieved results in the use of smaller or lighter components.

7. In impression die forging, the workpiece is shaped within the cavities of two dies that come together to completely enclose it.
Impression die forging is the shaping of hot metal within the cavities or walls of two dies that come together to completely enclose the workpiece. See Figure The impression for the forging can be entirely in either die or divided between the top and bottom dies. As the deformation continues, a small amount of metal begins to flow outside the die impression, forming flash. The thin flash cools rapidly, creating a pressure increase inside the workpiece, which assists the flow of metal into the unfilled portion of the impression.

8. Roll forging is used to produce components with long, symmetrical sections, often with a slight taper, by forcing the workpiece between two matched and grooved rolls that rotate in opposite directions.
Roll forging is the shaping of stock between two driven rolls that rotate in opposite directions and have one or more matched sets of grooves in the rolls. See Figure The stock enters the rolls when the grooves rotate into position. Roll forging is used to produce components with long, tapered, symmetrical sections. Crankshafts and connecting rods are frequently roll forged prior to being finish formed.

9. Upset forging is used to gather a large amount of material at one end of a bar, such as in the forming of a wrench socket.
Upset forging is the increase in cross section due to an applied pressure during fabrication. It is performed on a horizontal machine into which bar stock is inserted between an upsetting head and a fixed anvil plate.
The bar is held in place while the upsetting head pushes the end against an anvil plate, flattening it and developing maximum isotropy. See Figure Upset forging can be performed hot or cold.

10. Ring rolling is the shaping of seamless rings by reducing the cross section and increasing the circumference of a donut-shaped blank.
Ring rolling is the shaping of seamless rings by reducing the cross section and increasing the circumference of a heated, donut-shaped blank between two rotating rolls. See Figure The ring may be of any desired cross section. Seamless rings are produced in diameters that range from several inches to over 20 ft. Ring rolling is less expensive than closed die forging because there is less waste material and the ring is much closer to the finished shape. Ring rolling is often used to make flanges.

11. Rotary forging requires significantly less force compared with other forging processes.
Rotary forging is a process in which the workpiece is subjected to a combined rolling and pressing action between a bottom platen and a swiveling upper die with a conical working face. The cone axis is inclined so that the narrow sector in contact with the workpiece is parallel to the lower platen. See Figure As the cone rotates about its apex, the contact area also rotates. This causes the workpiece to be compressed by the rolling action.

12. Radial forging is used to produce components having a circular cross section.
Radial forging is a process using two or more moving dies or anvils for reducing the cross section of round billets, bars, and tubes. See Figure The workpiece is continuously rotated between the blows of the die.

13. Rolling is described by the roll design and roll configuration.
Rolling is the reduction of the cross-sectional area of metal stock, or the general shaping of metal products through the use of metal rolls. Rolling is described by the roll design and the roll configuration. See Figure The principal rolling processes are hot rolling and cold rolling. Hot rolling is the most common method of refining the cast structure of ingots and billets to make primary shapes. Cold rolling is most often a secondary forming process that is used to make bar, sheet, and strip. In softer metals, foil is also produced by cold rolling. The main reasons for cold rolling are to obtain a good surface finish and to improve mechanical properties. The machinability of steel improves with cold rolling. For this reason, cold-rolled bar stock is widely used for fast automatic-machining operations.

14. The primary methods of extrusion are forward extrusion and backward extrusion.
Extrusion is the conversion of ingots or billets into lengths of uniform cross section by forcing the metal to flow plastically through a die by means of a ram. The primary methods of extrusion are forward and backward extrusion. See Figure

15. Fast ram speeds and short strokes are used in impact extrusion, and, because of the impulsive force applied, high production rates are possible.
Impact extrusion is a form of cold extrusion that is performed backward, forward, or by a combination of the two processes. See Figure Fast ram speeds and short strokes are used in impact extrusion. High production rates are possible because of the impulsive force applied. Impact extrusion is used for nonferrous metals that have low melting points and good ductility. Applications of impact extrusion include the production of collapsible tubular containers (for example, toothpaste and artists’ paint tubes), battery cases, cartridge cases, and beverage cans.

16. The neutral axis is the boundary line between tensile and compressive stresses in bending.
Tube, pipe, and bar bending processes involve stressing the material beyond its yield strength but below its ultimate tensile strength. This is achieved by clamping the workpiece with the tooling while applying a bending force sufficient to cause a permanent set. When the applied force exceeds the yield strength of the material, the outside radius of the workpiece is stretched and the inside radius is compressed. The neutral axis is the boundary line between the tensile and compressive stresses in a workpiece. See Figure

17. Bending methods include draw bending, compression bending, and press bending.
Alloys that work harden rapidly have more tendency toward springback, which can be overcome by slight overbending. Springback is the tendency of a component to partially return to its original shape after bending, even after being plastically deformed. Bending methods include draw bending, compression bending, and press bending. See Figure

18. Roll bending is used to curve plate, sheet, and bars into cylinders or cylindrical segments using machines with two or three rolls.
Roll bending is the curving of plate, sheet, bars, and sections into cylinders or cylindrical segments by means of rolls. During roll bending, the material thickness does not change. Roll bending machines contain two or three rolls that rotate or bend the metal as it passes between them. See Figure A special version of the two-roll machine is used to form truncated cones. Roll bending is used to form curved shapes for the fabrication of tanks, pressure vessels, hoppers, bins, appliances, and ordinance components.

19. Spinning is a process of forcing disks or tubing into cones, disk shapes, hollow cylinders, and other circular shapes by combined forces of rotation and pressure.
Spinning is a process of forming disks or tubing into cones, dish shapes, hemispheres, hollow cylinders, and other circular shapes by the combined forces of rotation and pressure. Spinning does not result in any change in thickness. The operation is achieved by forcing the workpiece to conform to a shaped mandrel that rotates concentrically with the component. See Figure

20. Stretch forming is a process for forming sheet metal by applying tension to it and then wrapping it around a die of the desired shape.
Stretch forming is a process for forming sheet metal by applying tension, or stretch, and then wrapping it around a die of the desired shape. See Figure Stretch forming is used most extensively in the aircraft industry to produce components of large radius of curvature, frequently with double curvature.

21. Superplastic forming enables sheet metal to be deformed into complex shapes under relatively low pressure.
Superplastic materials include zinc, aluminum, titanium, steels, and duplex stainless steels. Superplastic forming of sheet metal is similar to vacuum forming of plastic sheet. See Figure Parts manufactured by superplastic forming include cash register housings, luggage compartments in trains, and complex aircraft component shapes.

22. In metal stamping operations, sheet metal is formed into the desired shape by the punch and the die.
The stamping of components from sheet metal consists of a wide variety of operations that cut or shape metal through deformation by shearing, punching, drawing, bending, etc. The workpiece is pressed into a die by a punch to form the desired shape. See Figure Production rates are high, and secondary machining operations are not usually necessary. Stampings are made from low-carbon steel, aluminum alloys, copper alloys, and other nonferrous metals. Stamping processes are widely used to make components for all kinds of equipment.

23. Cold heading is used to force metal to cold flow into enlarged sections by endwise squeezing.
Cold heading is the forcing of metal to cold flow into enlarged sections by endwise squeezing. See Figure It is similar to upset forging, which does much the same type of hot work on larger pieces and harder-to-work materials. Typical cold headed components are tacks, nails, rivets, screws, and bolts under 1" in diameter, and a large variety of machine components, such as small gears with stems.

24. In rotary swaging, tapered dies open and shut rapidly, as much as several thousand times a minute, as the workpiece is fed in.
Swaging is a squeezing process in which the material flows perpendicular to the applied force. Rotary swaging is the main swaging operation. Rotary swaging is a method of reducing the diameter of rods, bars, or tubes. A pair of tapered dies open and shut quickly while the workpiece is fed in rapidly as it rotates. The dies, which are inserted into slots in a spindle, are rotated and forced together repeatedly by the rollers around the periphery as much as several thousand times a minute. See Figure

25. Sizing, coining, and hobbing are finishing operations that change metal thickness and configuration by squeezing and working the metal beyond its yield point.
Sizing, coining, and hobbing are squeezing processes used for finishing. They change metal thickness and configuration by squeezing and working the metal beyond the yield point. See Figure

26. Thread rolling is the production of threads by rolling the workpiece between two grooved die plates and is most commonly used for making screw threads.
Thread rolling is the development of threads by rolling the workpiece between two grooved die plates or between rotating grooved circular rolls. See Figure The workpiece is depressed to the root (the root diameter of the thread) and raised to fill the crest of the thread. Thread rolling is most commonly used for making screw threads.

27. Ironing is a process for smoothing and thinning the wall of a shell or cup.
Ironing is a process for smoothing and thinning the wall of a shell or cup by forcing the component through a die with a punch. See Figure Cold working is severe, and annealing is often necessary between ironing steps.

28. The cutting tool is characterized by the rake angle and the relief angle.
In traditional machining processes, the cutting tool is driven through the component to remove chips from it by a shearing process and leave geometrically true surfaces. The cutting tool is characterized by two angles: the relief (a) angle and the rake (γ) angle. See Figure A relief angle is the angle that provides clearance between a cutting tool and a workpiece. A rake angle is the angle between a cutting tool face that contacts the chip and the vertical. An increase in rake angle increases the shear angle between the chip and the tool, decreasing the forces necessary to deform the workpiece but, at the same time, decreasing mass and strength of the tool point.

29. A lathe is a basic feature of any machine shop and performs a variety of turning functions.
Turning is a machining process for generating cylindrical forms by removing metal with a single-point cutting tool moving parallel to the axis of rotation of the workpiece. See Figure Turning is performed on a lathe. The rated size of a lathe indicates the largest diameter and length of workpiece it will handle. Thus a 14" diameter lathe will swing a 14" diameter workpiece.

30. Trepanning is a machining process for producing a circular hole or groove in solid stock, or a disk, cylinder, or tube from solid stock.
Trepanning is a machining process for producing a circular hole or groove, disk, cylinder, or tube from solid stock by a cutting tool revolving around a center. See Figure Trepanning is used for producing disks, large shallow through holes, circular grooves, and deep holes.

31. Milling is the removal of material from a workpiece by a rotary tool with teeth that engage the workpiece as it moves past the tool.
Milling is a machining process that removes material from a workpiece by the use of a rotary tool with teeth that engage the workpiece as it moves past the tool. Many sizes and types of cutters are used for the large variety of work that is done by milling. See Figure

32. Broaching is cutting with a bar that has a series of teeth on one face that is pushed or pulled over the surface of the workpiece.
Broaching is a machining process in which a bar that has a series of teeth on one face removes increasing amounts of material from the start to finish of the bar. The bar is pushed or pulled over the surface of the workpiece. See Figure Each tooth takes a thin slice from the surface. The entire cut is made in single or multiple passes of the broach over the workpiece to obtain the required surface contour. Internal or tunnel broaching is broaching of inside surfaces using several broach inserts. Typical internal broaching operations are the sizing of holes and cutting of serrations, straight or helical splines, gun rifling, and keyway shaping.

33. Electrical discharge machining is the removal of metal by rapid spark discharge between different polarity electrodes.
Electrical discharge machining (EDM) is the removal of metal by rapid spark discharge between two electrodes, one the workpiece and the other the tool, with different polarities. The electrodes are separated by a gap distance of mm to mm (0.0005" to "). This gap is filled with dielectric fluid and metal particles that are melted, partially vaporized, and expelled from the gap. See Figure Electrical discharge wire cutting is a form of EDM in which the electrode is a wire. EDM is a means of shaping hard metals and forming deep and complex-shaped holes in electrically conducting materials.

34. Electrochemical machining is the controlled removal of metal by anodic dissolution in an electrolytic cell in which the workpiece is the anode and the tool is the cathode.
Electrochemical machining (ECM) is controlled metal removal by the corrosion mechanism of anodic dissolution. Direct current passes through a flowing film of electrically conductive solution that separates the workpiece (anode) from the electrode-tool (cathode). See Figure The gap between the anode and cathode is a few thousandths of an inch. ECM cuts conductive materials of any hardness and is not confined to holes and cavities.

35. Chemical milling is the controlled dissolution of a workpiece surface by contact with chemical reagents varying in type and strength.
Chemical milling is the controlled dissolution of material by contact with chemical reagents of varying types and strengths, depending upon the particular alloy being machined. See Figure Electrochemical milling uses a low current to assist the dissolution process. The workpiece is first cleaned and areas that must not be machined are masked. The workpiece is then immersed in a tank of chemical solution. For example, caustic soda is used for aluminum. Metal is removed at a rate of about mm/min. (0.001 in./min.) when the proper concentration of chemical is used. After the required amount of metal is removed, the workpiece is removed, washed, and the masking taken off. Sometimes a mask is reapplied for a second chemical milling cut.

36. Grinding wheels are marked with symbols that designate their properties.
Grinding wheels are marked with symbols that designate their properties. See Figure The principal abrasive materials are aluminum oxide and silicon carbide. The grain size of the wheel determines the approximate surface roughness that can be obtained on the workpiece. Wheels with finer grain size usually produce surfaces having lower roughness with sacrifice of stock removal capability. Very smooth surfaces require abrasive grain sizes of 220 and finer.