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TALAT Lecture 3402: Forging Process
 

TALAT Lecture 3402: Forging Process

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This lecture helps to understand the basic principles of die forging and the characteristic features of special aluminium die forging processes. It aims at learning about the basic design of dies in ...

This lecture helps to understand the basic principles of die forging and the characteristic features of special aluminium die forging processes. It aims at learning about the basic design of dies in order to obtain optimum part qualities and tool life. General understanding of metallurgy and deformation processes is assumed.

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    TALAT Lecture 3402: Forging Process TALAT Lecture 3402: Forging Process Document Transcript

    • 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 aluminium 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
    • 3402 Forging Process Table of Contents 3402 Forging Process ....................................................................................................2 3402.01 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.............................................................................................................. 7 3402.02 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....................................... 10 Characteristic features of isothermal forging ............................................................ 10 3402.03 Forging Dies ................................................................................................. 11 Types of Forging Dies................................................................................................ 11 Parting of Forging Dies.............................................................................................. 12 Rules for design of partings of forging dies .............................................................. 13 Die Inserts .................................................................................................................. 14 Fabricating Forging Dies ........................................................................................... 15 Failure and Damaging of Forging Dies ..................................................................... 16 3402.04 Literature:....................................................................................................... 17 3402.05 List of Figures................................................................................................. 17 TALAT 3402 2
    • 3402.01 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 3402.01.01). 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 Shear forming Tensile forming Bend forming forming Bulge forming Forming by ben- Displacing Twisting ding with linear motion of tool Bulge forming Offsetting Setting with mandrel spiral twisting Die bending Bending by Free bending buckling Rolling Open-die forming Closed-die Extruding forming Indenting Rollforging Drawing out solid Drawing out in a Punching Solid forward and hollow bodies die impact extrusion Discrolling Through punching Expanding Fullering Backward cup Ringrolling Spreading Radial forging in Piercing with a impact extrusion a die hollow mandrel Lateral rolling Upsetting Upsetting in a die Solid lateral of shaped parts Heading Closed die forging Embossing impact extrusion Stepping without flash Cross rolling of Hobbing shaped parts Impression die Radial or rotary forging forging with flash Source: K. Lange alu Training in Aluminium Application Technologies Fabrication Process of Forging 3402.01.01 TALAT 3402 3
    • Processes for Changing Cross-Sections The processes for changing cross-sections build-up the fundamentals of forging, see Figure 3402.01.02. 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. Processes for Changing Cross-Sections Open-die forming Closed-die Forming Pushing through Rolling Solid forward impact extrusion Stretch rolling Material Drawing out Draw Drawing displace- out over a mandrel Backward cup impact extrusion ment Solid lateral Spreading Radial forging Upsetting in Radial forging impact extrusion Ring rolling Cross rolling a die in a die Material accumu- lation Heading in a Upsetting Heading Heading in horizontal up- a die setting machine Combined material displace- ment and accumu- Upset die forging lation Source: K.Lange alu Training in Aluminium Application Technologies Processes for Changing Cross-Sections 3402.01.02 TALAT 3402 4
    • Processes for changing direction These processes include bending processes (free bending, die bending) and shear forming processes (Figure 3402.01.03). Processes for Changing Direction Punch (Top die) Top die Bending die Forming (Bottom die) Bottom die Without elongation With elongation Bending in a die Preformed Shear workpiece Forming Top die Bottom die Displaced crank stroke pin Displacing Twisting Source: K.Lange alu Training in Aluminium Application Technologies Processes for Changing Direction 3402.01.03 Processes for Creating Hollow spaces Hollow spaces (cavities) are produced by the methods of indentation forming and extrusion forming (Figure 3402.01.04). Processes for Creating Hollow Spaces Hollow Punch(rounded, mandrel Punch not sharp) Indentation Forming Scrap Punching Through Piercing with a Punching hollow mandrel Holing punch Receiver Pressing Through Holed plate Counter punch Bulk Backward cup Source: K.Lange holing impact extrusion alu Training in Aluminium Application Technologies Processes for Creating Hollow Spaces 3402.01.04 TALAT 3402 5
    • Separating Processes Separating processes used for forging are mostly cutting processes (Figure 3402.01.05). Shear cutting processes are used for the loss-free cutting of raw parts with a given cross- section (extruded sections) and defined lengths or volumes. Separating Processes Shear cutting Cutting Punch Holing punch Closed Flash cutting with Holed plate cutting edge Cutting plate Flash removal Holing Chisel Chisel Chisel Wedge cutting Anvil Anvil Cutting with Anvil Chopping Slitting chisel Source: K.Lange alu Training in Alum inium Application Technologies Separating Processes 3402.01.05 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 3402.01.06. Die Forging Processes in a Narrow Sense Clamping jaws Upsetting Top die Flash Punch Workpiece Container Workpiece Stripper Bottom die Closed-die forging Closed-die forging Upsetting without flash with flash Source: K. Lange alu Die Forging Process 3402.01.06 Training in Aluminium Application Technologies TALAT 3402 6
    • Open-Die Forging The chacteristics of open-die forging are listed in Figure 3402.01.07. Characteristics of Open Die Forging Merits: # No special tools (costs, fabricating time) # Simple forms Problems: # High machining costs # Material not optimally used # Grain flow (fibre structure) not optimal Applications: $ For low production series $ Test samples and prototypes $ Especially large dimensions $ Shortest delivery times Alloys: ! Mainly medium and high-strength Source: H.G. Roczyn alu Characteristics of Open-Die Forging 3402.01.07 Training in Aluminium Application Technologies Die Forging The characteristics of die forging are listed in Figure 3402.01.08. Characteristics of Die Forging Merits: # Optimal microstructure # Grain flow (fibre structure) made to suit # Complicated forms # Low amount of machining # Efficient use of material Problems: # Tool costs Applications: $ For large production series $ Highest demands on strength + toughness $ Safety parts Alloys: ! Mainly medium to high-strength materials Source: H.G. Roczyn alu Training in Aluminium Application Technologies Characteristics of Die Forging 3402.01.08 TALAT 3402 7
    • 3402.02 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 3402.02.01) Special Forging Processes and their Aims Process Characteristics Advantages narrower tolerances, better 1. Precision forging better forging quality replication of final form 2. High precision forging best forging quality narrower tolerances, better replication of final form, better surfaces 3. Closed die forging forging in closed dies material savings without flash 4. Powder forging sintered raw parts material savings, fewer forming (mostly combined with 3.) process steps, narrower tolerances 5. Isothermal forging tool temperature ~ better replication of final form work temperature 6. Superplastic forging as in 5. ; very low material savings, fewer forming process (mostly combined with 3) forming speeds steps, better replication of final form 7.Squeeze casting pressing in pasty state fewer forming process steps, better replication of final form 8. Partial forging stepwise fabrication better replication of final form 9.Thermomechanical combined forging and better mechanical properties working structure change Soerce: K. Lange, H. Meyer-Nolkemper alu Training in Aluminium Application Technologies Special Forging Processes and their Aims 3402.02.01 TALAT 3402 8
    • List of characteristic features of precision forging (Figure 3402.02.02) 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: # 0° - 1° side tapers (draft) # Thinner work-piece sections # Narrower tolerances # Smaller radii $ High quality surface finish $ Shorter production times for finished product Problem: Higher tooling costs Source: K. Lange, H. Meyer-Nolkemper alu Training in Aluminium Application Technologies Characteristics of Precision Forging 3402.02.02 List of characteristic features of high precision forging (Figure3402.02.03) 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 alu Training in Aluminium Application Technologies Characteristics of High Precision Forging 3402.02.03 TALAT 3402 9
    • Characteristic features of closed die forging without flash (Figure 3402.02.04) 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 alu Closed-Die Forging 3402.02.04 Training in Aluminium Application Technologies Characteristic features of isothermal forging (Figure 3402.02.05) 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 alu Training in Aluminium Application Technologies Isothermal Forging 3402.02.05 TALAT 3402 10
    • 3402.03 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 3402.03.01: - 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 multiple die with flash closed die parts simple multiple-cavity die multi-stage die multi-stage die upsetting punch punch holder full die simple die holing insert with a punch number of block inserts identical cavities clamping die insert blocks Source: K. Lange alu Training in Aluminium Application Technologies Types of Forging Dies 3402.03.01 The elements of forging dies are shown in Figure 3402.03.02 for a hammer die. TALAT 3402 11
    • The Elements of Forging Dies (shown for a hammer die) h H impact area B a cavity reference areas clamping area hole for holding pin L support area a b a1 b a a h: cavity depth H: block height b: cavity width L: block length a, a1: web thicknesses B: block width Source: K. Lange alu The Elements of Forging Dies 3402.03.02 Training in Aluminium Application Technologies 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 (aluminium) 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 3402.03.03). TALAT 3402 12
    • 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 values 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 alu Training in Aluminium Application Technologies Partings of Forging Dies 3402.03.03 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 3402.03.04. TALAT 3402 13
    • Basic Rules for Designing Partings for Forging Dies Construction Principle Example aspects 1 symmetrical parting favourable, breaks rule 2 right wrong flash not at edge right of the workpiece wrong wrong 2 plane parting favourable, breaks rule 2 preferential, simple die breaks rule 3 fabrication favourable 3 parting for good material flow unfavourable favourable, breaks rule 2 "interrupted", flow optimised few areas with 4 parting for good side taper unfavourable favourable machining sufficient allow- ance on favourable, machining areas unfavourable breaks rule Source: K. Lange alu Design Rules for Partings Training in Aluminium Application Technologies of Forging Dies 3402.03.04 Die Inserts Die inserts are blocks which can accommodate the complete die cavity (see Figure 3402.03.05). 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 1.2713), thereby saving costs. The die insert is fastened to the die holder in a force or form locked type of joint. TALAT 3402 14
    • 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 alu Die Inserts 3402.03.05 Training in Aluminium Application Technologies Fabricating Forging Dies Fabricating forging dies Fabrication & rolling finishing block forging casting milling Heat treating diffusion annealing heating tempering annealing cold warm electro- Forming cavity machining eroding chemical hobbing hobbing milling Post treatment smoothing Heat treating stress- tempering relieving Post treatment pressure jet lapping Surface treatment Source: K.Lange / H. Meyer-Nolkemper alu Training in Aluminium Application Technologies Fabricating Forging Dies 3402.03.06 TALAT 3402 15
    • The steps of fabrication of forging dies are described in Figure 3402.03.06. • 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 aluminium has a lower flow stress than steel, the contours of the die cavity are reproduced more exactly on aluminium 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 3402.03.07). 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 alu Failure and Damaging of Forging Dies 3402.03.07 Training in Aluminium Application Technologies TALAT 3402 16
    • 3402.04 Literature: 1. Lange, K.: Benennung und Begriffsbestimmungen im Bereich der Umformtechnik. Ind. Anz. 86 (1964) 84-85 2. Lange, K.: Lehrbuch der Umformtechnik, Bd.2: Massivumformung, 2.Aufl. Berlin, Heidelberg, New York, Paris, Tokyo: Springer 1988. 3. Roczyn, H.G.: Vortrag Schmieden von Aluminium am IFU Stuttgart WS 92/93. 4. Lange, K.: Meyer-Nolkemper, H.: Gesenkschmieden, 2.Aufl. Berlin, New York: Springer 1977. 5. 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) 335 - 342, Metal forming 37(1970) 6-14 and 21. 3402.05 List of Figures Figure No. Figure Title (Overhead) 3402.01.01 Fabrication Process of Forging 3402.01.02 Processes for Changing Cross-Sections 3402.01.03 Processes for Changing Direction 3402.01.04 Processes for Creating Hollow Spaces 3402.01.05 Separating Processes 3402.01.06 Die Forging Process 3402.01.07 Characteristics of Open-Die Forging 3402.01.08 Characteristics of Die Forging 3402.02.01 Special Forging Processes and Their Aims 3402.02.02 Characteristics of Precision Forging 3402.02.03 Characteristics of High Precision Forging 3402.02.04 Closed Die Forging 3402.02.05 Isothermal Forging 3402.03.01 Types of Forging Dies 3402.03.02 The Elements of Forging Dies 3402.03.03 Partings of Forging Dies 3402.03.04 Design Rules for Partings of Forging Dies 3402.03.05 Die Inserts 3402.03.06 Fabricating Forging Dies 3402.03.07 Failure and Damaging of Forging Dies TALAT 3402 17