Materials & process selection

Materials selection is a critical component of the design process. REXP2 Research LLC has significant experience selecting, characterizing and qualifying new materials through the use of quantitative processes. Materials selection is about both quantifiable properties and qualitative experience using the materials. Materials selection must be integrated with process selection. Choosing the right heat treat conditions or forming processes is critical for achieving performance and cost control. We bring an extensive experience base across multiple material systems and alloys, as outlined in the table below, to new problems. 

When sufficient property data from materials characterization and quantifiable design requirements are available, the methods developed by Michael Ashby* provide an objective, quantifiable, and documentable process for selecting both a material and a process. The Ashby method is applicable to both the design engineer and the materials engineer. We have conducted professional development seminars on this subject and would be glad to prepare a workshop for your company or more simply work with you to implement the method in a design situation..

The Ashby method of materials sellection, an example

As an example of the Ashby approach, consider the the component shown below made from an aluminum alloy. The part is grounded at the point where the cylindrical shaft intersects the rectangular plate. The shaft is stressed in tension by load F, and the plate is in bending by load P.  The objective is to reduce the weight. 

The plot of log strength vs. log density shown at right is modified from the 1992 edition of M.F. Ashby’s book.  It shows the strength-density space occupied by various ranges of materials. Now, consider an aluminum alloy as the base line material, with a yield strength of 35 ksi (241 MPa). The three guidelines are for different types of loading, where σ/𝜌 is for uniaxial tension and σ^1/2/𝜌 is for a plate in bending. Skewing these lines to intersect the yield strength of the incumbent material allows us to identify lighter and heavier materials. 

Free body diagram for illustrating application of Ashby method in materials selection.

 

For the shaft in tension, those above the red line will be lighter, those below will be heavier. For the plate in bending, those above the blue line will be lighter and those below heavier. If the same material is to be used for both parts, then the options are limited to the green zone. Clearly, there are many more constraints in a design which must be considered, however, this method is designed to handle those situations. Charts can be created for fracture mechanics, stiffness, or thermall property driven designs.  The take-a-way here is that by using this approach, with design data for the actual product, a quantifiable and documentable series of decisions are possible. This method organizes the trade study.

* Michael F. Ashby, Materials Selection in Mechanical Design, Butterworth-Heinemann, 2011.

Ashby strength vs. density plot for light weight optimised material selection.
Adapterd fom a chart used with permission from the Appendix of: M.F. Ashby, "Materials Selection in Mechanical Design", Pergamon, 1992.

How process is an integral part of material selection

Consider the two piece assembly shown at right. Think of a c-clamp and the swivel connection. The red region of the head is swaged around the spherical end of the rod. Now, for this example, both the red and blue regions of the head require wear resistance and corrosion resistance. So, for initial materials selection, these would be two constraints on say, a strength vs. density analysis, as described above. However, to do the materials selection process correctly, the strengths used need to consider the effect of cold working the red and possibly the blue regions of the head.  

This is a challenging problem, as the materials must be wear resistant. The simplest way to achieve wear resistance is to harden the material. The swaged region in red could be hardened simply by the swaging process, but what about the blue region? For the blue region, a coating could be used, or it could be cold worked separately from the swaged region by upset forging only that region. Since cold working will account for wear resistance in ether one or two regions of this part, the materials selection process needs to assess cold workability and the extent of hardening. Not all materials respond to cold work in the same way.  An alternative way to make the head would be to start with cold worked and hardened metal and selectively anneal the red region to make it again workable for the swaging operation.

Schematic of cold headed and swaged coupling which work hardens the part for increased wear life.

Materials selection and characterization experience base

FerrousStainless steelAluminumNickel/CobaltTitaniumCeramics/glassMisc.Coatings
AISI 1010AISI 304LAA2014Inconel 718Ti-6-4Silicon nitrideCarbon-CarbonAnodized aluminum
AISI 1045AISI 304AA2024Inconel 625Ti-24-11Silicon carbideGraphiteAnodized magnesium
AISI 4340AISI 321AA2219Haynes 25Ti-25-10-3-1Zirconia toughened aluminaPitch polymers and carbonCarburizing
AISI 3130AISI 347AA6061Haynes 230Ti-6242AluminaParticle reinforce AA8009Nitriding
AISI 52100AISI 410AA7075WaspaloyBeta-CBoron nitrideRheniumBoriding
Aermet 100AISI 416AA8090Mar-M-247Phosphate glassTungstenCase hardening
InvarAISI 430AA8009Stellite alloysLanthanide glassMolybdenumParylene
Controlled expansion alloysAISI 440CC355Ni-Co electroformedTitanium nitrideFuran polymersSpark anodizing
BG4217-4 PHA357Nickel 200ParyleneChrome plating
Kronodor-3017-7 PHUdimet 700Photo polymersTungsten-cobalt plating
AISI M5015-15 PHHastelloy X750Fluoro-silicone polymersTungsten-nickel plating
AISI H11Greek AscoloyHastelloy XCDA 630Rhenium-nickel plating
CPM-10VNitronic 60Hastelloy CCDA 544Stellite coatings
A286Spinodal bronze ToughMet®