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The main aspects of precision forging Z GRONOSTAJSKI M HAWRYLUK Wroc aw University of Technology Wybrze e Wyspia skiego 25 50 370 Wroc aw Poland The article concerns the directions of development the forging and problems with precision forging like tool and preform temperature slug geometry press settings process speed lubrication and cooling and tool shape and quality It was


40 Z GRONOSTAJSKI M HAWRYLUK, The life span of the tools depends on the forging conditions workmanship of staff. tools material the shape of the preform and that of the slug etc The low durability of. the tools lowers the quality of the forgings in spite of their still control The most. common forging defect due to low tool durability are i e shorts laps burrs bends. cracks delamination micro and macro fractures and so on This in turn affects the. functionality of the final product made from the forging Because of the large number. and variety of factors and their interactions having an influence on precision forging. the process is very difficult to analyze Therefore a whole range of computer tools. such as CAD CAM CAE mostly based on FEM and physical modelling are used for. the design analysis and optimization of the forging process 2 7 11. Fig 2 Most common defects of tools,2 Precision forging. In conventional precision forging the material is formed at ambient temperature or. in semi hot conditions In the case of very complicated parts a properly prepared. charge is hot formed in isothermal conditions Sometimes the super plasticity of the. formed material is exploited Initially enclosed dies were used for forming 5 12. Thanks to the material savings and the lower costs of manufacturing products with en. hanced properties at competitive prices precision forging was increasingly applied to. alloys of light materials such as aluminium magnesium and titanium 10 12. The expression precision forging does not mean distinct forging process but rather. approach to forging The aim of this approach is to produce a net shape or at least. a near net shape parts Precision forging is sometimes described as close tolerance. forging to emphasize the aim of achieving required the dimensional and surface finish. tolerances only after forging 13, Precision forging at ambient temperature i e cold forging is preceded by making. a slug in a few hot forging operations In conventional hot forging in half dies a sub. stantial amount of the material is lost for the flash and allowances For this reason. closed die forging often employing complex formation i e forward and backward. The main aspects of precision forging 41, extrusion was adopted to make preforms for precision forging Extrusion forging has. this advantage that the stress is mostly triaxial compression whereby large deforma. tions can be obtained without losing material cohesion In addition no die drafts are. used in precision forming The whole manufacturing process is generally divided into. stages As an example the process of manufacturing a low carbon steel product is. shown in Figure 3,Fig 3 Flowchart of precision forging process.
Metal bundles are delivered from a storeroom to a machine which cuts the rods into. pieces with specified dimensions and weight Then the pieces are heated up to a tem. perature of about 900 C in an induction furnace A constant process temperature must. be maintained in order to ensure high quality of the forgings Preforms heated up to. a proper temperature are fed into a press where they are formed in 2 5 operations The. dies are preheated to a temperature close to the operating temperature to reduce the. risk of die cracking as a result of thermal shock Figure 4a. Fig 4a Dies are preheated Fig 4b Measurement and control. of forged object temperature, The forging process lasts only a few seconds The stability of the process and con. trol conducted according to plan ensure high quality of the forgings When they leave. 42 Z GRONOSTAJSKI M HAWRYLUK, the press the forgings are subjected to controlled cooling Figure 4b Then they go to. a shot peening machine where they are cleaned from graphite The clean forgings go. to cold working where they are oiled and cold formed In this way the precise shape of. the finished forged product is obtained After cold forming the forgings are washed. and greased,3 Choice of process parameters, As shown above each of the stages in the forging process is critical Any short. comings even at one of them may result in bad quality of the forged products press. jamming and production stoppages Therefore the process specifications must be ad. hered to during production which requires proper shopfloor customs and technical. culture For example if the hot forging die is improperly lubricated and preheated and. there is improper cooling during production the die heats up excessively which. quickly results in its plastic deformation or thermal fatigue In the available literature. much attention is devoted to the design and optimization of the whole forging process. or its key stages 2 12 16 Proper design of the particular process stages is the pre. condition for the optimization of forging parameters 3 18 Through the optimum. choice of process parameters one may significantly increase the life of the tools and. improve the quality of the forgings and consequently increase the productivity of the. whole process The main factors having an effect on the process of forging are tool. and preform temperature slug geometry press settings process speed lubrication and. cooling and tool shape and quality,3 1 Tool and preform temperature. Proper temperature is critical for the reliable operation of the tools It can affect the. die s narrow tolerance zones and the small spaces between the moving parts and the. fixed parts of the tools As a result of thermal expansion the clearance may decrease. and the tool components may lock up Also the thermal expansion of the die affects. the quality of the forgings Numerical simulations of the forging of CV joints showed. large temperature differences in the die in the contact area whereas already at a depth. of about 5 mm from the die s inner surface the temperatures are much lower Figure. 5a So large temperature gradients may adversely affect the state of stress inside the. tools This means that the temperature of the tools needs to be monitored for example. by a thermovision camera Figure 5b, An equally important parameter is the temperature of the slug since it has an effect.
on the forged object s microstructure and its material flow curve its formability and. through thermal expansion it causes a change in the slug volume The temperature. distribution in slugs Figure 6 for different preform diameters was examined in 8. The initial temperature of the preforms was 920 C Then the specimens were cooled. for 2 s at a temperature of 50 C in the press chamber as they were waiting to be. The main aspects of precision forging 43, forged In spite of such a short cooling time the temperature difference in the outer. layers amounted to about 50 C This can have a significant effect on the forging proc. Fig 5a Die thermal field distribution Fig 5b Thermovision camera photograph of die. determined by FEM 8, Fig 6 Temperature field distribution in slugs after Fig 7 Temperature field distribution in slugs after. 2 s long cooling at temperature of about 50 C in 1st forging operation about 4 5 s 8. press chamber 8,3 2 Slug geometry, In precision forging there is no flash gap and the charge material volume must be. the same as that of the finished part The allowable differences in mass can amount to. 0 5 1 the angle deviation in the cutting zone to 0 5 2 and the roundness deviation. to 2 and 6 This can be achieved by using special cutting It is necessary to maintain. so narrow tolerances for the preform in order to ensure high quality of the forged. product A too large slug volume may result in damage to the die or the press During. multioperation forging in closed dies proper distribution of slug material volume and. slug preparation through upsetting are critical for the proper filling of the die cavity. There exists a notion of an ideal metal body of revolution with appropriately distrib. 44 Z GRONOSTAJSKI M HAWRYLUK, uted material masses Therefore the design of preforms and slugs in forging processes. is an important activity aimed at improving product quality and reducing production. costs 1 3 11 The preform s shape and mechanical properties will affect the friction. conditions at the die slug interface while its geometry will have an effect on the die. and the slug, In 8 it was shown that the preform s initial dimensions have an effect on tempera.
ture distribution It was found that a larger drop in temperature occurs in a slenderer. slug since the surface of contact with the tools is then larger Figure 7. 3 3 Process speed, Precision Forging processes are usually conducted at high speeds Currently ten. dencies of increasing the speed are dictated by economic reasons The faster the forg. ing process the higher the productivity As many as 30 forgings per minute can be. produced in multioperation hot forging in closed dies in crank presses 7 The time in. which a single forming operation is performed in industrial precision forging is about. 0 2 to 1 second Hence as mentioned in pt 3 1 it is so critical to ensure a constant. preform temperature Figures 6 7 Currently attempts are made to maximally utilize. the capacity of the forming machines and to reduce the number of forging operations. If the number of strokes during forging is too large the whole time increases and so. do the costs 1 But the higher the speed of a single operation the higher the deforma. tion resistance and the greater the required forming forces. 3 4 Press settings, The settings of the forming machines are an important factor affecting the forging. process The main settings include, Press workspace The precision forging process and tools require that the tool. workspace between the anvil and the press slide should be sufficiently large for the. whole system of tools including the auxiliary components Moreover the cooling and. lubricating equipment should be integrated with the press. Constant forging energy Constant press energy is critical for reproducibility in. precision forging in every stroke Especially when speed is increased during redesign. a new forging process Excessive energy causes an increase in the pressure inside the. die which may result in the elastic deformation of some tools such as punches and. counterpunches and ultimately lead to an elastic rebound of the tool Elastic defor. mations of tool components should be avoided since they can cause changes in ge. ometry during forging, Precision guidance of tool components Precision guidance of the punch is required. when the die is closed by the upper punch during forging The gap between the punch. and the die is usually smaller than 0 1 mm in order to avoid a material flash More. over accurate positioning of tool components is necessary to obtain proper forged. product geometry,The main aspects of precision forging 45.
Accurate positioning of slug inside die In order to ensure proper die cavity filling. the slug must be accurately positioned inside the die especially when the slug is auto. matically delivered to the successive forging stations Using physical modelling to. analyze the flow of material it was found in 9 that too slender specimens incorrectly. positioned in the die impression would undergo buckling Despite the buckling the. forged object had the correct shape but the uneven flow of material may affect the du. rability of the end product Figure 8, Fig 8 Uneven flow of material caused by specimen buckling. a cross section and b view from punch, Reliable die closure Die closure is essential for the proper operation of the set of. tools at the machine settings For many precision forged products the split line lies in. the vicinity of the work surfaces and so they may be deformed at improper closure. The die should be closed by accessory elements or simultaneously with the motion of. the punches,3 5 Lubrication and cooling, To a large extent the correctness of forming process depends on the lubricant used. The latter is used to both lubricate and cool The lubricant should be characterized by. a high flash point so that it does not lose its tribological properties at high tempera. tures low heat conduction to prevent the object being forged from cooling down and. the tool from overheating proper viscosity at the operating temperature and a low co. efficient of friction Moreover an optimum lubricant should not contain any compo. nents having an adverse effect on the process 19 Graphite teflon glass and other. substances as well as intermediate metallic layers characterized by low flow stress are. usually used in hot forging, In order to ensure healthier working conditions in the forging industry and mini. mize its environmental impact as well as to increase the life of the tools the European. Community has recently funded an industry research project called Brite Euram aimed. at developing environmentally friendly systems of tool lubrication based on lubri. cants with optimum lubricating properties for the warm forging of steel and to pro. mote a wide use of forging with a smaller pollutant burden better working conditions. and higher productivity 19 It was found that in most hot forging processes no proper. 46 Z GRONOSTAJSKI M HAWRYLUK, tribological conditions were ensured This is mainly due to the high contact pressures.
and metal surface gains typical for hot forging especially forward and backward ex. trusion forging In all the cases it is necessary to provide additional lubrication in the. form of lubricating film on slugs Thanks to the project a comprehensive tool lubrica. tion system has been developed As regards the environmental impact and tool life the. best results have been achieved through the use of tool lubrication systems consisting. of a graphite based slug lubricating film and graphite free oil greases for dies. The choice of a tool material is a very difficult task for designers and process engi. neers The life of a tool and its suitability for production depend on many factors. which often have opposing consequences So far there are no clear cut criteria for se. lecting tool materials and to a large extent one must rely on the experience of the. manufactures and tool users Statistical data provided by different tool manufacturers. indicate the most common causes of tool failures to be tool fatigue cracking in cold. working and excessive abrasive wear material plastic flow and thermal fatigue in hot. working 20 The worst situation is in warm working since each of the phenomena. can be equally critical In such conditions the tools must withstand high pressures as. in cold working and at the same time must be made of heat resistant materials as in. hot working According to Lange et al 21 the life of a tool at high forging tempera. tures depends on wear in over 70 of cases Therefore tool materials their heat treat. ment and machining and tool design and fabrication accuracy must meet very high re. quirements Tool materials should be characterized by hardness in a range of 50 55. HRC considerably higher than that of the forged product good hardenability high. tensile strength high impact strength and low abrasibility. Currently warm and hot work tool steels WCL WWV WNLV which are. characterized by very good mechanical properties high tensile strength high hard. ness high abrasion resistance high yield point 2200 MPa are quite popular tool. materials Also other alternative tool materials are considered Moreover special. treatment such as nitriding surface coating and laser silicon carbide surface alloying. etc can significantly reduce the abrasive wear and increase the hardiness of the tool. materials particularly of their surface layer 24,4 Heat treatment. Heat treatment has a decisive effect on tool life Figure 9 shows a heat treatment. diagram for a hot work tool steel, For instance cracks which appear on the surface of a ground tool made of tool steel. can be caused by improper tempering or by overheating during austenitizing Such. heat treatment faults can limit the possibilities of grinding the tool even if proper pre. cautions are taken,The main aspects of precision forging 47. Fig 9 Exemplary tool material heat treatment diagram. Stress relief annealing Tool steels are usually delivered annealed further treat. ment is done by the user or in toolrooms The treatment causes stresses to arise which. during heating up to the hardening temperature are released and cause among others. changes in the dimensions Therefore tools which must meet high requirements for. retaining their dimensions should be annealed after mechanical pretreatment before. the final treatment, Hardening It is impossible for steels subjected to hardening to ideally fully retain. their dimensions A change in the structure from the annealed state structure to the. structure after hardening means an increase in the volume which as a result of tem. pering that follows is again reduced but not restored to the annealed state volume Be. cause of buckling it is recommended to use the gentlest possible cooling medium and. to perform intermediate cooling at 900 C for steels whose hardening temperature is. above 900 C Since there is a risk of strain cracking one should avoid cooling the. tools to room temperature The latter are usually cooled down to about 150 C Then. at a temperature of about 100 150 C equalization takes place aimed at eliminating. the differences in temperature and structure between the surface and the core Quench. ing in a hot bath and in vacuum as a hot bath simulation for hot work tool steels has. proved to be effective, Austenitizing keeping at hardening temperature The hardening temperatures speci.
fied by the manufacturers should be adhered strictly to If the hardening temperature is. too low the increase in hardness is insufficient If it is too high it results in brittleness. increased grain size a risk of cracks and so on Also soaking times are important An. approximate soaking time for hot work tool steel is 0 5 min per 1 mm of thickness. Tempering Immediately after hardening steel is tempered several times The aim. is to remove any stresses which arose during hardening and to reduce the often too. high postcooling hardness in favour of shock resistance Depending on the desired. hardness a tempering temperature should be chosen in accordance with the proper. 48 Z GRONOSTAJSKI M HAWRYLUK, tempering diagram It is reported in 7 that triple tempering gives the best results in. the case tool steel W303, Machining In order to increase tool life die cavities and punch work surfaces. should be made appropriately smooth During forging the heated up material rubs. against tool work surfaces and abrades them The degree of abrasion can be reduced. through the use of a proper material but this will not replace finishing consisting in. grinding and planishing of the work surfaces One should note that during such work. ing the tool surface is often overheated and weakened as a result In some cases the. tempering temperature of the tool steel may be exceeded by overheating Machining. may cause microcracks to appear in the tool The microcracks will grow during work. and result in a premature fracture, Thermochemical treatment Sometimes as a result of prolonged operation of the. tools at a high temperature the tool material suffers thermomechanical fatigue and its. properties sharply deteriorate whereby its elastic limit is exceeded or the metal spon. taneously heats up Then nitriding is applied to increase the life of the tools by in. creasing their surface hardness and abrasive wear resistance The treatment can be ap. plied several times,5 Optimum die profile, One of the main parameters having an influence on the forging process is the shape. of the adopted die The magnitude of deformation obtainable in a single operation is. limited not by material decohesion cracks but by material strength too large hoop. stresses For this reason instead of producing a forging in one operation multiopera. tion industrial precision forging is used Still this does not sufficiently reduce the great. forming forces and thereby the wear of the tools, Die shape optimization receives much attention in the literature 2 4 11 15 22 It.
turns out that one of the ways of reducing excessive pressures on the die or the punch. is the choice of an optimum die profile outline Three types of die profile stream. line curvilinear and conical are generally used in industrial applications In forward. extrusion the streamline profile ensures the smallest excessive deformation the small. est extrusion forces and the least deformation inhomogeneity in the cross section But. the problem is with determining the correct profile and fabricating it the profile is. usually to long time For this reason the profile is often replaced by any curvilinear. outline which often leads to even worse results than when conical profile dies are. used A conical profile with an angle minimizing excessive deformations may result. in good extrusion parameters and good product quality and it can be very easily made. Today research on die profile optimization aimed at improving product quality and. productivity combines classical mathematical optimization methods with FEM The. die profile optimization problem can be solved by applying one of the mathematical. programming methods 18 Usually B zier curves and other specially adapted optimi. zation methods are employed to mathematically describe die profiles Ales and Boris. The main aspects of precision forging 49, 12 investigated tool design optimization for stationary extrusion by imposing two. criterion optimization Wife et al 16 proposed a special incremental computing. method combined with FEM in order to obtain the distribution of pressure on the die. walls for the adopted die profile during the hot extrusion of a bar Thanks to studies. of optimum die profiles by Lee et al 18 a more uniform structure of hot forged prod. ucts can be obtained In 4 the die profile was optimized by sequential quadratic pro. gramming SQP which is now successfully applied to solve nonlinear optimization. The present authors et al in 8 assessed the flow of material in arch and conical. dies for forging CV joint tulips, CV joints are irreplaceable components of front wheel drive cars They transmit. torque from the gearbox to the front wheels Their production has been steadily in. creasing in recent years A CV joint consists of a spider a race and a casing The most. difficult to manufacture is the casing the more so that because of its irregular shape. machining is made very difficult Currently hot and cold multioperation forging in. closed dies with a complex formation scheme forward and backward extrusion is. used in long run production The authors employed both physical modelling using. soft modelling materials synthetic filia wax and Plasticine and numerical modelling. by means of the MSC software package to study material flows A macroscopic analy. sis revealed differences in the flow of material in the particular tools In the case of. arch tools the material flow in the specimen s cross section is more uniform than when. for conical tools Figure 10 This is confirmed by the fact that the mesh line curves. for the object forged in the arch die are less elongated than the ones for the object. forged in the conical die La Lb, Fig 10 Comparison of model material flow in a arched tools and b conical tools 8. The investigations showed that the material flow in the arched dies was more uni. form than in the conical dies whereby the distribution of strains was more uniform too. Figure 11 In addition the punch forces were smaller for the arched dies. Figure 12 shows a vector distribution of unit pressures for the tools obtained by. means of the SuperForm2005 software The unit pressures at the first reduction stage. were similar for the two kinds of dies but at the next stages they were much higher for. the conical die,50 Z GRONOSTAJSKI M HAWRYLUK, Fig 11 Distribution of plastic strains in a arched tools and b conical tools 8. Fig 12 Vector distribution of unit pressures in a conical b arched tools 8. 6 Die design, Currently in order to increase the crack resistance of precision forging tools pre.
stressed dies i e reinforced with a single concentric ring or a larger number of such. rings with thermo compression or forced in joints in between are used It turns out that. by changing the state of stress one can significantly reduce tool cracking 7 The ap. plication of an appropriately high pre stress compressive hoop stress during assembly. should compensate for the very high tensile hoop stresses which occur during forging. The main aspects of precision forging 51, The present authors et al analyzed using numerical modelling and actual negative. allowance measurements the effect of negative allowance in pre stressed dies used in. the production of the CV joint casing on their strain 7. Numerical modelling was done using the MSC Marc2005 and Super Form 2005. software It was carried out in two stages 1 forcing of a compensation ring together. with a die onto an elastic sleeve and 2 actual forging based on the first stage condi. tions Figure 12 shows the analyzed set of tools The function of the compensating. ring is to pre stress the die via the slit elastic sleeve. Fig 12 Schematic of die prestressed by single ring 0. The nominal negative allowance was wnom 0 4 mm But it was found that the ac. tual negative allowance depended on the tools dimensions determined by their toler. ances and wear, From the field of execution tolerances for the particular tool system components it. followed that the negative allowance could range from wmin 0 225 mm to wmax 0 45. mm Hence one could assume that the expected average negative allowance would be. wav 0 34 mm i e much less than the nominal negative allowance. Fig 13 Distributions of hoop stress in die during forging at negative allowance. a w 0 25 b w 0 4 c w 0 7 mm 7,52 Z GRONOSTAJSKI M HAWRYLUK. The aim of the research was to determine how the change in negative allowance. resulting from the tolerance fields can affect the strain of the dies during forging. A negative allowance of 0 25 mm close to the minimum negative allowance the. nominal negative allowance of 0 4 mm and an increased negative allowance of 0 7. mm were analyzed Figure 13 shows hoop tensile stress distributions for the different. negative allowance values The maximum hoop stresses occur at the second reduction. stage the circle marks this place, The hoop tensile stresses at a negative allowance of 0 25 mm are very high amount. ing to 1000 MPa This means that at such low negative allowances the dies may. quickly fail, Then reduced stresses were analyzed Figure 14 shows reduced stress von Misses.
stress distributions for the assumed negative allowances. Fig 14 Distributions of von Mises stress in die during forging at negative allowance. a w 0 25 b w 0 4 c w 0 7 mm 7, Figure 15 shows the dependence between reduced stress in the most heavily loaded. places the first stage of cross section reduction and negative allowance Even though. all the flow stress values are below the yield point of the die material the smallest. negative allowance of 0 25 mm may be too small since in the places where the die is. most heavily loaded the temperature significantly rises As a result the yield point in. those areas may go down to below 2000 MPa In real experiments plastic flow of die. material would be observed at the first stage of reduction The analysis showed that. a negative allowance below 0 4 mm did not ensure sufficient pre stressing of the dies. This was confirmed by observations of the real process It was found that the wear of. the compensating ring and that of the elastic sleeve and a wide range of execution tol. erances of the tools may result in an allowance much smaller than 0 4 mm. The main aspects of precision forging 53, Fig 15 Reduced stress versus negative allowance at forging stage 7. Fig 16 a Method of measuring distance Z and, b Relation between distance Z and negative allowance 7. Therefore it is necessary to control negative allowance when the die is forced into. the compensating ring For this purpose a simple indirect method of determining nega. tive allowance i e on the basis of the distance between the upper surface of the die. and the upper surface of the compensating ring forced into the die by the force of. gravity Figure 16a is proposed It has been found that if this distance is larger than. 15 mm the negative allowance is larger than 0 4 mm Figure 16b shows the relation. between distance Z and negative allowance which allows one to determine the. negative allowance value in a simple way during the assembly of the tools. 7 Conclusion, The growing market demand particularly from the automotive industry has led to. very intensive development of precision forging Its advantage over other technologies. is that it offers considerable material savings owing to the fact that there is no flash. and that the end product is almost finished whereby finishing is not needed or reduced. to minimum Currently precision forging is used mainly as multi operation industrial.

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