Lesson 13 - Material Selection Process - Rev. 0 1x6l6u

LESSON 13 MATERIAL SELECTION PROCESS

OVERVIEW THE SELECTION OF THE CORRECT MATERIAL for a design is a key step in the process because it is the crucial decision that links computer calculations and lines on an engineering drawing with a working design. The importance of materials selection in design has increased in recent years. World pressures of competitiveness have increased the general level of automation in manufacturing to the point where materials costs comprise 50% or more of the cost for most products. Finally, the great activity in materials science worldwide has created a variety of new materials and focused attention on the competition between six broad classes of materials: metals, polymers, elastomers, ceramics, glasses, and composites. This presents the opportunity for innovation in design by utilizing these materials in products that provide greater performance at lower cost.

RELATION OF MATERIALS SELECTION TO DESIGN An incorrectly chosen material can lead not only to failure of the part but also to unnecessary cost. Selecting the best material for a part involves more than selecting a material that has the properties to provide the necessary performance in service; it is also intimately connected with the processing of the material into the finished part (Fig. 1).

RELATION OF MATERIALS SELECTION TO DESIGN A poorly chosen material can add to manufacturing cost and unnecessarily increase the cost of the part.

Also, the properties of the material can be changed by processing (beneficially or detrimentally), and that may affect the service performance of the part. With the enormous combination of materials and processes to choose from, the task can be done only by introducing simplification and systemization. Design proceeds from concept design, to embodiment (configuration) design, to detail (parametric) design, and the material and process selection then becomes more detailed as the design progresses through this sequence (Fig. 2).

RELATION OF MATERIALS SELECTION TO DESIGN

THE PROCESS OF MATERIAL SELECTION A materials selection problem usually involves one of two situations: • Selection of the materials and the processes for a new product or design. • The evaluation of alternative materials or manufacturing routes for an existing product or design. Such a redesign effort usually is taken to reduce cost, increase reliability, or improve performance.

Materials Selection for a New Design In this situation, these steps must be followed: 1. Define the functions that the design must perform, and translate these into required materials properties such as stiffness, strength, and corrosion resistance, and such business factors as the cost and availability of the material. 2. Define the manufacturing requirements in of such parameters as the number of parts required, the size and complexity of the part, its required tolerance and surface finish, general quality level, and overall fabricability of the material. 3. Compare the needed properties and parameters with a large materials property data base (most likely computerized) to select a few materials that look promising for the application. It is helpful to establish a screening property. A screening property is any material property for which an absolute lower (or upper) limit can be established. No trade-off beyond this limit is allowable. It is a go-no go situation.

Materials Selection for a New Design

4. Investigate the candidate materials in more detail, particularly in of trade-offs in product performance, cost, fabricability, and availability in the grades and sizes needed for the application. Material property tests and other testing often is done at this stage. Step 4 results in the selection of a single material for the design and a suggested process for manufacturing the part. 5. Develop design data and/or a design specification. In most cases, this results in establishing the minimum properties through defining the material with a generic material standard such as those issued by the American Society for Testing and Materials (ASTM), the Society of Automotive Engineers (SAE), the American National Standards Institute (ANSI), and the United States military (MIL specs).

Materials Substitution for an Existing Design. In this situation, the following steps pertain: 1. Characterize the currently used material in of performance, manufacturing requirements, and cost. 2. Determine which characteristics must be improved for enhanced product function. Often failure analysis reports play a critical role in this step. 3. Search for alternative materials and/or manufacturing routes. Use the idea of screening properties to good advantage.

4. Compile a short list of materials and processing routes, and use these to estimate the costs of manufactured parts. 5. Evaluate the results in step 4, and make a recommendation for a replacement material.

PERFORMANCE CHARACTERISTICS OF MATERIALS The performance or functional characteristics of a material are expressed chiefly by physical, mechanical, thermal, electrical, magnetic, and optical properties. Material properties are the link between the basic structure and composition of the material and the service performance of the part (Fig. 3).

PERFORMANCE CHARACTERISTICS OF MATERIALS The goal of materials science is to learn how to control the various levels of structure of a material (electronic structure, defect structure, microstructure, macrostructure) so as to predict and improve the properties of a material. An important role of the materials engineer is to assist the designer in making meaningful connections between materials properties and the performance of the part or system being designed.

PERFORMANCE CHARACTERISTICS OF MATERIALS Table 3 shows the relationships between standard mechanical properties and the failure modes for materials. For most modes of failure, two or more material properties act to control the material behavior. Also, it must be kept in mind that the service conditions met by materials are in general more complex than the test conditions used to measure material properties. Usually simulated service tests must be devised to screen materials for critical complex service conditions. Finally, the chosen material, or a small group of candidate materials, must be evaluated in prototype tests or field tests to determine their performance under actual service conditions.

PERFORMANCE CHARACTERISTICS OF MATERIALS

Example: Materials Selection for an Automotive Exhaust System The product design specification for the exhaust system must provide for the following functions: •

Conducting engine exhaust gases away from the engine

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Preventing noxious fumes from entering the automobile

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Cooling the exhaust gases

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Reducing the engine noise

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Reducing the exposure of automobile body parts to exhaust gases

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Affecting engine performance as little as possible

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Helping control undesirable exhaust emissions

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Having a service life that is acceptably long

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Having a reasonable cost, both as original equipment and as a replacement part

Example: Materials Selection for an Automotive Exhaust System In its basic form, the exhaust system consists of a series of tubes that collect the gases at the engine and convey them to the rear of the automobile. The size of the tube is determined by the volume of the exhaust gases to be carried away and the extent to which the exhaust system can be permitted to impede the flow of gases from the engine. An additional device, the muffler, is required for noise reduction, and a catalytic converter is required to convert polluting gases to less-harmful emissions. The basic lifetime requirement is that the system must resist the attack of hot, moist exhaust gases for some specified period. In addition, the system must resist attack by the atmosphere, water, mud, and road salt.

Example: Materials Selection for an Automotive Exhaust System The location of the exhaust system under the car requires that it be designed as a complex shape that will not interfere with the running gear of the car, road clearance, or the enger compartment. The large number of automobiles produced each year requires that the material used in exhaust systems be readily available at minimum cost. This system requires numerous material property requirements. The mechanical property requirements are not overly severe: suitable rigidity to prevent excessive vibration and fatigue plus enough creep resistance to provide adequate service life. Corrosion is the limiting factor on life, especially in the cold end, which includes the resonator, muffler, and tail pipe.

Example: Materials Selection for an Automotive Exhaust System Several properties of unique interest, that is, where one or two properties dominate the selection of the material, are found in this system. These pertain to the platinum-base catalyst and the ceramic carrier that s the catalyst. The majority of the tubes and containers that comprise the exhaust system were for years made of readily formed and welded low-carbon steel, with suitable coatings for corrosion resistance. With the advent of greater emphasis on automotive quality and longer life, the material selection has moved to specially developed stainless steels with improved corrosion and creep properties. Ferritic 11 % Cr alloys are used in the cold end components, with 17 to 20% Cr ferritic alloys and austenitic Cr-Ni alloys in the hot end of the system.

STANDARDS AND SPECIFICATIONS Materials properties usually are formalized through standards and specifications. The distinction between these entities is that a standard is intended for use by as large a body as possible, for example ASTM or ANSI standards, whereas a specification, though dealing with similar technical content, is intended for use by a more limited group, for example a company specification. There are two types of standards or specifications: performance standards and product standards. Performance standards delineate the basic functional requirements of a product and set out the basic parameters from which the design can be developed. Product standards define the conditions under which the components of a design are purchased and manufactured. Materials standards are invariably product standards. They stipulate performance characteristics, quality factors, methods of measurement, tolerances, and dimensions.

RELATION OF MATERIALS SELECTION TO MANUFACTURING The selection of a material must be closely coupled with the selection of a manufacturing process. This is not an easy task for there are many processes that can produce the same part. The goal is to select the material and process that maximizes quality and minimizes the cost of the part. The overall guide should be to select a primary process that makes the part as near to final shape as possible (near-net shape forming) without requiring expensive secondary machining or grinding processes. Sometimes the form of the starting material is important. For example, a hollow shaft can be made best by starting with a tube rather than a solid bar.

RELATION OF MATERIALS SELECTION TO MANUFACTURING Figure 4 gives a breakdown of manufacturing processes into nine broad classes.

COSTS AND RELATED ASPECTS OF MATERIAL SELECTION The decision on materials selection ultimately will come down to a trade-off between performance and cost. The total cost of a part includes the cost of the material, the cost of tooling (dies, fixtures), and the processing cost. The unit cost of a part, C, can be expressed by:

where Cm is the material cost; Cc is the capital cost of plant, machinery, and tooling required to make the part; CL is the labor cost per unit time; n is the batch size; and n is the production rate (parts produced per unit time).