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We will go through the process of material selection; the manufacturing process and testing required verifying both chemical and mechanical properties. We will discuss use of materials, stampings and materials required.
Raw material in the form of a metal strip appears at first glance to be a fairly uncomplicated product. A coil or roll of metallic ribbon is taken for granted by most that see it. The reality is however, that a very controlled and precise process is involved in its manufacture.
In the alloying process for material, elements in varying amounts are melted together to for a blended alloy. Processing of this melt requires many steps to get to a final form and dimension. This process can take as long as 12 to 18 weeks in some cases.
A stamping, made from this material, is designed to perform certain functions. There are mechanical as well as electrical and thermal requirements that are specified in order for the product to perform as designed. Material selection is crucial to meeting these design standards.
The metal manufacturing sequence is as follows:
- Determine elements and quantities required.
- Melt and cast ingot
- Surface cut ingot/square up
- First breakdown by rolling
- Anneal (batch process)
- Second breakdown by rolling
- Anneal (batch process)
- Third breakdown by rolling
- Strand anneal (continuous process)
- Fourth breakdown by rolling
- Pre-Finish anneal (continuous process)
- Slit to width (mill harden if required)
- Tension level if necessary
- Final spooling (or cutting) and QA testing
Flat Stock: Monolithic or Composite
The preceding manufacturing process was specific to an individual alloy at a prescribed thickness and temper. This material is known as Monolithic (uniform, continuous) material. The alloy is consistent throughout thickness, length and width.
A composite material is prepared with one or more alloy in its geometry.
Composite materials come in many forms. Materials of different alloys can be combined allowing for stamping part geometries that address different applications within the same stamping. One of the major reasons to go to a composite is to control costs when using precious metals. Materials to be joined must be compatible and share similar rolling and thermal processing capabilities. In the automotive sensor industry a common bimetal used to reduce cost but still have durability and desired mechanical/electrical properties is Paliney 6 (made with Palladium and Platinum) or Paliney 7 (made with Palladium Platinum and Gold) and Pfinodal (a lower cost material primarily of copper content).
Methods of joining vary, from different types of welding, plasma, electron beam or laser, to skiving and joining using hot or cold (pressure) bonding techniques. Breakdown rolling of composite materials although similar vary with the attachment methods but in general follow the same rolling/annealing sequence as solid monolithic materials.
Most commonly used alloys:
- Base Metal
- Copper
- Brass
- Bronze
- Stainless Steel
- Nickel
- Nickel Silver
- Pfinodal
- Precious Metal
- Paliney 6
- Paliney 7
- Gold
Alloys are selected for various reasons, some for their strength, others, for electrical properties, or resistance to corrosion.
Temper Designations Defined
Annealed
Material supplied at finished size in its softest state. This temper is chosen when the material must be malleable to allow for heavy coining reductions or severe bends.
Cold Worked
Material has some reduction from its last anneal point , thereby introducing some degree of cold working to effectively change the mechanical properties of the material. Material can be cold worked by varying percentages, 10%, 20%, etc. These percent reductions relate to the terms ¼ hard, ½ hard, etc. As hardness and strength increase, formability decreases.
Mill Hardened
Material that goes through thermal processing as a final stage to increase its mechanical properties.
Post Heat Treated
Post heat treating is generally related to the processing of finished stampings that need improvement in mechanical strength or hardness. In some cases, the final requirements of a part design may only be accomplished by stamping in a softer temper an then post heat treating to desired mechanical properties.
Mechanical Properties
Tensile Strength
Actual fracture point of the material specimen when pulled to its breaking point.
Yield Strength
The point at which the material specimen begins to stretch when pulled.
Elongation
The reduction in area of the material specimen at the poijnt of fracture.
Hardness
The material’s resistance to wear
Formability
The relationship of the bending radius to the thickness of the material specimen.