New to the metalworking industry, especially anyone working in machining operations, one will need to recognize the variety raw materials forms to assure success in finishing operations. Different bulk forming methods produce different results, especially in surface condition and metallurgy. Each will require different approaches to assure consistent high-quality results.
Visual clues are the first things to observe to determine how raw material was produced. Many people confuse forgings, castings, and powder metal parts because they do not understand what to look for. Each process has unique approaches in producing a bulk form. Observation of features can provide a means to be able to tell one process from another. The differences can be subtle and, if significant grinding, polishing or machining has been performed, many telltale indications may be removed, making process identification more difficult.
Exterior features such as tapered side walls, called draft, corner radii and fillets, marks from ejector pins and even surface marks and roughness can provide the information that allows identification. So, what are some things to look for?
Open die forgings, if unmachined, will typically exhibit tool marks on exterior surfaces where the forging tool has pressed against the workpiece to shape it. Often, these are a multiplicity of “flat” marks where the workpiece has been hammered or pressed repeatedly during the forging process. Impression die forgings, where material is hammered or pressed into die cavities will have more uniform, smooth surfaces. Most impression die forging is done with two or more mating dies. Where those dies come together there will be “parting lines” into which excess material will flow out of the cavity, between the dies, during the forging operation. This excess material, which forms around the periphery of the cavities, is removed in a “trimming” operation, where the excess is sheared off the main body of the forging. The parting line and trimming operation can be obvious signs the item is a forging. Additionally, as material must have accommodation to release from the die, side walls of the cavities usually have draft. This means the cavity tapers from a smaller size in the bottom of the cavity to become larger at the parting line where the die faces meet. The more difficult an alloy is to form, the more draft is needed and the larger the fillets need to be in the depth of the die cavity.
Castings come is a variety of forms, produced by different variations of the casting process. Many types of casting require assembling a mold in pieces. In the process called sand casting, where a mixture resembling sand, hardened with a binder, is packed around objects called “patterns”, the assembly of those pieces to form a complete mold will typically result in parting lines. Liquid material may flow between the mold halves, requiring removal once the casting is broken from the mold. Typically, such excess material will be ground off the casting. Grinding and polishing of castings will usually have a different appearance compared to flash trimming of forgings. With experience, the differences become obvious. Other types of casting processes have different features and appearance. Investment castings will not exhibit parting lines, but like all castings can be identified by the openings to the mold, typically called gates and risers, where liquid must enter the mold and gasses must exit when the liquid flows in. The material that solidifies in those opening must be removed, and locating those places, visually, identifies the item as a casting. The material attached to the casting is cut off by sawing and grinding. Permanent mold or die castings are another type of casting process that result in more variations of appearance. In die casting, tooling is mounted in a machine that opens and closes the dies for each piece (each shot). With dies closed, liquid material is forced, under pressure into the die cavities. Flash can form at the parting lines that will need to be removed, often in a trimming operation somewhat like trimming a forging, in a die made especially for that operation. In permanent mold castings, liquid material may be drawn into the mold by vacuum as opposed to die casting where it may be forced in, under pressure. Again, in all cases, raw casting will show evidence of the gates, sprues and risers needed to assure the mold fills properly as well as evidence of marks used to eject the casting from the mold.
The techniques of consolidating powder result in telltale signs of the powder process, different for each method used but very different from forging or casting. There are a variety of methods for producing components from powders. The dominant method is compacting powder in a die and subsequently sintering the item to cause the constituents to melt together. Such powder compacts may be subsequently coined, isostatically pressed, or even forged to achieve full density and improve properties. Press compacted parts will generally lack significant three-dimensional aspects with cross section thickness limiting the ability to achieve adequate compaction of powders. Press compacted items will not have parting lines or evidence of places material entered the die. In metal injection molding, very fine powders are injected into more complex configurations than can be achieved in conventional press compacting. Metal injection mold items tend to be small, high volume, complex net shapes that would be difficult or impossible to produce using any other method. Evidence of locations where material is injected into a die or gasses escape may be undetectable to the naked eye. Using powders in additive manufacturing, where the powder is directly melted to build a configuration, layer by layer, particles are melted into a large mass in a process somewhat like welding. These processes will result in surface features different from other powder, casting or forging processes. Items produced in powder bed additive manufacturing will tend to have variations of surface roughness depending upon the speed of the process selected. Typically, the more rapidly the process runs, the rougher the surfaces. AM items are built on a base surface and are usually released from the base by saw cutting. Many configurations also require additional supporting structures that are created along with the desired item. Such supports must be removed and scrapped when the AM processing is complete.
Metallurgy and Microstructure
The most significant differences, one type of forming process versus another, is the resulting metallurgy. Wrought properties, the mechanical properties created with deformation processes such as forging, are the outcomes often sought and claimed by every process competing with forging. Virtually all casting and powder metal processes create isotropic microstructure or metallurgy that may be degraded by the kinetics of thermal transfer gradients. Solidification variables and subsequent high temperature processes designed to assure densification may cause variability of properties through the cross section of a component. Often, these dynamics can result in internal voids or structures that weaken mechanical properties or limit fatigue life. Wrought properties are achieved through deformation energies that alter the entire cross section of a forging. Engineering the deformation process can create anisotropic properties to strengthen a product in the areas of greatest stress during service. Forgings will achieve this result with minimum material mass compared to other processes which would need to add bulk and weight to assure sufficient strength. When fatigue properties are paramount, however, wrought properties provide unmatched durability.
Having identified the way in which an item was produced, what additional processes can be successfully performed to make the finished part required? Investment cast items, and most produced from powdered materials, usually require little additional processing other than simple machining and finished steps. Other types of castings and forgings often require more extensive machining, although with creative designs and additional processes such as coining, many net surfaces can be created. Castings and powder metal items may not provide strength or consistency to sustain productivity in machining operations. Voids, inclusions, or locations of inadequate densification may be present that prevent successful machining, plating, anodizing or other finishing operation. Wrought product, produced using deformation processes, will provide consistent machining and finishing due to the fully dense metallurgical structure.
The difference between these processes is one that may be identified by looking at the outer surface, identifying one process versus another. The important differences are revealed when peeling back the surface as part of subsequent finishing or when comparing properties related to strength and durability. Learning to recognize a basic bulk forming process by visual inspection is useful to understand what to expect as additional work is performed. Knowing what to look for on the outside provides confidence, knowing what strengths or weaknesses can be expected from the structure on the inside.