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I can't begin to remember how many times over the past 30 years I've felt the bile rise in my system as I've heard design engineers suggest that it doesn't matter what material(s) are used to build parts and components. I suppose it's natural for any of us to feel unloved and underappreciated in our professional endeavors. As a materials engineer working for two different major manufacturers over my career I often felt that materials selection was not given as much attention during the design process as it deserved. There are 5 repetitive problems I have observed related to material(s) selection over my career: 1. Material Selection after the part/component is 'designed'. 2. Material Selections not reviewed over time. 3. Materials copied from old designs. 4. Material substitutions fail. 5. Bad material selections - the gift that keeps on taking. In the remainder of this article I'll describe each of these problems in greater detail. 1. Material Selection after the part/component is 'designed' or The 4th Dimension In Material Selection Many times a design engineer comes to the materials engineer after lines are committed to paper and the part is 'designed' - the question often is 'Okay, what should we make this out of?' The shortcoming of this approach is that once parts are designed, materials options are limited. Done right, Material Selection is integral with the design process and parts are designed around specific material characteristics. Since material cost is typically the primary driver in piece part cost, it seems logical that materials should be selected early in the design process. Yet, more often than not, materials are chosen after the part is 'designed'. When this happens, the part design winds up dictating the material selection, rather than vice versa and not only do we lose control over one of the most effective levers we have to control piece part cost, we also lose some control over manufacturability and reliability. In order to move material selection 'up' in the design process, I think it is useful to start to think of the material as another dimension of the part, just like the dimensions that describe its size and shape. In order to optimize part design we need to think of all of its dimensions as interrelated. We need to start asking materials questions early on: What material properties are critical - strength, toughness, fatigue resistance, thermal conductivity, corrosion.....?
1. What physical characteristics matter - density, magnetism, color....? As we start to narrow the list of candidate materials, we can start to define the physical dimensions of the part and then we can start to ask second level questions: What are the limitations of the candidate materials - do we sacrifice toughness for strength, wear resistance for corrosion resistance......? 1. What are our manufacturing options - are the materials weldable, machinable, castable.....? 2. What about cost and availability? By asking these questions early in the design process we can make rational choices and optimize part design. We select materials with the characteristics we believe are important and utilize those characteristics to help shape a part that is functional, reliable and cost effective. 2. Material Selections are not reviewed over time or The Times, They Are Changing. Even when Material Selection is done right, the materials world is a dynamic place and what makes sense today, may not make sense tomorrow. For many years copper was less expensive per pound than aluminum (for most of my career, copper was $0.65/pound while aluminum was around $1.00/pound), today copper is 4.5 times more expensive than aluminum per pound - does copper still make sense for heat exchanger tubing or headers or connecting tubing now that aluminum is so much less expensive? Maybe, maybe not, but it is obviously a different question Continuing the investigation of common materials selection issues let's discuss the importance of regularly reviewing material selections. Things change! Copper, aluminum and zinc are often used in applications where corrosion resistance to water is important. There are a whole host of factors which need to be considered when selecting the 'right' material. Strength, density, resistance to specific contaminants, etc., etc. all can influence a material choice but cost is obviously an important parameter. If in 1998 our analysis showed that copper was the right choice for a given application when it was on cost parity with aluminum and zinc, is it still the right choice in 2011 when the price of copper has risen four times as rapidly as the cost of the other two metals? Cost, obviously, is not the only thing that changes with time. Technology changes, too. New materials are being commercialized daily. Originally, charred oak was the material of choice for plain bearing applications. Have you seen many wooden bearings lately? Of course not, but we still do see a lot of lead-tin, or antimony-tin babbit bearings. Are these the right materials when metal matrix composite bearings are stronger, lighter and often more cost effective? They may or may not be the right choice today, but we can't know without doing the analysis. New technology in manufacturing is allowing us to use 'old' materials in new ways. For example, for many, many years gray cast iron was not used in some bearing and sealing applications because it was not possible to obtain the required surface finish. But as machining technology improved, it became possible to utilize this old, inexpensive material in new applications. Clearly, the point is that the materials world changes. The material we chose for a given
application years ago, may not only be sub-optimum today, it may be absolutely wrong and we may be letting our competition get an unnecessary advantage in the marketplace if they are more nimble than we are at changing materials as the times change. If we only consider new or alternative materials when we design new equipment, we're missing out on opportunities to reduce cost, improve profitability and solve technical problems on existing equipment. 3. Materials copied from old designs or Where Imitation is not the Highest Form of Flattery Problems can arise when we simply copy material selections from similar parts. Reportedly it's common for mapmakers to intentionally include minor errors, such as non-existent, or misspelled streets or rivers, in their published maps so they can determine if their competitors are merely copying their products rather than legitimately creating maps themselves. Clearly, this is a risk engineers may be taking when they copy previous designs - the risk of copying and promulgating errors that were made in the originals. But there are less obvious risks in copying material selections from one part to another similar part. Material properties such as hardness, strength and toughness do not necessarily scale geometrically. For example, during World War II a new class of cargo ships, called Liberty Ships, was created to meet the overwhelming demand for shipping materiel across the Atlantic Ocean in wartime. Unfortunately a number of these ships fractured and sank in the North Atlantic because the water temperature fell below the ductile to brittle transition temperature of the steel used to construct the hulls. The steel used to construct these ships was the same as had been used historically to build similar ships which did not fracture and sink, so what changed? A couple of things: 1) The new ships were much larger than previous ships and subsequently required thicker steel to provide adequate strength. 2) To speed the construction of the ships, they changed from riveted to welded construction. The steel which had adequate toughness for small riveted ships did not have adequate toughness for larger welded ships. Fortunately most of the problems which arise from inappropriately copying material selections from similar parts do not have the tragic consequences of sinking Liberty Ships, however they can lead to premature component failure or excessive costs. The bottom line is to always consider what is different between new and old part designs and discuss the material selection with a qualified materials engineer. 4. Materials substitutions fail or You Don't Always Get What You Want Selecting Substitutions This is similar to #2 above, but it also can create problems for other reasons: What is the right material for a one pound widget may not be the right material for an 'identical' 10 pound widget material properties do not necessarily scale with the geometry of a part. Where a part is manufactured may impact the material selection - what makes sense for a part made in the US, may not be appropriate for a part made in China. Whether we try to convert parts from metal to plastic, or from metal castings to powdered metal, materials substitutions fail. The primary reason is that when materials change, part design needs
to change. A plastic gear, even though it may serve the same function as a metal gear, if designed properly, should look different than a plastic gear. If we simply try to drop in a plastic material into the same geometry as a part that was originally made from metal we are almost assured of failure. There are numerous reasons why manufacturers consider material substitutions. Most frequently the driver for material substitution is cost reduction, but sometimes it is a change in supply base, or changes in material availability or manufacturing issues with the original material. The scope of the substitutions can vary from minor (e.g., changing from one alloy to another alloy in the same alloy family) to major (e.g., dramatically changing materials and processing such as changing from an aluminum die casting to a plastic injection molding), with infinite gradations in between. Obviously, material substitutions of any complexity can be and are regularly accomplished, but in over 30 years I've witnessed many material substitution failures and here are some of the reasons: 1. Predicted cost savings aren't realized. Not all material substitution failures are technical, some are economic. As an example, with wildly increasing alloy costs over the past decade manufacturers sought out lower-alloy content stainless steels to minimize cost. A common substitution was to replace Types 304 or 302 stainless with a lower nickel alternative, Type 201. For many applications this was an excellent technical solution, Type 201 provided good corrosion protection at significantly reduced cost from the steel mill. However, for manufacturers buying steel from service centers rather than mills, there actually could be cost increases because the service centers did not normally stock Type 201 and charged a premium for obtaining it in small quantities. 2. Critical properties change. Historically for many types of machine parts made out of steel, the critical property in design was strength. When lower weight and/or lower cost aluminum and plastic materials started to appear with equivalent strength to plain carbon steel there were many wholesale changeover programs from steel to these alternatives and many of these changeovers were miserable failures. Those failures usually occurred because with the alternative materials some other property (often stiffness) had not even been a consideration in the steel part design, now governed the integrity of the part. The bottom line is that properly designed parts made from alternative materials may look considerably different than the original part made from the traditional material. 3. We get greedy. As mentioned above, the most common driver for material substitution is cost reduction. With that being the case it is natural for us to consider material substitutions in those applications where the cost savings are the greatest. When making major changes, this may not be the best approach. If you're considering using a technology that is new to you, it might be well to start with an application that is fairly simple, one where you can learn and build on your experience. Early in my career I became an advocate for powder metallurgy technology and started out to apply P/M on several high profile projects. The parts I chose to work on were complex and P/M offered very substantial cost savings over the traditional steel fabrication methods that had been used to make prototype parts. Unfortunately, we ran in continual problems which, due to our inexperience, we didn't know how to solve and the part designs reverted to traditional manufacturing methods in order to meet project schedules. It took many years for me to get design engineers interested in applying P/M again after these spectacular failures. Any subsequent success we had with P/M came from building our experience starting with simpler parts and working our way up the learning curve. In conclusion, successful material substitutions occur every day, but without attention to detail,
designing for the specific material we're using and building our experience base, even the simplest substitution can fail. 5. Bad Material Selection - The Gift That Keeps on Taking Oftentimes during the development process, problems are discovered too late in the game to 'redesign' parts before going into production. Instead, we call on some materials 'magic' and we implement special coatings, or heat treatments or exotic materials into the existing designs just to get us into production. Our intentions are always good - we say we'll come back later and redesign the parts so that we can revert to more standard/affordable materials - but the reality is that we rarely have the time to go back and redesign and instead we pay for the premium material throughout the life of the component design, and in those cases where we simply copy material selections from old designs into new designs, we pay the premium forever. With raw material costs accounting for 50-80% of piece part cost it seems intuitive that choosing the 'right' material is critical to managing cost, yet we often treat this part of the design process as an afterthought. Let's explore each of the repetitive problems described above in more detail. There are lots of reasons we may have less than optimum materials selected for the products we produce: 1) Time or supply constraints forced us to use 'premium' materials during development and we never got back to cost reduce the product in production. 2) Times and markets changed and what was a good material selection years ago is no longer an appropriate choice. 3) We simply copied the material selection from older parts into newer parts without giving it any thought..... The list can go on and on. My experience has shown that once we have 'institutionalized' a poor material selection it is very difficult to change. This, too, happens for a lot of reasons, but mostly it happens because of our natural engineering conservatism which tells us not to mess with success. If we're successfully producing parts and they're not breaking in the field, why change materials? Changing materials entails risk, I can't tell you how many times I've had design engineers tell me they wouldn't even consider a material change because, 'One failure will eat up a whole year's material cost savings.' The answer to that is to do our homework and clearly and objectively define what the current situation actually is and what the real risks and rewards of change are. Often times, the current situation isn't as great as we think it is, it's just that we're used to it and we know how to deal with the problems and shortcomings of the traditional material. Also, objective consideration of the risks and rewards of a material change can have surprising results. It's hard for us to realize just as there is a risk in making a change, there is risk in not changing. If our competitors can figure out how to use materials more effectively and economically than we do we are conceding an economic advantage to them that will be very difficult to overcome. The goal should be to select and implement the 'best' material for every application and to review those selections regularly to assure we're not giving our competitors an unearned advantage.
Initially working for Caterpillar as a research engineer in the metalurgy field I moved to The Trane Company, a world class HVAC manufacturer, in 1983. My focus engineering areas including materials and chemistry, materials sourcing and strategic supply chain, retiring from Trane in
2010. Currently a Sr. associate for GEA Consulting, a small but highly expert consulting group supporting the HVAC industry. Following is additional background information. Professional License: Â• Registered Professional Metallurgical Engineer - Wisconsin Â• Certificate Number E-255995 / 1988 - present Expertise Â• Failure analysis - determination of failure causes for components and for complex machinery including air conditioning compressors, chillers, unitary products, fans, air handlers, earthmoving equipment, diesel engines, transmissions, electric motors, rolling element and plain bearings Â• Material selection - selection of materials for engineered products to optimize reliability, manufacturability, cost and availability Â• Corrosion - evaluation of corrosive environments and selection of materials to survive exposure to those environments Â• Heat treatment - development of heat treatment processes for ferrous and non-ferrous metals to improve material strength, toughness, wear resistance and corrosion resistance Â• Brazing - selection of brazing filler metals and processing to join a wide variety of metals used in HVAC equipment. Provided brazing training to manufacturing facilities in the US, Asia and Europe Â• Bearing metallurgy - selection and processing of specialized materials used in rolling element and plain bearings. Development of tribological tests for bearing materials Â• Sheet metal forming - selection of materials for sheet metal forming applications including deep drawing, bending and stretching. Application of circle-grid analysis to facilitate application of appropriate materials
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Published on Mar 5, 2012
1. Material Selection after the part/component is 'designed'. 2. Material Selections not reviewed over time. 3. Materials copied from old de...