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Casting and Other Forming Processes
DeGarmo, E. Paul et al. Materials and Processes in Manufacturintg. John Wiley & Song, 1998. P205-211
Casting
Casting is the introduction of molten metal into a cavity or mold where, upon solidification, it becomes an object whose shape is determined by mold configuration. Casting offers several advantages over other method of metal forming: it is adaptable to intricate shapes, to extremely large pieces, and to mass production; it can provide parts with uniform physical and mechanical properties through out and, depending on the particular material being cast, the design of the part, and the quantity being produced, its economic advantages can surpass other processes.
Categories
Two broad categories of metal-casting processes exist: ingot casting (which includes continuous casting) and casting to shape. Ingot castings are produced by pouring molten metal into a permanent or reusable mold. Following solidification these ingots (or bars, slabs, or billets, as the case may be) are then further processed mechanically into many new shapes. Casting to shape involves pouring molten metal into molds in which the cavity provides the final useful shape, followed only by machining or welding for the specific application.
Ingot casting Ingot castings make up the majority of all metal castings and are separated into three categories: static cast ingots, semi-continuous or direct-chill cast ingots, and continuous cast ingots.
Static cast ingots Static ingot casting simply involves pouring molten metal into a permanent mold. After solidification, the ingot is withdrawn from the mold and the mold can be reused. This method is used to produce millions of tons steel annually.
Semi-continuous cast ingots A semi continuous casting process is employed in the aluminum industry to produce most of the cast alloys from which rod, sheet, strip, and plate configurations are made. In this process molten aluminum is transferred to a water-cooled permanent mold which has a movable base mounted on a long piston. After solidification has progressed from the mold surface so that a solid "skin" is formed, the piston is moved down. Finally the piston will have moved its entire length, and the process is stopped. Conventional practice in the aluminum industry utilizes suitably lubricated metal molds. However, technological advances have allowed major aluminum alloy producers to replace the metal mold (at least in part) by an electromagnetic field so that molten metal touches the metal mold only briefly, thereby making a product with a much smoother finish than that produced conventionally.
Continuous cast ingots Continuous casting provides a major source of cast material. In the steel and copper industry and is growing rapidly in the aluminum industry. In this process molten metal delivered to a permanent mold, and the casting begins much in the same way as in semi continuous casting. However, instead of the process ceasing after a certain length of time, the solidified ingot is continually sheared or cut into lengths and removed during casting. Thus the process is continuous, the solidified bar or strip being removed as rapidly as it is being cast. This method has many economic advantages over the more conventional casting techniques; as a result, all modem steel mills produce continuous cast products.
Casting to shape Casting to shape is generally classified according to the molding process, molding material or method of feeding the mold. There are four basic types of these casting processes: sand, permanent-mold, die, and centrifugal
Sand casting This is the traditional method which still produces the largest volume of cast-to-shape pieces. It utilizes a mixture of sand grains, water, clay, and other materials to make high-quality molds for use with molten metal. This "green sand" mixture is compacted around a pattern (wood, plaster, or metal), usually by machine, to 20-80% of its bulk density. The two halves of the mold (the cope and drag) are closed over cores necessary to form internal cavities, and the whole assembly is weighted or damped to prevent floating of the cope when the metal is poured.
Other casting processes which utilize sands as a basic component are the shell, carbon dioxide, investment casting, ceramic molding and plaster molding processes. In addition, there are a large number of chemically bonded sands which are becoming increasingly important.
Permanent-mold casting Many high-quality castings are obtained by pouring molten metal into a mold made of cast iron, steel, or bronze. Semi permanent mold materials such as aluminum, silicon carbide, and graphite may also be used. The mold cavity and the gating system are machined to the desired dimensions after the mold is cast: the smooth surface from machining thus gives a good surface finish and dimensional accuracy to the casting. To increase mold life and to make ejection of the casting easier, the surface of the mold cavity is usually coated with carbon soot or a refractory slurry; these also serve as heat barriers and control the rate of cooling of the casting. The process is used for cast iron and nonferrous alloys with advantages over sand casting such as smoother surface finish, closer tolerances, and higher production rates.
Die casting A further development of the permanent molding process is die casting. Molten metal is forced into a die cavity under pressures of 100 -100, 000 psi. Two basic types of die-casting machines are hot-chamber and cold-chamber. In the hot-chamber machine, a portion of the molten metal is forced into the cavity at pressures up to about 2, 000 psi. The process is used for casting low-melting-point alloys such as lead, zinc, and tin.
In the cold-chamber process the molten metal is ladled into the injection cylinder and forced into the cavity under pressures which are about 10 times those in the hot-chamber process. High-melting-point alloys such as aluminum-, magnesium-, and copper-base alloys are used in this process. Die casting has the advantages of high production rates, high quality and strength, surface finish on the order of 40—100-microinch rms (root mean square), and close tolerances, with thin sections.
Rheocasting is the casting of a mixture of solid and liquid. In this process the alloy to be cast is melted and then allowed to cool until it is about 50% solid and 50% liquid. Vigorous stirring promotes liquid like properties of this mixture so that it can be injected in a die-casting operation. A major advantage of this type of casting process is expected to be much reduced die erosion due to the lower casting temperatures.
Centrifugal casting Inertial forces of rotation distribute molten metal into the mold cavities during centrifugal casting, of which there are three categories: true centrifugal casting, semi centrifugal casting, and centrifuging. The first two processes produce hollow cylindrical shapes and parts with rotational symmetry respectively. In the third process, the mold cavities are spun at a certain radius from the axis of rotation; the centrifugal force thus increases the pressure in the mold cavity.
The rotational speed in centrifugal casting is chosen to give between 40 and 60g acceleration. Dies may be made of forged steel or cast iron. Colloidal graphite is used on the dies to facilitate removal of the casting.
Successful operation of any metal-casting process requires careful consideration of mold design and metallurgical factors.
Sheet and Plate Bending
Bending is a method of producing shapes by stressing metal beyond its yield strength, but not past its ultimate tensile strength. The forces applied during bending are in opposite directions, just as in the cutting of sheet metal. Bending forces, however, are spread farther apart, resulting in plastic distortion of metal without failure.
The bending process appears to be simple; yet, in reality, it is a rather complex process involving a number of technical factors. Included are characteristics of the work piece material flow and reactions during various stages of deformation, the effect of tooling design on force required to form the bend, and the type of equipment used.
In the large, varied field of sheet metal and plate fabricating, several types of bending machines are used. Press brakes predominate in shops that process heavy-gage materials, because they are well suited to such applications and also because they are adaptable to other metalworking operations, such as punching, piercing, blanking, notching, perforating, embossing, shearing, and drawing.
Light-gage metal typically is formed with specialized bending machines, which are also described as leaf, pan, or box brakes; as wing folders; and as swivel benders. Equipment of this type is often manually operated.
The principal kinds of equipment used to bend sheet metal and plate can be grouped into the following categories:
Mechanical press brakes—elongated presses with numerous tooling options. Work is performed by means of energy released from a motor-driven flywheel. These machines normally have a 3" or 4" stroke length.
Hydraulic press brakes—stretched C-frame presses that are likewise compatible with a wide range and diversity of tooling. High-pressure oil in hydraulic cylinders supplies the force, which is directed downward in most models. The stroking length usually exceeds 6".
Hydraulic-mechanical press brakes—presses with drives that combine hydraulic and mechanical principles. In operation, oil forces a piston to move arms that push the ram toward the bed.
Pneumatic press brakes—low- tonnage bending machines that are available with suitable tooling options.
Bending brakes—powered or manual brakes commonly used for bending light-gage sheet metal.
Special equipment—custom-built benders and panel formers designed for specific forming applications.
Terms used to describe various aspects of sheet metal bending are illustrated
Bend Allowance
Bend allowance is the dimensional amount added to a part through elongation during the bending process. It is used as a key factor in determining the initial blank size.
The length of the neutral axis or bend allowance is the length of the blank. Since the length of the neutral axis depends upon its position within the bend area, and this position is dictated by the material type and thickness and the radius and degree of bend, it is impossible to use one formula for all conditions. However, for simplicity a reasonable approximation with sufficient accuracy for practical usage when air bending is given by the following equation:
L=A/360*2π(R+kt)
or
L=0.017453A(R+kt)
where:
L = bend allowance (arc length of the neutral axis) in. or mm
A =bend angle, de
R = inside radius of part, in. or mm
t = metal thickness, in. or mm
k = constant, neutral-axis location
Theoretically, the neutral axis follows a parabolic arc in the bend region; therefore, the k factor is an average value that is sufficiently accurate for practical applications. A value of 0.5 for k places the neutral axis exactly in the center of the metal. This figure is often used for some thickness. One manufacturer specifies k according to sheet thickness and inside radius of the bend; when R is less than 2t, k = 0.33; when R is 2t or more, t=0. 50.
Types of Bending
The basic types of bending applicable to sheet metal forming are straight bending, flange bending and contour bending.
Straight Sending During the forming of a straight bend the inner grains are compressed and the outer grains are elongated in the bend zone. Tensile strain builds up in the outer grains and increases with the decreasing bend radius. Therefore, the minimum bend radius is an important quantity in straight bending since it determines the limit of bending beyond which splitting occurs.
Flange Bending Range bend forming consists of forming shrink and stretch flanges. This type of bending is normally produced on a hydrostatic or rubber-pad press at room temperature for materials such as aluminum and light-gage steel.
Parts requiring very little handwork are produced if the flange height and free-form-radius requirements are not severe. However, forming metals with low modulus of elasticity to yield strength ratios, such as magnesium and titanium, may result in unde sirable buckling, a and b. Also, splitting may result during stretch-flange forming as a function of material elongation. Elevated temperatures utilized during the bending operation enhance part formability and definition by increasing the material ductility and lowering the yield strength, providing less spring back and buckling.
Contour Bending Single-contour bending is performed on a three-roll bender or by using special feeding devices with a conventional press brake. Higher production rates are attained using a three-roll bending machine. Contour radii are generally quite large; forming limits are not a factor. However, spring back is a factor because of the residual-stress buildup in the part; therefore, over forming is necessary to produce a part within tolerance.
Stretch Bending Stretch bending is probably the most sophisticated bending method and requires expensive tooling and machines. Furthermore, stretch bending requires lengths of material beyond the desired shape to permit gripping and pulling. The material is stretched longitudinally, past its elastic limit by pulling both ends and then wrapping around the bending form. This method is used primarily for bending irregular shapes; it is generally not used for high production.
Drawing
Drawing is an operation wherein the workpiece is pulled through a die, resulting in a reduction in outside dimensions.
Wire and Bar Drawing
Among the variables involved in the drawing of wires and bars are properties of the original material, percent reduction of cross-sectional area, die angle and geometry, speed of drawing, and lubrication. The operation usually consists of swaging the end of a round rod to reduce the cross-sectional area so that it can be fed into the die; the material is then pulled through the die at speeds as high as 8, 000 feet per minute. Short lengths are drawn on a draw bench, while long lengths (coils) are drawn on bull blocks. Most wire drawing involves several dies in tandem to reduce the diameter to the desired dimension.
Die angles usually range from 6 to 15°, the actual angle depending on die and workpiece materials. Reductions in cross-sectional area vary 10—45% in one pass, although theoretically the maximum reduction per pass for a perfectly plastic material is 63%.
Die materials are usually alloy steels, carbides, and diamond. Diamond dies are used for drawing fine wires. The purpose of the die land is to maintain dimensional accuracy. The force required to pull the workpiece through the die is a function of the material, die angle, coefficient of friction, and reduction in cross-sectional area. The work applied to the process comprises three components: ideal work of deformation, friction work, and redundant work due to no uniform deformation within the material. Depending on a number of factors, there is an optimum die angle for which tile drawing force is a minimum. In cold drawing, the strength of the material increases due to work hardening.
Temperature rise in drawing is important because of its effect on die life, lubrication, and residual stresses. Also, a defect in drawn rods is the rupturing of the core, called cuppy core. The tendency for such internal rupturing increases with increasing die angle, friction, and inclusions in the original material, and with decreasing reduction per pass.
The magnitude of residual stresses in a drawn material depends on the die geometry and reduction. The surface residual stresses are generally compressive for light reductions and tensile for intermediate or heavy reductions.
Extensive study has been made of lubrication in rod and wire drawing. The most common lubricants are various oils containing fatty or chlorinated additives, chemical compounds, soap solutions, and sulfate and oxalate coatings. The original rod to be drawn is usually surface-treated by pickling to remove scale, which can be abrasive and thus considerably reduce die life. For drawing of steel, chemically deposited copper coatings are also used. If the lubricant is applied to the wire surface it is called dry drawing; if the dies and blocks are completely immersed in the lubricant, the process is called wet drawing.
Tube Drawing
Tubes are also drawn through dies to reduce the outside diameter and to control the wall thickness. The thickness can be reduced and the inside surface finish can be controlled by using an internal mandrel (plug). Various arrangements and techniques have been developed in drawing tubes of many materials and a variety of cross sections. Dies for tube drawing are made of essentially the same materials as those used in rod drawing.
Deep Drawing
A great variety of parts are formed by this process, the successful operation of which requires a careful control of factors such as blank-holder pressure, lubrication, clearance, material properties, and die geometry. Depending on many factors, the maximum ratio of blank diameter to punch diameter ranges from about 1.6 to 2.3.
This process has been extensively studied, and the results show that two important material properties for deep draw ability are the strain-hardening exponent and the strain ratio (anisotropy ratio) of the metal. The former property becomes dominant when the material undergoes stretching, while the latter is more pertinent for pure radial drawing. The strain ratio is defined as the ratio of the true strain in the width direction to the true strain in the thickness direction of a strip of the sheet metal. The greater this ratio, the greater is the ability of the metal to undergo change in its width direction while resisting
Anisotropy in the sheet plane results in earring, the appearance of wavy edges on drawn cups. Clearance between the punch and the die is another factor in this process: this is normally set at a value of not more than 1.4 times the thickness of the sheet. Too large a clearance produces a cup whose thickness increases toward the top, whereas correct clearance produces a cup of uniform thickness by ironing. Also, if the blank-holder pressure is too low, the flange wrinkles; if it is too high, the bottom of the cup will be punched out because of the increased frictional resistance of the flange. For relatively thick sheets it is possible to draw parts without a blank holder by special die designs.
Punching and blanking
Though punching and blanking are the most common sheet metal operations involving H shearing of the metal strips, (2) lancing, (3) slitting, (4) nibbling, and (5) trimming.
In the notching operation, material is removed from the side of a sheet metal, whereas lancing makes cuts partway through the metal without producing any scrap. Lancing is frequently combined with bending to form tabs. Slitting is an operation to cut a coiled sheet metal lengthwise to produce narrower strips. In the nibbling operation, complicated shapes are cut out from a sheet metal by producing overlapping notches, starting either from the outer boundary or from a punched hole. Without using any special tool, a simple, round or triangular punch of small dimensions is reciprocated at a fixed location. The sheet metal is guided to obtain the desired shape of the cut. Trimming refers to the removal of the excess material in a flange or flash.
In reducing the operation time and cost, the design of the die and punch for blanking plays an extremely important role. An accurate relative location of the punch and the die is maintained with the help of a set of guide posts. The stripper helps in removing the sheet metal workpiece from the