Introduction to Metal Forming Process

Metal Forming Metal Forming Large group of manufacturing processes in which plastic deformation is used to change the shape of metal workpieces (V1 = V2, S1 > S2) The tool, usually called a die, applies stresses that exceed the yield strength of the metal The metal takes a shape determined by the geometry of the die

Comparison of Crystal Structures Metal Forming Comparison of Crystal Structures (a) 절삭가공 섬유조직 (a) 소성가공

Stresses in Metal Forming Stresses to plastically deform the metal are usually compressive Examples: rolling, forging, extrusion However, some forming processes Stretch the metal (tensile stresses) Others bend the metal (tensile and compressive) Still others apply shear stresses

Metal Forming Stresses in Metal Forming

Material Properties in Metal Forming Desirable material properties: Low yield strength High ductility These properties are affected by temperature: Ductility increases and yield strength decreases when work temperature is raised Other factors: Strain rate, friction, and lubrication

Basic Types of Deforming Processes Metal Forming Basic Types of Deforming Processes Bulk deformation Rolling Forging Extrusion Wire and bar drawing Sheet metalworking Bending Deep drawing Cutting Miscellaneous processes

Bulk Deformation Processes Metal Forming Bulk Deformation Processes Characterized by significant deformations and massive shape changes "Bulk" refers to workparts with relatively low surface area‑to‑volume ratios Starting work shapes include cylindrical billets and rectangular bars

Figure. Basic bulk deformation processes: (a) rolling Metal Forming Rolling Figure. Basic bulk deformation processes: (a) rolling

Metal Forming

Figure. Basic bulk deformation processes: (b) forging Metal Forming Forging Figure. Basic bulk deformation processes: (b) forging

Metal Forming Forging FIGURE 6.14 Schematic illustrations of stages in impression-die forging. Note the formation of a flash, or excess material that subsequently has to be trimmed off.

Metal Forming Forging FIGURE 6.15 Typical load-stroke curve for impression-die forging. Note the sharp increase in load when the flash begins to form. Source: After T. Altan.

Metal Forming Open Die Forging FIGURE 6.19 (a) Schematic illustration of a cogging operation on a rectangular bar. Blacksmiths use a similar procedure to reduce the thickness of parts in small increments by heating the workpiece and hammering it numerous times along the length of the part. (b) Reducing the diameter of a bar by open-die forging; note the movements of the die and the workpiece. (c) The thickness of a ring being reduced by open-die forging.

Metal Forming Roll Forging FIGURE 6.21 Two illustrations of roll forging (cross-rolling) operations. Tapered leaf springs and knives can be made by this process using specially designed rolls. Source: After J. Holub.

Metal Forming Grain Flow ??? Grain flow lines in upsetting a solid, steel cylindrical specimen at elevated temperatures between two flat cool dies. Note the highly inhomogeneous deformation and barreling, and the difference in shape of the bottom and top sections of the specimen. The latter results from the hot specimen resting on the lower die before deformation proceeds. The lower portion of the specimen began to cool, thus exhibiting higher strength and hence deforming less than the top surface. Source: After J.A. Schey.

Metal Forming Forged Wrenches

Metal Forming

Figure. Basic bulk deformation processes: (c) extrusion Metal Forming Extrusion Figure. Basic bulk deformation processes: (c) extrusion

Metal Forming

Metal Forming Extrusion Parts

Figure. Basic bulk deformation processes: (d) drawing Metal Forming Wire and Bar Drawing Figure. Basic bulk deformation processes: (d) drawing

Metal Forming Sheet Metal Working Forming and related operations performed on metal sheets, strips, and coils High surface area‑to‑volume ratio of starting metal, which distinguishes these from bulk deformation Often called pressworking because presses perform these operations Parts are called stampings Usual tooling: punch and die

Metal Forming

Metal Forming Stamping Parts

Figure. Basic sheet metalworking operations: (a) bending Metal Forming Sheet Metal Bending Figure. Basic sheet metalworking operations: (a) bending

Metal Forming Sheet Metal Bending

Figure. Basic sheet metalworking operations: (b) drawing Metal Forming Deep Drawing Figure. Basic sheet metalworking operations: (b) drawing

Metal Forming Deep Drawing Parts

Figure. Basic sheet metalworking operations: (c) shearing Metal Forming Shearing of Sheet Metal Figure. Basic sheet metalworking operations: (c) shearing

Metal Forming Shearing of Sheet Metal > Clearance

Material Behavior in Metal Forming Plastic region of stress-strain curve is primary interest because material is plastically deformed In plastic region, metal's behavior is expressed by the flow curve: where K = strength coefficient; and n = strain hardening exponent  Flow curve based on true stress and true strain

Metal Forming Material Behavior in Metal Forming (기계재료 자료참조)

Metal Forming Flow Stress For most metals at room temperature, strength increases when deformed due to strain hardening Flow stress = instantaneous value of stress required to continue plastic deforming the material where Yf = flow stress, that is, the yield strength as a function of plastic strain

Metal Forming Average Flow Stress Determined by integrating the flow curve equation between zero and the final strain value defining the range of interest For calculation of forming pressure using the average flow stress where = average flow stress; and  = maximum strain during deformation process

Metal Forming Flow Stress & Average Flow Stress

Temperature in Metal Forming For any metal, K and n in the flow curve depend on temperature Both strength (K) and strain hardening (n) are reduced at higher temperatures In addition, ductility is increased at higher temperatures

Temperature in Metal Forming Any deformation operation can be accomplished with lower forces and power at elevated temperature Three temperature ranges in metal forming: Cold working Warm working Hot working

Metal Forming Cold Working Performed at room temperature or slightly above Many cold forming processes are important mass production operations Minimum or no machining usually required These operations are near net shape or net shape processes

Advantages of Cold Forming Metal Forming Advantages of Cold Forming Better accuracy, closer tolerances Better surface finish Strain hardening increases strength and hardness Grain flow during deformation can cause desirable directional properties in product No heating of work required

Disadvantages of Cold Forming Metal Forming Disadvantages of Cold Forming Higher forces and power required in the deformation operation Surfaces of starting workpiece must be free of scale and dirt Ductility and strain hardening limit the amount of forming that can be done In some cases, metal must be annealed to allow further deformation In other cases, metal is simply not ductile enough to be cold worked

Metal Forming Multi-step stamping

Metal Forming Warm Working Performed at temperatures above room temperature but below recrystallization temperature Dividing line between cold working and warm working often expressed in terms of melting point: 0.3Tm, where Tm = melting point (absolute temperature) for metal

Advantages of Warm Working Metal Forming Advantages of Warm Working Lower forces and power than in cold working More intricate work geometries possible Need for annealing may be reduced or eliminated

Metal Forming Hot Working Deformation at temperatures above the recrystallization temperature Recrystallization temperature = about one‑half of melting point on absolute scale In practice, hot working usually performed somewhat above 0.5Tm Metal continues to soften as temperature increases above 0.5Tm, enhancing advantage of hot working above this level

Metal Forming Hot Working

Metal Forming Why Hot Working ? Capability for substantial plastic deformation of the metal ‑ far more than possible with cold working or warm working Why? Strength coefficient (K) is substantially less than at room temperature Strain hardening exponent (n) is zero (theoretically) Ductility is significantly increased

Heat treatment conditions Metal Forming Effect of annealing (In case of sheet metal) Original material Heat treatment conditions 900 ℃, holding time 3min 800 ℃, holding time 3min 700 ℃, holding time 3min Material Y.S (MPa) T.S El (%) Hardness (Hv) STS 304-1/2H Original material 651.4 851.8 38.4 270.07 700℃ 618.5 845.7 52.5 258.76 800℃ 552.5 827.3 57.2 237.32 900℃ 272.1 691.3 60.9 155.08

Advantages of Hot Working Metal Forming Advantages of Hot Working Workpart shape can be significantly altered Lower forces and power required Metals that usually fracture in cold working can be hot formed Strength properties of product are generally isotropic No strengthening of part occurs from work hardening Advantageous in cases when part is to be subsequently processed by cold forming

Disadvantages of Hot Working Metal Forming Disadvantages of Hot Working Lower dimensional accuracy Higher total energy required (due to the thermal energy to heat the workpiece) Work surface oxidation (scale), poorer surface finish Shorter tool life

Strain Rate Sensitivity Metal Forming Strain Rate Sensitivity Theoretically, a metal in hot working behaves like a perfectly plastic material, with strain hardening exponent n = 0 The metal should continue to flow at the same flow stress, once that stress is reached However, an additional phenomenon occurs during deformation, especially at elevated temperatures: Strain rate sensitivity

Metal Forming What is Strain Rate ? Strain rate in forming is directly related to speed of deformation v (m/s) Deformation speed v = velocity of the ram or other movement of the equipment Strain rate is defined: (m/s/m or simply s-1) where = true strain rate; and h = instantaneous height of workpiece being deformed (m)

Evaluation of Strain Rate Metal Forming Evaluation of Strain Rate If deformation speed v is constant during the operation, strain rate will change as h changes. In most practical operations, valuation of strain rate is complicated by Workpart geometry Variations in strain rate in different regions of the part Strain rate can reach 1000 s-1 or more for some metal forming operations

Effect of Strain Rate on Flow Stress Metal Forming Effect of Strain Rate on Flow Stress Flow stress is a function of temperature At hot working temperatures, flow stress also depends on strain rate As strain rate increases, resistance to deformation increases This effect is known as strain‑rate sensitivity

Strain Rate Sensitivity Metal Forming Strain Rate Sensitivity C: strength coefficient, similar to K m: strain-rate sensitivity exponent cold working -0.05<m<0.05 hot working 0.05<m<0.3 superplasticity 0.3<m<0.7 Newtonian fluid m=1 Figure. (a) Effect of strain rate on flow stress at an elevated work temperature. (b) Same relationship plotted on log‑log coordinates.

Strain Rate Sensitivity Equation Metal Forming Strain Rate Sensitivity Equation where C = strength constant (similar but not equal to strength coefficient in flow curve equation), and m = strain‑rate sensitivity exponent The value of C is determined at a strain rate of 1.0 and m is the slope of the curve in the previous figure. Increasing temperature decreases the value of C and increases the value of m.

Effect of Temperature on Flow Stress ??? Metal Forming Effect of Temperature on Flow Stress ??? Figure. Effect of temperature on flow stress for a typical metal. The constant C, as indicated by the intersection of each plot with the vertical dashed line at strain rate = 1.0, decreases, and m (slope of each plot) increases with increasing temperature.  This is important in hot working because deformation resistance of the material increases so dramatically as strain rate is increased.

Observations about Strain Rate Sensitivity Metal Forming Observations about Strain Rate Sensitivity Increasing temperature decreases C and increases m At room temperature, effect of strain rate is almost negligible  Flow curve is a good representation of material behavior As temperature increases, strain rate becomes increasingly important in determining flow stress

Friction in Metal Forming In most metal forming processes, friction is undesirable: Metal flow is retarded Forces and power are increased Tooling wears faster Friction and tool wear are more severe in hot working

Friction in Metal Forming 입력에너지의 50%이상이 마찰 극복에 사용 제품의 표면조도와 치수정밀도는 마찰에 직접 관련 윤활의 변화 ⇒ 마찰변화, 재료유동의 형태를 바꿈 ⇒ 최종제품의 물성을 변화 ※ 압연의 경우는 충분한 마찰 필요

Lubrication in Metal Forming Metalworking lubricants are applied to tool‑work interface in many forming operations to reduce harmful effects of friction Benefits: Reduced sticking, forces, power, tool wear Better surface finish Removes heat from the tooling

Considerations in Choosing a Lubricant Metal Forming Considerations in Choosing a Lubricant Type of forming process (rolling, forging, sheet metal drawing, etc.) Hot working or cold working Work material Chemical reactivity with tool and work metals Ease of application Cost