week 1 - introduction to metal forming processes

Week 1 – Introduction to Metal Forming Processes

Unit of study code HES3281

Unit of study name Materials and Manufacturing 2

Teaching Term/Semester & Year

Semester 1 / 2012

Contact Hours (hrs/wk) or total contact hours

5 Hours / Week

Prerequisites HES2281 Materials and Manufacturing 1

Corequisites Nil

Credit Points 12.5

AimsThe unit aims to provide students the understanding of avariety of metal and polymeric manufacturing processes andthe importance of fatigue failure and failure of materialsLearning ObjectivesAfter successfully completing this unit, you should be able to:1.To demonstrate an understanding in the manufacturing ofplastic components by recognising the processes andcalculating the forces required to produce such components2.To develop an understanding in fatigue and failure bycalculating the stress and strain involved3.To demonstrate an understanding in the manufacturing ofmetal components by recognising the processes andcalculating the forces required to produce such components

ContentMetal Forming Processes: • Extrusion, wire drawing, strip forming, forging,rolling, sheet metal forming – mathematical modelling and process parameters.Polymers and composite: manufacture and processes: • Crystalline andamorphous microstructures, physical properties, Mechanical properties ofpolymers and composites. Forming and moulding techniques, extrusion andinjection moulding: effect of process parameters. Blow moulding: output dierequirements, parisons dimensions, swelling considerations.Fatigue Failure and Failure of Materials: • Static failure of materials, Fatiguefailure, fatigue/fracture, life estimation. Analysis and prediction of failure, Non‐destructive testing.Advanced Manufacturing Processes: • Materials selection, powder metallurgy.Laboratory experiments: Cold rolling, stress concentration and polymerprocessing.

AssessmentThis subject contains the following assessments: Examination (50%) ‐ 3 hrs. at the end of the semester Tests (2 written test) (10%) – 5% each Case study (Oral Presentation ) (10%)Practical laboratory work (20%) – attendance (5%) and lab reports (15%). Tutorial Participation (10%) – Attendance is compulsory.

Minimum requirements to pass this unit of study:In order to achieve a pass in this unit of study, you must:(a) at least 35% of the possible final marks for eachAssessment Component plus(b) an aggregate mark for the subject of 50% or more.If you do not achieve at least 35% of the possible finalmarks for each Major Assessment Component you willreceive a maximum of 44% as your mark for the subjectconcerned. The Major Assessment Components carries aweighting of at least 15% of the total mark available.

Resources and Reference Material

TEXT There is no specific text for this subject. However, most notes are obtained from Groover, M.P., Fundamentals of Modern Manufacturing Methods‐Materials, Processes and Systems, Prentice Hall Inc, 2007, and other references listed below.This text (Groover, M.P., Fundament….) is also used in the subject HES2281 Materials and Manufacturing 1REFERENCESGroover, M.P., Fundamentals of Modern Manufacturing Methods‐Materials, Processes and Systems, Prentice Hall Inc, 2007Tlusty, J., Manufacturing Processes and Equipment, Prentice Hall 2000.Schey, J.A., Introduction to Manufacturing Processes, 3rd Edn., McGrawHill, 2000.El‐Wakil, S.D., Processes and Design for Manufacturing, 2nd Edn., PWS Pub. Company, 1998Crawford, R.J., Plastic Engineering, 3rd Edn., Maxwell Macmillan, 1997Kalpakjian, S., and Schmid, S.R., Manufacturing Engineering and Technology, Prentice Hall, 5th Edition, 2006Jones, D. R. H., Ashby, M., Engineering Materials, Volume 1, 3rd Edn., Elsevier, London, 2005

1. Solidification processes ‐ starting material is a heated liquid or semifluid

2. Particulate processing ‐ starting material consists of powders

3. Deformation processes ‐ starting material is a ductile solid (commonly metal)

4. Material removal processes ‐ starting material is a ductile or brittle solid 

Large group of manufacturing processes in whichplastic deformation is used to change the shape ofmetal work pieces.» 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

Deformation processes exploit a remarkableproperty of metals, which is their ability toflow plastically in the solid state withoutdeterioration of their properties.

With the application of suitable pressures, thematerial is moved to obtain the desiredshape with almost no wastage.

FormingProcesses

Forming processes tend to be complexsystems consisting

Independent variables,Dependent variables, andIndependent-dependent interrelations.

» 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

Independent Variables

Starting materialThe engineer is often free to specify the chemistryand condition. These may also be chosen forease in fabrication or they may be restrictedby the final properties desired for the product.

Starting geometry of the workpieceThis may be dictated by previous processing or itmay be selected by the engineer from a variety ofavailable shapes. Economics often influence thisdecision.

Independent Variables

Tool or die geometryThis are has many aspects such as the diameter of arolling mill roll, the die angle in wire drawing and the cavitydetails when forging. Since tooling will produce andcontrol the metal flow, success or failure of a processoften depends on tool geometry.

Independent Variables

LubricationSince lubricants also acts as coolants, thermal barriers, corrosion inhibitors, and parting compounds, their selection is an aspect of great importance. Specification includes type of lubricant amount to be applied and the method of application.

Independent Variables

Starting temperatureMany material properties vary greatly with temperature,so its selection and control may well dictate the successor failure of an operation.

Speed of operation Since speed can directly influence the lubricanteffectiveness, the forces required for deformationand the time available for heat transfer. It isobvious that its selection would be significant in aforming operation.

Independent Variables

Amount of deformation while some processes control this variable through die design, others, such as rolling permits its selection at the discretion of the engineer.

Dependent Variables

Force or power requirements Engineers cannot directly specify the force or power;

they can only specify the independent variables andthen experience the consequences of the selection.The ability to predict the forces or powers however isextremely important for only by having this knowledgewill the engineer be able to specify or select the

equipment for the process.

Dependent Variables

Material properties of the productThe customer is not interested in the starting

properties but is concerned with our ability to produce thedesired final shape with the desired final properties

Exit temperatureEngineering properties can be altered by both themechanical and thermal history of the material thusit is important to know and control the temperatureof the material throughout the process

Dependent Variables

Surface finish and precision Both are characteristics of the resultant product that are dependent on the specific details of the process.

Nature of the material flowsince properties depend on deformation history,control here is vital the customer is satisfied only ifthe desired geometric shape is produced with theright set of companion properties and withoutsurface or internal defects.

Independent-Dependent Interrelations

ExperienceThis requires long time exposure to the process and isgenerally limited to the specific materials, equipmentand products encountered in the realm of past contact.

ExperimentWhile possibly the least likely in error directexperiment is both time consuming and costly.

Independent-Dependent Interrelations

Process modelingHere one approaches the problem with a high speedcomputer and one or more mathematical models of theprocess numerical values are provided for the variousindependent variables and the models are used to

compute predictions for the dependent variables

» 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 and friction

1. Bulk deformation˃ Rolling˃ Forging˃ Extrusion˃ Wire and bar drawing

2. Sheet metalworking˃ Bending˃ Deep drawing˃ Cutting˃ Miscellaneous 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 18.2 Basic bulk deformation processes: (a) rolling

Rolling

Figure 18.2 Basic bulk deformation processes: (b) forging

Forging

Figure 18.2 Basic bulk deformation processes: (c) extrusion

Extrusion

Figure 18.2 Basic bulk deformation processes: (d) drawing

Wire and Bar Drawing

» 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

Figure 18.3 Basic sheet metalworking operations: (a) bending

Sheet Metal Bending

Figure 18.3 Basic sheet metalworking operations: (b) drawing

Deep Drawing

Figure 18.3 Basic sheet metalworking operations: (c) shearing

Shearing of Sheet Metal

» 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: 

nK

where K = strength coefficient; and n = strain hardening exponent

Flow curve based on true stress and true strain

» For most metals at room temperature, strength increases when deformed due to strain hardening

» Flow stress = instantaneous value of stress required to continue deforming the material

where Yf = flow stress, that is, the yield strength as a function of strain

nf KY

» Determined by integrating the flow curve equation between zero and the final strain value defining the range of interest 

where       = average flow stress; and  = maximum strain during deformation process

nKY

n

f

1_

_fY

» 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

» 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

» 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 shapeprocesses 

» 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

» 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

» 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

» Lower forces and power than in cold working» More intricate work geometries possible» Need for annealing may be reduced or eliminated 

» Deformation at temperatures above therecrystallization 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 

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 

» 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

» 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 

» 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

» Strain rate in forming is directly related to speed of deformation v

» Deformation speed v = velocity of the ram or other movement of the equipment

» Strain rate is defined:

where = true strain rate; and h = instantaneous height of workpiece being deformed

hv

.

.

» 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

» 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

Figure 18.5 (a) Effect of strain rate on flow stress at an elevated work temperature. (b) Same relationship plotted on log-log coordinates.

Strain Rate Sensitivity

where C = strength constant (similar but not equal to strength coefficient in flow curve equation), and m = strain‐rate sensitivity exponent

mf CY ε=

Figure 18.6 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.

Effect of Temperature on Flow Stress

» 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

» 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

» 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

» 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