Cost-competitive lightweight materials are a significant and relative objective for commercial-vehicle (CV) manufacturers and component suppliers. This focus has expanded significantly in all areas of CV segments, impacting both powertrain and chassis applications. The primary challenge to determine the optimal lightweight material is multifaceted in an effort to meet all imperative load criteria, while simultaneously providing mass savings in a globally scalable, cost-competitive solution.
Grede Holdings presents a comparative analysis of lightweight material options in both strength and cost unit values. Further comparisons qualify the improved value performance of advanced ultrahigh-strength cast ductile iron materials such as the SiboDur grades that provide a lightweight and cost-competitive solution vs. the traditional nonferrous cast aluminum materials.
General industry trends reveal that low-density materials such as aluminum, magnesium, and composite derivatives have begun to replace iron and steel. These materials make sense when simply looking at mass density values and typically are the first materials considered in such mass-savings initiatives; however, there can be cost and design ramifications when balancing modulus differences, material strength, ductility, and stiffness.
Mechanical property and cost comparatives
Typically using an aluminum casting vs. an iron casting will result in cutting the component mass by half. Most applications make use of the A300 series aluminum, particularly the hypoeutectic alloys: A356, nominally 7% silicon, 0.35% magnesium; and A357, nominally 7% silicon, 0.55% magnesium. The silicon gives good fluidity when casting, enabling thin sections to be successfully cast. The magnesium provides strength through heat treatment, which also adds considerable cost to the overall manufacturing process in attempts to offer reasonable ductility.
The lowest-cost general-purpose alloy is A356-T6, which is commonly used in many industry applications.
Comparative strength values of cast aluminum, cast ductile iron, cast SiboDur, and cast austempered ductile iron (ADI) materials offer one of the primary determination factors to qualify the material strength parameters for selection of a lightweighting design initiative. A key parameter identified is the limitations of aluminum in both tensile strength and elongation vs. the higher strength characteristics found in the ductile iron derivatives.
The key objective in the material selection process is to match applied structural load criteria to the tensile and elongation values within acceptable safety factors of the design criteria, applied operating environments, and given product life cycles.
Five different SiboDur alloys are available, each offering select chemistries that can improve fatigue strength and functional design optimizations to match the requirements of specific applications. For example, SiboDur’s ultimate tensile strength ranges from 450 to 800 MPa, with elongation percentage ratios of 6 to 23%.
Regarding the comparative strength values of these same materials in Rp0.2%-yield stress (N/mm²) vs. elongation % values, again limitations for aluminum are revealed. Limitations of the material selection criteria are imperative to fully consider the dynamic strength characteristics of the material choices, as both tensile and elongation values can be considered as interdependent strength values in the product design and material selection process.
To determine the optimal cost-competitive lightweight material option, it is necessary to fully understand and consider the unit strength value of the select material properties with a quantifiable understanding of the associated cost/lb of a given material and its correlating mechanical-property strength value.
The fundamental costs of material options can be evaluated by comparing the true cost/lb vs. tensile unit strength value of measurement. Aluminum A356-T6 market cost position for a traditional high-pressure die cast component carries a cost burden rate of approximately $2.50/lb, with a defined tensile unit strength property value of about 450 MPa. In comparison, an ultra-high-strength cast ductile iron alloy such as SiboDur 700-10 carries a cost burden rate of approximately $1.20/lb with a tensile unit strength value of 700 MPa.
With the perspective of material cost valuation, it is prudent to further consider the cost value per lb of mass savings that a given lightweighting program may offer. This cost value of mass savings per lb will certainly vary by a number of basic factors, which largely depend on OEM platform efficiencies and/or governmental mandates driving such initiatives.
Design envelope and component integration
When considering the total cost competitiveness of a lightweight material, it is important to fully consider the design envelope requirements and implications of utilization from one material alloy to another. Further prudence should be taken when considering the total design-for-manufacturing process and integration of mating ancillary adaptive components that comprise the subsystem. Careful examination of the total cost value stream, considering component, manufacturing, and total subsystem integration cost values, is paramount.
That being said, it is critical to understand the implications of the available design envelope that typically expands to accommodate the lower-density materials, such as aluminum, which contain a lower Young’s modulus value.
For example, a steering knuckle design that originated from an aluminum die-casting process was transitioned progressively into a higher-density material such as standard GJS450 cast ductile iron material. The design envelope and cross-sectional areas are reduced in overall size comparatively. Additional improvements in reducing the component design envelope and taking additional cast integration of the ancillary brake anchor components and shock tower adaptation features, which were previously considered separate bolt-on componentry, can be realized in a one-piece, ultra-high-strength cast ductile iron solution.
The aluminum design necessitated a much larger design envelope to compensate for deltas of Young’s modulus between the material options as compared, and certainly a compensated approach to offset the loss in stiffness was required in the suspension application. By comparison, total cost value stream improvements are possible by utilizing a higher-strength material such as SiboDur 700-10 to achieve part consolidation and streamlined manufacturing operations, as well as overall mass reduction values.
Fundamentally, aluminum offers what is perceived to be mass savings with the delta of material mass density. However, with further consideration to the associated costs for adaptation methodologies required for wheel bearings, brake caliper brackets, and ancillary components, coupled with the increase in the total design envelope required to accommodate the lesser-strength material, it is commonly found that OEMs have dramatically realized cost inflation to overcome design inefficiencies.
SiboDur handles the load
Commercial vehicles are subjected to a wide range of loads, operating environments, and thermal conditions, which include high shear and bending (due to the vehicle mass and high loads distributed), torsion (caused by irregular roads, putting one wheel on a curb), lateral loading (caused by cornering), and fore and aft loading (caused by acceleration and deceleration). In practice, these load events occur in combination together with both dynamic and transient effects in a wide array of temperature and environmental conditions.
Typically a CV structure will be designed so that under the worst envisaged dynamic load condition, there will still be a factor of safety based on the yield strength of 1.5. This factor is common and usually adequate for guarding against fatigue failure; however, fatigue calculations are still used where stress concentration effects are likely to be significant.
In practice, vehicle design is normally governed by stiffness requirements rather than maximum permitted stress levels.
To maximize SiboDur’s capabilities under such conditions, Grede uses topology and FEA to create precise casting geometry to match the individual load case and stress strain vectors exactly where needed, effectively reducing mass and improving fatigue strength.
Truck makers worldwide are using, developing, and testing SiboDur in a variety of components for current and future vehicle programs. These include safety-critical parts with high stress and torque requirements, such as steering knuckles, axles, control arms, hubs, brake rotors, engine mounts, crankshafts, and suspension links.
SiboDur was developed by Swiss-based Georg Fischer Automotive AG; Grede has an exclusive license to use the alloy in North America. In addition to the commercial truck industry, the company also plans to supply SiboDur-based castings for cars, light-duty trucks, and industrial applications.
This article is based on SAE International technical paper 2013-01-2419 written by Jeffrey W. Nichols, Director of Business Development, Grede Holdings, LLC.