Module 8 Overview of processes 1

Module 82 Metal forming Principle of the process Structure Process modeling Defects Design For Manufacturing (DFM) Process variation

Module 8 Principle of Metal Forming 3

Module 84 Metal Forming Metal forming includes a large group of manufacturing processes in which plastic deformation is used to change the shape of metal work pieces Plastic deformation: a permanent change of shape, i.e., the stress in materials is larger than its yield strength Usually a die is needed to force deformed metal into the shape of the die

Module 85 Metal with low yield strength and high ductility is in favor of metal forming One difference between plastic forming and metal forming is Plastic: solids are heated up to be polymer melt Metal: solid state remains in the whole process - (temperature can be either cold, warm or hot) Metal Forming

Module 86 Metal forming is divided into: (1) bulk and (2) sheet Metal Forming Bulk: (1) significant deformation (2) massive shape change (3) surface area to volume of the work is small Sheet: Surface area to volume of the work is large

Module 87 Bulk deformation processes Rolling Forging Extrusion Drawing Traditionally Hot

Module 88 Sheet deformation processes (Press working/ Stamping) Bending Drawing Shearing Actually Cutting

Module 89 In the following series of lecture, we discuss: 1.General mechanics principle 2.Individual processes: - mechanics principles - design for manufacturing (DFM) rules - equipment

Module General mechanics principle The underlying mechanics principle for metal forming is the stress-strain relationship; see Figure 1. Figure 1

Module 811 True strain: Instantaneous elongation per unit length of the material L0: the initial length of a specimen L: the length of the specimen at time t the true strain at time t True Stress: Applied load divided by instantaneous value of cross-section area

Module 812 In the forming process we are more interested in the plastic deformation region (Figure 1) Plastic deformation region

Module 813 The stress-strain relationship in the plastic deformation region is described by Where K= the strength coefficient, (MPa)  = the true strain, σ=the true stress n= the strain hardening exponent, The flow stress (Y f ) is used for the above stress (which is the stress beyond yield) Called FLOW CURVE

Module 814 As deformation occurs, increasing STRESS is required to continue deformation (shown in curve) Flow Stress: Instantaneous value of stress required to continue deforming the material (to keep metal “flowing”) FLOW STRESS

Module 815 For many bulk deforming processes, rather than instantaneous stress, average stress is used (extrusion) The average flow stress can be obtained by integrating the flow stress along the trajectory of straining, from zero to the final strain value defining the range of interest AVERAGE FLOW STRESS Average flow stress Max. strain during deformation Strength Coefficient Strain hardening exponent

Module 816 Example 1: Determine the value of the strain-hardening exponent for a metal that will cause the average flow stress to be three- quarters of the final flow stress after deformation. According to the statement of the problem, we have of

Module 817 The above analysis is generally applicable to the cold working, where the temperature factor is not considered. The metal forming process has three kinds in terms of temperature: (1) cold, (2) warm, (3) hot In the case of warm and hot forming, the temperature factor needs to be considered, in particular Temperature up  The (yield) strength down and ductility up

Module 818 Strain rate (related to elevated temperatures) - Rate at which metal is strained in a forming process - In the hot forming or warm forming, the strain rate can affect the flow stress h Speed of deformation (could be equal to velocity of ram) Instantaneous height of work-piece being deformed h Flow stress Strain Rate

Module 819 where C  strength constant m  strain-rate sensitivity exponent C and m are determined by the following figure which is generated from the experiment Strength coefficient but not the same as K

Module 820

Module 821 C and m are affected by temperature Temperature Up C Down m Up

Module 822 Even in the cold work, the strain rate could affect the flow stress. A more general expression of the flow stress with consideration of the strain rate and strain is presented as follows: A is a strength coefficient, a combined effect of K, C All these coefficients, A, n, m, are functions of temperature

Module 823 Example 2: A tensile test is carried out to determine the strength constant C and strain-rate sensitivity exponent m for a certain metal at 1000 o F. At a strain rate = 10/sec, the stress is measured at 23,000 lb/in2; and at a strain rate = 300/sec, the stress=45,000 lb/in2. Determine C and m 23000=C(10)^m 45000=C(300)^m From these two equations, one can find m= Solution: