Metal Shaping Processes. Vukota Boljanovic
processes can be conveniently classified into bulk deformation processes (rolling, extrusion, and forging) and sheet metal processes (shearing, bending and drawing, and forming). In both cases the surfaces of the deforming material and of the tools are usually in contact, and the friction between them has a major influence. In bulk forming the input material is in billet, rod, or slab form, and a considerable increase in the surface-to-volume ratio occurs in the formed part. In sheet metal processes a sheet blank is plastically deformed into a complex three-dimensional configuration, usually without any significant change in sheet thickness and surface characteristics.
Bulk deformation processes have the following characteristics:
•The workpiece undergoes large plastic deformation, resulting in an appreciable change in shape or cross section.
•The portion of the workpiece undergoing permanent plastic deformation is generally much larger than the portion undergoing elastic deformation. Therefore, elastic recovery or spring-back after deformation is negligible.
The characteristics of sheet metal processes are as follows:
•The workpiece is a sheet or a part fabricated from a sheet. The deformation usually causes significant changes in shape but not in the cross-section of the sheet.
•In some cases the magnitudes of permanent plastic and recoverable elastic deformations are comparable; therefore, elastic recovery or springback may be significant.
Material removal processes. Machining processes are frequently used as primary or secondary processes. In these processes the size of the original workpiece is large enough so that the final geometry of the finished piece can be achieved by employing one or more removal operations. The chips or scrap are necessary to obtain the desired geometry, tolerance, and surface finish. The amount of scrap may vary from a few percent to more than 70% of the volume of the starting workpiece material. Machining as a primary process used for low-production volume parts, for the production of prototypes, and for production of the tooling used in processes such as stamping, injection molding, and other processes, generates a part’s shape by changing the volume of the workpiece through the removal of material.
Machining is also used as a secondary process. Thus, machining is used to improve a basic shape that has been produced by casting or deformation processing, as well as to produce the basic shape itself.
PRODUCTION EQUIPMENT AND TOOLING
Equipment. The type of equipment and especially the tools used in manufacturing depends on the manufacturing process. Machine tools are among the most versatile of all production equipment. They are used not only for producing tools and dies, but also for producing components for other products and production equipment, such as presses and hammers for deformation processes, rolling mills for rolling sheet metal, and so on.
The name of the equipment usually follows from the name of the process. The productivity, reliability, and cost of equipment used for shaping processes are extremely important factors, since they determine the economics and practical application of a given process. For example, in both sheet and bulk forming the stroking rate of forming machines continues to get faster. Because of this machine dynamics and machine rigidity and strength are of increasing concern. As in other unit processes, the use of sensors for process monitoring and control is essential and continues to increase. Sensors are also being increasingly used to monitor the condition of the tooling during operations. Such monitoring systems cannot only improve part quality but can also enable longer tool life.
Tooling. The design and manufacture of tooling are essential factors determining the performance of shaping processes. The key to a successful shaping process lies in the tool design, which generally, to a very large extent, is based on the experience of the designer(s). Innovative multi-action tool designs have recently been developed that are capable of near-net shaping of increasingly complex parts, such as gears and universal joint components. These tooling approaches can be expanded. Many companies are already using computer-aided engineering and computer-aided manufacturing to design and fabricate process tooling. Advanced heat treatment and coating techniques can extend tool life. Based on a developing understanding of the mechanisms of erosive tool wear, studies are being conducted to measure and predict lubricant behevior and heat transfer at the tool material interface. This is an extremely important area, since tool life directly influences the economics of deformation processes.
In this part we discuss casting, molding, and other processes in which the starting material is a heated liquid. The three chapters deal with the fundamentals of metal casting, metal casting processes, and metal casting design and materials (Fig. P.1). Special attention is given to the heating of metals, an analysis of the processes from an engineering perspective, solidification and cooling of metals, defects in casting, sand casting, permanent mold casting, and other important factors in casting processes.
Fig. P.1 An outline of the casting and molding processes described in Part I.
1
FUNDAMENTALS OF METAL CASTING
1.5 Solidification and Cooling of Metals
Metal casting is a process whereby molten metal is poured into a mold the shape of the desired finished product. Casting is one of the oldest metal shaping processes; according to Biblical records, casting technology reaches back almost 5500 years BCE, or 7500 years ago. Gold was the first metal to be discovered and shaped according to prehistoric people’s fancy. The Chinese made iron castings around 1000 BCE, and steel was fabricated in India about 500 BCE.
Four main elements are required in the process of casting: a pattern, a mold, cores, and the work-piece. The pattern, the original template from which the mold is prepared, creates a corresponding cavity in the casting material. Cores are used to produce tunnels or holes in the finished mold, and the workpiece is the final output of the process.
The cast metal remains in the mold until it has solidified, and it is then ejected or revealed to show the fabricated part or casting.
The casting process is divided into two broad types of casting:
•expendable-mold casting processes, and
•nonexpendable-mold casting processes.
Expendable-mold casting is a general classification that includes sand, plastic, shell, and investment (lost wax technique) moldings. All of these involve the use of temporary and nonreusable molds, and they all need gravity to help force the molten fluid into the casting cavities. In this process, the mold in which the molten metal solidifies must be destroyed in order to remove the