Fundamentals of Metal Forming

Bulk deformation vs Sheetmetal working Fundamentals of Metal Forming Bulk deformation vs Sheetmetal working Bulk deformation * Low surface area to volume ratios * Operations increase surface area to volume ratios Operations are: rolling extrusion forging drawing This group of machines - which are over a half mile long - roll the slab into sheet steel using tremendous pressure. Aluminum extrudate – a simple angled shape. The die shown is more complex.

Fundamentals of Metal Forming Bulk deformation Rolling characteristics * compressive deformation * 2 or more rolls * strong frictional relationship Extrusion characteristics * compressive deformation * force metal through die opening * strong frictional relationship * hot and cold working process

Fundamentals of Metal Forming Sheet metalworking Characterized by: * large surface area to volume ratios * little ratio change after process * machines called presses * parts called stampings * tools are punch and die Operations: bending drawing shearing

Material Behavior – Flow Stress = K en Concept of flow stress, Yf – the instantaneous stress required to deform the material Yf = K en Process force considerations: * during compression, determine instantaneous force from Yf * max force is often all that is required, typically at end of stroke * force analysis may be based on average stresses and strains Average flow stress = Yf = K en /(1 + n)

Fundamentals of Metal Forming Temperature and Metal Forming K and n depend on working temperature, characterized by 3 ranges: Cold working Warm working Hot working

Fundamentals of Metal Forming Fundamentals of Metal Forming Temperature and Metal Forming – cold working Advantages: * good accuracy and tolerances * better surface finish * strain hardening increases strength and hardness * directional properties * energy economy Disadvantages: * higher forces/power * strain hardening limits deformation * may need to anneal

Fundamentals of Metal Forming Temperature and Metal Forming – warm working Advantages: * above room temp., but below re-crystallization temp. (0.3 Tm) * lower forces and power * reduced strain hardening * more difficult geometry * no need for annealing Disadvantages: * more energy * limited geometry

Fundamentals of Metal Forming Temperature and Metal Forming – hot working Advantages: * above re-crystallization temp. (0.5 Tm < T < 0.75 Tm) * lower forces and power * no strain hardening * difficult geometry * isotropic properties Disadvantages: * more energy * poorer surface finish * shorter tool life

Fundamentals of Metal Forming - Strain Rate Note: Strain rate a strong function of working temperature (high values -> 1000 s-1) where v = speed of testing head h = instantaneous height of part being worked Strain rate equation: C = strength constant m = strain-rate coefficient 0.1 1.0 10 102 103 104 Strain rate Flow stress Room temp. 400º C 800º C 1200º C We will mostly assume that strain rate is negligible at room temperature! Yf = C em

Friction and lubrication * undesirable * retards metal flow (residual stresses & defects) * increases force and power requirements * wears tooling * high temperature stiction Lubricants: * reduce frictional effects * remove heat and material * lengthen tool life * may react chemically with tool or work * concerns about toxicity, flammability….(mineral oils, emulsions, oils, graphite, glass..) Lubricants are usually oil-based, and when used in extreme pressure situations, sulfur, chlorine and phosphorus in the lubricant may chemically react with the metal surfaces (tool, chip). The reactions form a surface boundary layer that is more effective than the lubricant itself in reducing friction. Lubricants are most effective at cutting speeds < 400 ft/min.

Example 20.6 For a metal with K = 50,000 lb/in2 and n = 0.27, determine the average flow stress that the metal experiences if it is subjected to a stress that is equal to its strength coefficient K. Solution: Yf = K = 50,000 = 50,000 en Thus e = 1 Yf = 50,000 (1) 0.27/1.27 = 39,370 lb/in2

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