22nd March 2013 by Mike Heskitt
This post has been contributed by Regu Ramoo, Director of Engineering at Altair ProductDesign
Many studies on automotive mass reduction have been undertaken over the years by various steel, aluminum, magnesium, and composites consortia, all expounding the virtues of substituting a particular material. Altair has participated in studies with all these organizations over the years and has understood the strengths, limitations, and constraints of working with various materials.
AHSS, HSS, Al, Mg, Ti, GFRP, CFRP…
High Strength Steels, Aluminum, and Magnesium all have certain advantages in specific applications. Understanding when to exploit the unique advantages of these materials while concurrently minimizing the associated cost penalty is key in any weight reduction challenge.
Where energy mitigation is the driver, High Strength Steels (HSS) traditionally have an advantage but where strength and stiffness are dominant and durability is of less concern, significant weight save can be realized by using aluminum and magnesium. Depending on the part geometry, processing, and manufacturing constraints, one may yield a lower cost penalty and greater weight save.
Degradation of strength & fatigue properties due to welding, as well as issues related to joining dissimilar materials, such as expansivity mismatch and galvanic corrosion are some issues that must be considered in the overall design. When the design freedom allows for expanding the package space to accommodate larger sections or when certain net-shape re-designs to consolidate features can be exploited, composites can have a significant advantage.
Understanding Dominant Loading Characteristics
Except for idealized theoretical models, most real world structures see a multitude of loads making selection of the optimal material usage difficult without the right toolset, and materials and manufacturing knowledge. Front crash rails are predominantly loaded in the axial direction requiring the capacity to accommodate high plastic strains while side impact structures are loaded mostly bending requiring high yield strength.
The passenger compartment and green-house structure must minimize intrusion and provide adequate stiffness to meet NVH requirements.
When designing with different materials, the existence of varying dominant loads in different regions of the vehicle structure must be exploited. The use of Topology Optimization early in the design development phase can help distinguish regions that are stiffness dominant and those that are strength dominant. Regions that are strength dominant benefit the most from using high specific strength materials.
Steel body in white (BIW) structures have employed Boron steels, HSLA, dual phase materials, foam injected joints, composite inserts, and advanced processing methods like tailor-welded blanks, 4-t welds, deep draw reinforcements, and many other innovative methodologies to improve the stiffness and strength to weight ratios.
While much research has gone into developing AHSS with higher strengths and more formability, the specific stiffness of all grades of steels are still very comparable. Vehicle NVH attributes are almost totally driven by specific bending stiffness which gives aluminum and magnesium a significant advantage. Durability attributes are also somewhat driven by vehicle stiffness but high strength local reinforcements can make up for any local compliance to meet design targets.
So what is a more systematic way for deploying multi-material optimization?
Here is a system that if conformed to, can get you the greatest gains. Start with a high level topology optimization to see where is the best place to put material. Then consider what the dominant load sets for different regions are. Put together a spread-sheet of all the materials at your disposal and attributes you have to design to meet:
Cost and weight should certainly be a consideration but you would need to understand the manufacturing methods (stamped, extruded, roll formed, die cast) and volumes, as more often than not, those are larger cost drivers than the material cost per pound. A cursory look at the specific material performance will provide some insight on the weight and associated cost:
If you are looking for bending stiffness, magnesium or aluminum would be my choice. If you need high threshold strength to meet fatigue loads or ultimate loads, HS steel may be good a choice or aluminum if you have the space claim available to package the section size or thickness you need but be mindful of the degradation around the heat affected zones. Of course the complexities associated with joining dissimilar materials (which by itself warrants another blog) must be thought through early.
If you are still not sure on how to mix it up to get the best bang for your buck, contact us at Altair ProductDesign and we’ll be glad to help!