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 Editor's Note: CR4 would like to thank PJ Sikorsky of GEA Consulting for contributing this blog entry, which originally appeared here.
In this blog we'll discuss the fourth of the five traps we
can fall into with Material Selection,
"Why Material Substitutions Fail".
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.
PJ Sikorsky
9/27/11
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Re: Material Selection: Why Material Substitutions Fail
