TALAT Lecture Forging Process. 17 pages, 20 figures. Basic Level

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1 TALAT Lecture 3402 Forging Process 17 pages, 20 figures Basic Level prepared by K. Siegert, R. Malek and R. Neher, Institut für Umformtechnik, Universität Stuttgart Objectives: to understand the basic principles of die forging and the characteristic features of special minium die forging processes to learn about the basic design of dies in order to obtain optimum part qualities and tool life Prerequisites: general understanding of metallurgy and deformation processes Date of Issue: 1994 EAA - European Aluminium Association

2 3402 Forging Process Table of Contents 3402 Forging Process Principles of the Forging Process... 3 Fabricating Processes of Forging...3 Processes for Changing Cross-Sections...4 Processes for changing direction...5 Processes for Creating Hollow spaces...5 Separating Processes...6 Die Forging Processes...6 Open-Die Forging... 7 Die Forging Special Forging Processes for Aluminium... 8 List of the aims of various special die forging processes...8 List of characteristic features of precision forging...9 List of characteristic features of high precision forging...9 Characteristic features of closed die forging without flash Characteristic features of isothermal forging Forging Dies Types of Forging Dies Parting of Forging Dies Rules for design of partings of forging dies Die Inserts Fabricating Forging Dies Failure and Damaging of Forging Dies Literature: List of Figures TALAT

3 Principles of the Forging Process Fabricating processes of forging Processes for changing cross-sections Processes for changing direction Processes for creating hollow spaces Separating processes Die forging processes Fabricating Processes of Forging The term forging is used to define a group of processes which are mainly forming processes (see Figure ). Additionally included are processes of separating (splitting) and joining, if large or complicated workpieces are built up out of individual parts. The exact processes of separating and joining are not listed here in detail. According to the characteristic differences in free forming (or unrestricted forming) and die forming (restricted forming), forging can be divided into open-die forging and die forging. Fabrication Process of Forging Separating Forming Joining Compressive forming Tensile forming Bend forming Shear forming Bulge forming Bulge forming with mandrel Forming by bending with linear motion of tool Displacing Offsetting Twisting Setting spiral twisting Free bending Die bending Bending by buckling Rolling Open-die forming Closed-die forming Indenting Extruding Rollforging Drawing out solid and hollow bodies Discrolling Expanding Ringrolling Spreading Lateral rolling Upsetting of shaped parts Heading Cross rolling of shaped parts Stepping Radial or rotary forging Drawing out in a die Fullering Radial forging in a die Upsetting in a die Closed die forging without flash Impression die forging with flash Punching Through punching Piercing with a hollow mandrel Embossing Hobbing Solid forward impact extrusion Backward cup impact extrusion Solid lateral impact extrusion Source: K. Lange Fabrication Process of Forging TALAT

4 Processes for Changing Cross-Sections The processes for changing cross-sections build-up the fundamentals of forging, see Figure According to the law of constant volumes, changes in cross-section lead to corresponding changes in length. The cross-section can be changed by material displacement and material accumulation, whereby the processes of material displacement dominate. Material displacement Processes for Changing Cross-Sections Open-die forming Closed-die Forming Pushing through Drawing out Drawing out over a mandrel Draw Backward cup impact extrusion Solid forward impact extrusion Rolling Stretch rolling Spreading Radial forging Upsetting in a die Radial forging in a die Solid lateral impact extrusion Ring rolling Cross rolling Material accumulation Combined material displacement and accumulation Upsetting Heading Heading in a die Heading in a horizontal upsetting machine Upset die forging Source: K.Lange Processes for Changing Cross-Sections TALAT

5 Processes for changing direction These processes include bending processes (free bending, die bending) and shear forming processes (Figure ). Processes for Changing Direction Forming Without elongation Punch (Top die) Bending die (Bottom die) With elongation Bending in a die Top die Bottom die Shear Forming Displacing Preformed workpiece Top die Bottom die Displaced crank stroke pin Twisting Source: K.Lange Processes for Changing Direction Processes for Creating Hollow spaces Hollow spaces (cavities) are produced by the methods of indentation forming and extrusion forming (Figure ). Processes for Creating Hollow Spaces Indentation Forming Punch Punch(rounded, not sharp) Hollow mandrel Punching Scrap Through Punching Piercing with a hollow mandrel Holing punch Receiver Pressing Through Source: K.Lange Bulk holing Holed plate Counter punch Backward cup impact extrusion Processes for Creating Hollow Spaces TALAT

6 Separating Processes Separating processes used for forging are mostly cutting processes (Figure ). Shear cutting processes are used for the loss-free cutting of raw parts with a given crosssection (extruded sections) and defined lengths or volumes. Separating Processes Shear cutting Cutting Closed cutting with cutting edge Punch Flash Cutting plate Holing punch Holed plate Flash removal Holing Chisel Chisel Chisel Wedge cutting Cutting with chisel Anvil Chopping Anvil Slitting Anvil Source: K.Lange Separating Processes Die Forging Processes These are processes which are used to produce a defined workpiece geometry having good dimension and volume accuracy. Such processes are: form pressing with flash, form pressing without flash and compressing in a die, see Figure Die Forging Processes in a Narrow Sense Clamping jaws Upsetting Punch Top die Flash Workpiece Workpiece Container Stripper Bottom die Upsetting Closed-die forging without flash Closed-die forging with flash Source: K. Lange Die Forging Process TALAT

7 Open-Die Forging The chacteristics of open-die forging are listed in Figure Characteristics of Open Die Forging Merits: Problems: # No special tools (costs, fabricating time) # Simple forms # High machining costs # Material not optimally used # Grain flow (fibre structure) not optimal Applications: Alloys: $ For low production series $ Test samples and prototypes $ Especially large dimensions $ Shortest delivery times! Mainly medium and high-strength Source: H.G. Roczyn Characteristics of Open-Die Forging Die Forging The characteristics of die forging are listed in Figure Characteristics of Die Forging Merits: Problems: Applications: Alloys: # Optimal microstructure # Grain flow (fibre structure) made to suit # Complicated forms # Low amount of machining # Efficient use of material # Tool costs $ For large production series $ Highest demands on strength + toughness $ Safety parts! Mainly medium to high-strength materials Source: H.G. Roczyn Characteristics of Die Forging TALAT

8 Special Forging Processes for Aluminium List of characteristic features of precision forging List of characteristic features of high precision forging Characteristic features of closed die forging without flash Characteristic features of isothermal forging List of the aims of various special die forging processes (Figure ) Special Forging Processes and their Aims Process 1. Precision forging 2. High precision forging 3. Closed die forging without flash 4. Powder forging (mostly combined with 3.) 5. Isothermal forging 6. Superplastic forging (mostly combined with 3) 7.Squeeze casting 8. Partial forging 9.Thermomechanical working Soerce: K. Lange, H. Meyer-Nolkemper Characteristics better forging quality best forging quality forging in closed dies sintered raw parts tool temperature ~ work temperature as in 5. ; very low forming speeds pressing in pasty state stepwise fabrication combined forging and structure change Advantages narrower tolerances, better replication of final form narrower tolerances, better replication of final form, better surfaces material savings material savings, fewer forming process steps, narrower tolerances better replication of final form material savings, fewer forming process steps, better replication of final form fewer forming process steps, better replication of final form better replication of final form better mechanical properties Special Forging Processes and their Aims TALAT

9 List of characteristic features of precision forging (Figure ) Characteristics of Precision Forging Precision Forging is a die forging process which saves at least one finishing or supplementary operation compared to conventional die forging. Its merits are: Problem: # 0-1 side tapers (draft) # Thinner work-piece sections # Narrower tolerances # Smaller radii $ High quality surface finish $ Shorter production times for finished product Higher tooling costs Source: K. Lange, H. Meyer-Nolkemper Characteristics of Precision Forging List of characteristic features of high precision forging (Figure ) Characteristics of High Precision Forging - Special case of precision forging - Production of "ready-to-use" parts # 0-1 side tapers # Thin work-piece regions # Narrow tolerances # Small radii $ High quality surface finish $ Shorter production times for finished product $ Little or no machining required $ Weight savings $ Good reproducibility Source: K. Lange, H. Meyer-Nolkemper Characteristics of High Precision Forging TALAT

10 Characteristic features of closed die forging without flash (Figure ) Closed Die Forging without Flash Characteristics: = Die forms in closed tools from which no material is lost # Constant volume of hot starting, intermediate and final form # Exact mass distribution # Exact positioning # No flash $ Weight savings $ No flash $ Shorter production times for finished part Source: K. Lange, H. Meyer-Nolkemper Closed-Die Forging Characteristic features of isothermal forging (Figure ) Isothermal Forging - Form pressing with die temperatures almost equal to the work temperature # Melted pockets due to local overheating caused by too high forming rates # Low temperature gradient tool / work piece # No flashed removal $ High quality parts in almost "ready-to-use" shape $ Shorter production times for finished part Source: K. Lange, H. Meyer-Nolkemper Isothermal Forging TALAT

11 Forging Dies Types of forging dies Parting of forging dies Rules for design of partings of forging dies Die inserts Fabricating forging dies Failure and damaging of forging dies Types of Forging Dies The following types of forging dies are encountered in die forging: Figure : - Single-cavity die - Multiple-cavity die: a number of identical cavities in one die - Multiple-stage die: more than one forming step for the workpiece in a die Types of Forging Dies die with flash closed die die with multiple parts simple multiple-cavity die multi-stage die multi-stage die upsetting punch punch holder full die simple die die insert insert with a number of identical cavities holing punch block inserts clamping blocks Source: K. Lange Types of Forging Dies The elements of forging dies are shown in Figure for a hammer die. TALAT

12 The Elements of Forging Dies (shown for a hammer die) h H impact area cavity reference areas clamping area hole for holding pin support area a B a L b a 1 b a a Source: K. Lange h: cavity depth b: cavity width a, a 1 : web thicknesses H: block height L: block length B: block width The Elements of Forging Dies While designing forging dies, the following aspects must be taken into consideration: Design to meet the stresses Forging dies are mostly subjected to repeated stress. The fatigue strength depends on the surface, cracks, residual stress and top-layer hardness. Design for dimensions Shrinkage of the formed part is taken into account. Particular attention must be given to the fact that both die (steel) and workpiece (minium) have different coefficients of thermal expansion and that the die geometry has various sources of errors. Design for machining Machining tolerances are to be considered. Design for optimal material flow The tool stress can be reduced by avoiding sharp edges, abrupt transitions, long narrow fins (ribs) etc. (see also TALAT Lecture 3403). Parting of Forging Dies Dividing the cavity between top and bottom die (parting line of a forging) is of particular importance. The position of the parting line influences the tolerance of the forging and several other properties of the forging as well as die wear (see Figure ). TALAT

13 Partings of Forging Dies The parting plane of dies determines the proportion of volume of the forging in the upper and lower die. The parting has an influence on further ves and properties: - geometry of the forging - fibre structure - strength properties - mass of material - forging process (force required, form filling) - further working - wear of dies Source: H.G. Roczyn Partings of Forging Dies Rules for design of partings of forging dies Rules for design of partings of forging dies: 1. Symmetrical parting: The effort for making the tool with a given wall taper is lowest. 2. Plane parting: The die block height is the lowest; the mechanical working is simplified. 3. Parting for good material flow: This makes the material flow easy. 4. Parting for good machining: This makes it easy to machine or to remove flash. Basic rules for the positioning of the parting of forging dies are collected in Figure TALAT

14 Basic Rules for Designing Partings for Forging Dies Construction aspects Principle Example 1 symmetrical parting flash not at edge of the workpiece 2 plane parting 3 parting for good material flow right wrong wrong preferential, simple die fabrication favourable unfavourable "interrupted", flow optimised favourable, breaks rule 2 right wrong favourable, breaks rule 2 breaks rule 3 favourable, breaks rule 2 4 parting for good machining few areas with side taper unfavourable favourable Source: K. Lange sufficient allowance on machining areas unfavourable favourable, breaks rule Design Rules for Partings of Forging Dies Die Inserts Die inserts are blocks which can accommodate the complete die cavity (see Figure ). One can save expensive die steel for the die holder if die inserts are used. Die inserts are made of tempered alloy steel. due to the lowered stress acting on the die holders, these can be made of low alloy steel or tempered steel (e.g. steel ), thereby saving costs. The die insert is fastened to the die holder in a force or form locked type of joint. TALAT

15 Die Inserts Die insert in a die holder Merits of die inserts % Easy handling due to reduced weight % Insert can be constructed with high-strength materials % Die holder can be made of unalloyed or low alloyed material % Cost savings with replacements Source: Thyssen Die Inserts Fabricating Forging Dies Fabricating forging dies Fabrication & finishing block rolling forging milling casting Heat treating annealing heating diffusion tempering annealing Forming cavity cold hobbing warm hobbing machining eroding electrochemical milling Post treatment smoothing Heat treating Post treatment tempering pressure jet lapping stressrelieving Surface treatment Source: K.Lange / H. Meyer-Nolkemper Fabricating Forging Dies TALAT

16 The steps of fabrication of forging dies are described in Figure The die block is produced by casting or rolling/forging and finally mechanically working. The cavity is fabricated by machining, cold hobbing, eroding, electrochemical milling, etc.. The die inserts (and die holder) are heat-treated to improve strength. After polishing, the cavity has a surface roughness of 3 µm. Since minium has a lower flow stress than steel, the contours of the die cavity are reproduced more exactly on minium forgings. Failure and Damaging of Forging Dies Damages occur on forging dies due to thermal and mechanical fatigue as well as due to wear and permanent deformation (see Figure ). The individual effects occur in combinations of: 1. Warm fatigue cracks can occur due to thermal stresses in the tool. The temperature gradients depend on the geometry and forming conditions. 2. Plastic deformation occurs as a result of local stresses exceeding the yield strength, as may be the case in protruding form elements. 3. Fatigue cracks are initiated due to repeated die stresses occurring over the forming operation cycle. Thus, notches and abrupt transitions in the die should be avoided as far as possible. 4. Wear occurs due to small particles which detach from the surface. The degree of wear depends on the tribological system between tool and workpiece. Failure and damaging of forging dies Damage is a result of : - the mechanical loading (repeated) due to forming resistance and geometrical conditions, - the thermal stressing due to workpiece and tool temperature as well as pressure contact time, - the tribological conditions at the contact zone between workpiece and tool. mechanical fatigue permanent deformation Wear (abrasion) thermal fatigue Source: A. Kannappan Failure and Damaging of Forging Dies TALAT

17 Literature: 1. Lange, K.: Benennung und Begriffsbestimmungen im Bereich der Umformtechnik. Ind. Anz. 86 (1964) Lange, K.: Lehrbuch der Umformtechnik, Bd.2: Massivumformung, 2.Aufl. Berlin, Heidelberg, New York, Paris, Tokyo: Springer Roczyn, H.G.: Vortrag Schmieden von Aluminium am IFU Stuttgart WS 92/ Lange, K.: Meyer-Nolkemper, H.: Gesenkschmieden, 2.Aufl. Berlin, New York: Springer Nussbaum, A.I.: Forging Technology for the Nineties. Light Metal Age, 50 (1992) 7/8. 6. Thyssen Edelstahlwerke AG, Werkstoffblätter für Warmarbeitsstähle 7. Kannappan: Wear in forging dies. A review of world experience. Metal forming 36(1969) , Metal forming 37(1970) 6-14 and List of Figures Figure No. Figure Title (Overhead) Fabrication Process of Forging Processes for Changing Cross-Sections Processes for Changing Direction Processes for Creating Hollow Spaces Separating Processes Die Forging Process Characteristics of Open-Die Forging Characteristics of Die Forging Special Forging Processes and Their Aims Characteristics of Precision Forging Characteristics of High Precision Forging Closed Die Forging Isothermal Forging Types of Forging Dies The Elements of Forging Dies Partings of Forging Dies Design Rules for Partings of Forging Dies Die Inserts Fabricating Forging Dies Failure and Damaging of Forging Dies TALAT