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Sheet Metal Powder Processes

Metal ForgingCONTENTS

Hot Vs. Cold Forging Process Classification

Open Die Forging Cogging Fullering Edging

Impression Die Forging Precision Forging Flashless Forging Metal Forgeability

Metal Forging Defects Lubrication

Forging Die Material Forging Die Design Formation Of Flash

Ribs And Webs Fillet Radius

Draft Angle In Forging Die Parting Line Location

Metal Forging Process Design

Metal forging is a metal forming process that involves applying compressive forces to a workpiece to deform it, and create a desired geometric change to the material. The forging processis very important in industrial metal manufacture, particularly in the extensive iron and steelmanufacturing industry. A steel forge is often a source of great output and productivity. Workstock is input to the forge, it may be rolled, it may also come directly from cast ingots orcontinuous castings. The forge will then manufacture steel forgings of desired geometry andspecific material properties. These material properties are often greatly improved.

Metal forging is known to produce some of the strongest manufactured parts compared toother metal manufacturing processes, and obviously, is not just limited to iron and steelforging but to other metals as well. Different types of metals will have a different factorsinvolved when forging them, some will be easier to forge than others. Various tests aredescribed latter to determine forging process factors for different materials. Aluminum,magnesium, copper, titanium, and nickel alloys are also commonly forged metals. It isimportant to understand the principles of manufacturing forged products, including differenttechniques and basic metal forging design. The following will provide a comprehensiveoverview of the metal forging process.

Metal forging, specifically, can strengthen the material by sealing cracks and closing emptyspaces within the metal. The hot forging process will highly reduce or eliminate inclusions inthe forged part by breaking up impurities and redistributing their material throughout themetal work. However, controlling the bulk of impurities in the metal should be a considerationof the earlier casting process. Inclusions can cause stress points in the manufactured product,something to be avoided. Forging a metal will also alter the metal's grain structure withrespect to the flow of the material during its deformation, and like other forming processes,can be used to create favorable grain structure in a material greatly increasing the strength offorged parts. For these reasons, metal forging manufacture gives distinct advantages in themechanical properties of work produced, over that of parts manufactured by other processessuch as only casting or machining.

Metal forgings can be small parts, or weigh as much as 700,000 lbs. Products manufacturedby forging in modern industry include critical aircraft parts such as landing gear, shafts for jetengines and turbines, structural components for transportation equipment such as automobilesand railroads, crankshafts, levers, gears, connecting rods, hand tools such as chisels, rivets,screws, and bolts to name a few. The manufacture of forging die and the other high costs ofsetting up an operation make the production of small quantities of forged parts expensive on aprice per unit basis. Once set up, however, operation costs for forging manufacture can berelatively low, and many parts of the process may be automated. These factors makemanufacturing large quantities of metal forgings economically beneficial.

Figure:154

Hot Die Vs. Cold Die ForgingMost metal forging operations are carried out hot, due to the need to produce large amounts ofplastic deformation in the part, and the advantage of an increased ductility and reducedstrength of the work material. Hot die forging also eliminates the problem of strain hardeningthe metal. In cases where it is desirable to create a favorable strain hardening of the part, colddie forging may be employed. Cold die forging manufacture, while requiring higher forces,will also produce greater surface finish and dimensional accuracy than hot die forging. Somespecific metal forging processes are always performed cold, such as coining.

Classification Of Metal Forging ProcessesMetal forging processes can be classified by the degree to which the flow of material isconstrained during the process. There are three major classifications in metal forgingmanufacture. First, open die forging, in which the work is compressed between two die thatdo not constrain the metal during the process. Secondly, impression die forging, in whichcavities within the die restrict metal flow during the compression of the part, causing thematerial to deform into a desired geometric shape. Some material in impression die forging isnot constrained by the cavities and flows outward from the die, this metal is called flash. Inindustrial metal forging, a subsequent trimming operation will be performed to remove theflash. The third type of metal forging is flashless forging. In flashless forging manufacture theentire work piece is contained within the die in such a way that no metal can flow out of thedie cavity during the compression of the part, hence no flash is produced.

Open Die ForgingThe manufacturing process of metal forging has been performed for at least 7,000 years,perhaps even 10,000 years. The most basic type of forging would have been shaping somemetal by striking it with a rock. Latter the employment of different materials, such as bronzethen iron and steel, and the need for forged metal products such as swords and armor, led wayto the art of blacksmithing or blacksmith forging. Blacksmithing is an open die forgingprocess where the hammer and anvil surfaces serve as opposing flat die. Bronze forgings,followed by iron and steel forgings, mark some of man's earlier manufacturing prowess.

A simple type of open die forging is called upsetting. In an upsetting process the work isplaced between two flat die and its height is decreased by compressive forces exerted betweenthe two die. Since the volume of a metal will remain constant throughout its deformation, areduction in height will be accompanied by an increase in width. Figure 155 shows a flat dieupsetting process, under ideal conditions.

Figure:155

In real conditions during industrial manufacturing, friction plays a part in the process. Frictionforces at the die-work interface oppose the spreading of the material near the surfaces, whilethe material in the center can expand more easily. The result is to create a barrel shape to thepart. This effect is called barreling in in metal forging terms. Barreling is generallyundesirable and can be controlled by the use of effective lubrication. Another consideration,during hot forging manufacture, that would act to increase the barreling effect would be theheat transfer between the hot metal and the cooler die. The metal nearer to the die surfaceswill cool faster than the metal towards the center of the part. The cooler material is moreresistant to deformation and will expand less than the hotter material in the center, alsocausing a barreling effect.

Figure:156

Another common open die forging process performed in industrial metal forging manufacture,involves using flat die to round an ingot. With the use of mechanical manipulators, a workpiece is compressed and rotated in a series of steps eventually forming the metal into acylindrical part. The compressions affect the material of the forging, closing up holes andgaps, breaking down and reforming weak grain boundaries, and creating a wrought grainstructure. As this open die forging process progresses the material of the part will be alteredfrom the outside first, progressing inward. It is important that when manufacturing a metalforging by this process, the part is worked significantly enough to change the structure of thematerial in the center of the work piece. Large shafts for motors and turbines are forged thisway from cast ingots.

Cogging, or drawing out, is often used in manufacturing industry. Cogging is an open dieforging process in which flat or slightly contoured die are employed to compress a work piece,reducing its thickness and increasing its length. In a cogging operation, the forging is largerelative to the size of the die. The part is forged in a series of steps. After each compression ofthe material, the open die advance along the length of the work piece and perform anotherforging compression. The distance the die travel forward on the work piece between eachforging step is called the bite, and is usually about 40 to 75 percent of the width of the die, inindustrial practice. A greater reduction in the thickness of the forged part can be accomplishedby decreasing the width of the bite. Cogging allows for smaller machinery with less powerand forces to form work of great length. Often in commercial manufacture of metal products,cogging may be just one metal forging process in a series of metal forging processes requiredto form a desired part. Sometimes formed products such as metal fences may be produceddirectly from cogging.

Figure:157

A typical open die forging process performed in metal forging manufacture is fullering.Fullering is mostly used as an earlier step to help distribute the material of the work inpreparation for further metal forging operations. This often occurs when a manufacturingprocess requires several forging operations to complete. The metal forging process designsection will discuss this concept later. In fullering, open die with convex surfaces are used todeform the work piece. The result is to cause metal to flow out of one area and to both sides.

Figure:158

Edging is also an open die forging process often used in manufacturing practice, to prepare awork for sequential metal forging processes. In edging, open die with concave surfacesplastically deform the work material. Edging acts to cause metal to flow into an area fromboth sides. Edging and fullering both are used to redistribute bulk quantities of the metalforging's material.

Figure:159

Impression Die ForgingImpression die forging manufacture involves compression of a work piece by the use ofimpression die, (a mold), that contain cavities that act to restrict the flow of metal within thedie during the deformation of the work. The metal will fill the space within the die cavity as itis plastically compressed into the mold. Closing of the mold completes the deformation, henceimpression die forging is also referred to as closed die forging. The forged metal part will nowhave the geometric dimensions of the mold, provided a complete filling of the die cavityoccurred during the process. The operation of forcing metal to flow into and fill theimpressions in the die will also alter the grain structure of the metal. The creation of favorablegrain structure through controlled material deformation should always be a consideration inthe design of an impression die forging process.

One characteristic of impression die forging manufacture is the formation of flash or finaround the forged part. During the design of the metal forging operation, the volume of thestarting work piece is made slightly higher than that of the closed die cavity. As the die close,and the work metal flows into and fills the contours of the impression, some excess materialwill flow out of the die and into the area between the two die. This will form a thin plane ofmetal all around the work at the parting line, (where the two die meet when they close), of theforged product. Flash is trimmed from the forging in a latter process.

Figure:160

Precision ForgingModern technological advances in the metal forging process and in the design of die, haveallowed for the development of precision forging. Precision forging may produce some or noflash and the forged metal part will be at or near its final dimensions, requiring little or nofinishing. The number of manufacturing operations is reduced as well as the material wasted.In addition, precision forging can manufacture more complex parts with thinner sections,reduced draft angles, and closer tolerances. The disadvantages of these advanced forgingmethods are that special machinery and die are needed, also more careful control of themanufacturing process is required. In precision forging, the amount of material in the work, aswell as the flow of that material through the mold must be accurately determined. Otherfactors in the process such as the positioning of the work piece in the cavity must also beperformed precisely.

Flashless ForgingFlashless forging is a type of precision forging process in which the entire volume of the workmetal is contained within the die and no material is allowed to escape during the operation.Since no material can leave the mold as the part is forged, no flash is formed. Like otherprecision forging processes, flashless forging has rigorous process control demands,particularly in the amount of material to be used in the work piece. Too little material and thedie will not fill completely, too much material will cause a dangerous build up of forces.

Figure:161

Metal ForgeabilityMetal selection must be considered carefully in forging manufacture. The ability of a metal toexperience deformation without failure or cracking is an important characteristic to considerin its selection as a material for a forging process. In metal forging industry, several tests havebeen developed to try and quantify this ability. The amount of deformation a particular metalcan tolerate without failure is directly related to that metal's forgeability. The higher theamount of deformation, the higher the forgeability.

One popular test involves compressing a cylindrical work stock between two flat die. This iscalled upsetting the work, thus this test is called the upsetting test. In an upsetting test, thework stock is compressed by flat open die, reducing the work in height until cracks form. Theamount of reduction can be considered a measurement of forgeability. Upsetting tests can beperformed at different temperatures and different compression speeds. Testing varioustemperatures and strain rates will help determine the best conditions for the forging of aparticular metal.

Another common test used in modern industry is called the hot twist test. In a hot twist test, around bar is twisted in one direction until material failure occurs. The amount of rotation istaken as a quantitative measurement of metal forgeability. This test is often conducted on amaterial at several different temperatures. Other tests are also used in industrial metal forgingmanufacture. Impact testing is sometimes used to gauge the forgeability of a material. Cracksin the metal are the common criteria for failure for most tests, however, forgeability tests canalso determine other negative effects that a material may exhibit under different conditions ofstress, strain rate, and temperature.

Defects In Metal Forging

Inspection is an important aspect of metal forging manufacture. All parts should be checkedfor defects after the manufacturing process is complete. Defects of metal forged productinclude exterior cracking, interior cracking, laps, cold shuts, warping of the part, improperlyformed sections and dead zones. Cracking both interior and exterior is caused by excessivestress, or improper stress distribution as the part is being formed. Cracking of a forging can bethe result of poorly designed forging die or excess material in the work piece. Cracks can alsobe caused by disproportionate temperature distributions during the manufacturing operation.High thermal gradients can cause cracks in a forged part.

Laps or folds in a metal forging are caused by a buckling of the part, laps can be a result of toolittle material in the work piece. Cold shuts occur when metal flows of different temperaturesmeet, they do not combine smoothly, a boundary layer, (cold shut), forms at their intersection.Cold shuts indicate that there is a problem with metal flow in the mold as the part is beingformed. Warping of a forged part can happen when thinner sections cool faster than the rest ofthe forging. Improperly formed sections and dead zones can be a result of too little metal inthe work piece or flawed forging die design resulting in incorrect material distribution duringthe process.

In general, defects in parts manufactured by metal forging can be controlled first by carefulconsideration of work stock volume, and by good design of both the forging die, (mold), andthe process. The main principle is to enact the right material distributions, and the rightmaterial flow to accomplish these distributions. Die cavity geometry and corner radius play alarge roll in the action of the metal. Forging die design, and forging process design will bediscussed in later sections.

Lubrication In Industrial Metal ForgingManufacture

Frictional forces within the mold, between the work and the surfaces of the die cavity, have alarge influence over the flow of material in a metal forging operation. Lubricants are used inindustrial metal forging production in order to lower frictional forces, and enact a smootherflow of metal through the mold. In addition, they are used to slow the cooling of the work andreduce temperature gradients, in hot forging manufacture, serving as a thermal barrier betweenthe metal work and the die. Lubricants also help keep the metal and die surfaces from stickingtogether and assist in the removal of the metal forging from the die. Common lubricants usedin modern forging industry include, water, mineral oil, soap, saw dust, graphite, molybdenumdisulfide, and liquid glass.

Forging Die MaterialThe exact material used to make a forging die,(mold), is dependant upon all the details of thatparticular metal forging process. In general, a forging die must be tough, possess high strengthand hardness at elevated temperatures, good shock resistance, resistance to thermal gradients,hardenability and ability to withstand abrasive wear. During the manufacture of a hot forgedpart, the forging die is usually preheated before the operation begins. Preheating forging diereduces thermal cycling that can cause cracks in the die.

Metal forging die are hardened and tempered. Forging die dimensions must account forshrinkage of the work, as well as extra material allowances for the finishing of the part. Theabrasive wear present in hot forging operations is due largely to the scale on the work stock.Much of the scale can be removed from the blank immediately after heating in the furnace,prior to the forging of the part. Adequate lubrication can also greatly mitigate wear.Sometimes a forging die may be assembled using different die sections. These sections, calleddie inserts, are manufactured separately and may be of different materials. Complex cavitiescan be produced easier with die inserts, also different sections of the forging die can beindividually replaced.

Some factors to consider when determining the material composition of a forging die are, typeof operation, number of die forgings, size of forged parts, complexity of forged parts, type ofmachinery to be used, temperature that the metal will be forged at, and the cost of materials.Forging die are made from tool steels that, depending upon process criteria, are alloyed withvarious levels of one or more of these materials, chromium, molybdenum, vanadium, andnickel. Die blocks are cast from the alloy, forged themselves, then machined, and finished.

Forging Die DesignForging die design will always depend on the factors and requirements of the manufacturingprocess. However, there are some general principles to consider for good forging die design.During the forging process metal is flowing under pressure to fill the impression within the diecavity, (mold). Similar to the metal casting process of die casting, in metal forging, anincrease in pressure on the metal within the die will increase the ability to fill the die cavitycompletely. One main difference between the processes is that in die casting the metal isliquid, while in forging the work is a solid metal above or below its recrystallizationtemperature. Smaller, thinner, longer, and more complex sections can be produced with morepressure, but too much pressure within the die cavity is bad because it can damage the die andmachinery.

The formation of metal flash is an important part of impression die forging manufacture. First,flash provides a way for excess material from the work stock to exit the forging die. If thismaterial could not escape during the compression the build up of pressure, as the volume ofwork metal exceeded the volume of the die cavity, could easily crack the die. Flash, whileallowing material to escape, does increase the pressure within the die cavity. Flash must travelthrough a narrow passage, called land, before it opens up into a gutter.

As it flows through land, the friction between the metal flash and the mating surfaces resistsfurther flow of material out of the die cavity, increasing pressure within the forging die. Inaddition, the cooling of the flash from the mating surfaces increases resistance to flow ofmaterial out of the die, thus also increasing pressure within the die cavity. A longer land willcause the metal flash to have to flow further under resistance, increasing the die pressure.Decreasing the width of land will also increase pressure within the forging die by increasingthe cooling rate of the flash, as the temperature goes down the metal's resistance to flow goesup. More resistance to metal flow will cause a thinner land to create higher die pressure. Thepressure within the forging's die cavity is often controlled by varying the width of land.

Figure:162

One of the main principles to remember when designing a forging die for a specificmanufacturing process is that while deformation of the metal is occurring, the material willtend to flow in the direction of least resistance. Proper metal flow within the die is importantin ensuring a complete filling of the die cavity, preventing defects, and in controlling the grainstructure of the forged part. Friction in the die is an important consideration in metal forgingmanufacture. Friction will act to resist the movement of the material and increase the forcesrequired to fill the die cavity during the process. More forces, in turn, mean more stress andwear on the mold and equipment.

Another critical factor in the movement of material within the die cavity during the forming ofthe part, is the interior geometry of the die cavity. The size of the forged part, work material,complexity of the forged part, size and thickness of different part features, and distancedifferent areas are from the parting line, are some of the important factors concerning thestructure of the forging die. Basically thinner more complex features will be more difficult tofill completely, as would areas further from the parting line or out of the way of thepredominant flow of metal.

Thin portions of a metal forging are called ribs and webs. A rib is a section that runsperpendicular to the forging plane as determined by the parting line. Long narrow ribs areharder to fill and require more forces, increasing the width of a long rib will better facilitatethe filling of the rib with material during the process. A web is a portion of the metal forgingthat runs parallel to the forging plane. The thickness of webs can be minimized as much aspractical. When designing a forging die, web thickness should not be too small or else theremay be trouble completely filling the web with metal. Webs that are too thin may also coolfaster than the rest of the metal forging, the resulting shrinkage could cause tears or warpingof the part.

Figure:163

As the work material fills the die cavity, the flow of metal will have to change directionsdepending upon the part's geometry. Smooth, large filleted turns will allow the metal flow tochange directions while adhering to the die's dimensions. If corners within the metal forgingare too sharp then the material may not completely follow the path of those corners, resultingin vacancies, laps, or cold shuts. Sharp corners will also act as stress raisers within the diecavity. Good forging die design should provide adequate enough fillet and corner radius toallow for easy metal flow.

Figure:164

Figure:165

Draft angle, in metal manufacturing processes, is the taper around the internal and externalsides of a part. Draft angle is necessary to include in the forging die design in order to allowthe removal of the work from the die after the part has been forged. The larger the draft angle,the better it will facilitate the metal forging's removal. As the metal forging cools, it shrinksaway from the outer surfaces of the die cavity, therefore exterior draft angles are usually madesmaller than interior angles.

In general, easier to forge metals, such as aluminum and magnesium, require less draft anglesthan harder to forge materials, such as steel, nickel, and titanium alloys. Often in metal forgingoperations, there is an ejector to help push the part from the die cavity. However, ejectors arenot used in drop forging. Draft angle effects the complexity of the forging that may beproduced. The greater the draft angle, the more it limits metal forging complexity. Someprecision forging operations produce a forged part with no draft angle. Common draft anglesused in manufacturing industry are 3, 5, 7, and 10 degrees.

Figure:166

Similar to the pattern in metal casting, the size of the die cavity in metal forging manufactureshould account for the size of the part, shrinkage of the part during cooling, and allowancesfor machining and other finishing operations that may follow the metal forging process.

Location of the parting line is of primary importance in metal forging die design. The partingline, which defines the forging plane of the operation, is a large determinant in how metalflows through the die during the forging's compression. The parting line dictates where flashwill be formed, and effects the grain structure of the manufactured part. It is easier to fillsections closer to the parting line than further away. In determining a parting line themaximum periphery of the metal forging should be considered.

Figure:167

Figure 167 shows a metal forging with three possible locations for a parting line. The locationof the parting line of C will better facilitate the flow of metal through the die cavity, sinceunlike A or B, location C makes use of the maximum periphery of the forging. It is easier tofill material near the forging plane than in the further recesses of the die cavity. In addition tobeing a major factor in the flow of metal during the forging process, the location of the partingline is also critical in the formation of the grain structure of the forged work. The parting lineacts to disrupt the metal's grain structure.

Figure:168

Figure 168 also shows three possible parting line locations for a metal forging. The placementof the parting line in A and B acts to disrupt the grain structure of the metal at the planethrough which it passes. Locating the parting line at the top of the forging as in C eliminatesthe rupture of the forging's grain structure. Also this particular location of the parting line willallow for the entire impression to be formed in one die, while the other die can be flat. Designof the die as in C is both more economical and provides superior grain structure of the metalforging.

Forging Process DesignIn modern manufacturing industry, metal parts of complex geometry are often forgedcompletely with the need for only minor finishing operations. These parts can not bemanufactured with a single forging. The work stock is taken through a series of metal forgingoperations that, in steps, alter the geometric shape of the material until the final processcreates the desired forging. In these types of design sequences each operation must be plannedin such a way as to prepare the work piece for the next forging process. Together the series ofmetal forging operations that are required to create a part, make a larger single process andeach individual forging operation has its place within the larger process.

When designing a complex metal forging process, great consideration should be taken witheach step and how it relates to the final product. Also, design the chosen path for theredistribution of the work material from the start of the process to the end of the last step,concentrating on smooth metal flow. Forging design, in general, should first accomplish arough redistribution of the material, then the more detailed impression die forging operations,and finally finishing operations. In addition to providing a smooth transition of material theforging processes, as a whole, should be designed to produce a controlled grain structure inthe final product. When choosing a path for material redistribution, a metal forging designshould consider how this particular method of metal deformation will effect and change thegrain structure of the part. It is desirable that the final product contain a favorable grainorientation throughout the structure of its material. Such a grain structure should strengthenthe part, particularly with respect to that part's application.

Figure:169

Open die forging often plays a roll in the early stages, providing a general mass redistributionof the work metal. Before the more detailed impression forgings can shape the work, metalmust be formed in such a way as to place higher concentrations of material in regions that willrequire more material. Fullering and edging of the metal, discussed in the open die forgingsection, are very important open die forging processes used to accomplish a rough transfer ofmaterial. Fullering and edging will squeeze more metal into some areas of the work, whilecausing other areas to have less depending on the needs of the process. Figure 170 shows tworough forms, one was subject to fullering the other to edging, the nature of the differentprocesses should be apparent.

Figure:170

Impression die forging occurs after the rough form has been shaped. This closed die forgingprocess will create the geometric features of the part on the work. The flow of metal must becarefully designed both before and during this phase. Finishing processes, such as sizing,create less but very accurate geometric change to the forging in the final stages of partmanufacture. Figure 171 shows the different steps in the metal forging process used tomanufacture a complex part.

Figure:171

Most industrial metal forging products will be processed by further manufacturing operationsthat will impart higher tolerances and dimensional accuracy than forging manufacture alone.These operations, (such as machining), although more accurate than forging, do not producethe stronger material of forged metal work. By combining different types of processes such asmachining and metal forging, a manufacturer can utilize the benefits of both processes,creating very accurate parts, good surface finish and superior mechanical properties.

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