Engineering Essays - Aluminum Alloys for Automotive Castings

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The Use of Aluminum Alloys for Automotive Castings

1.      Introduction

Aluminum and its alloys have long been in use as the material of choice for a wide variety of applications ranging from common household utensils to very sophisticated applications like space crafts, airplanes, etc. The automobile industry, searching for a lighter and superior corrosion resistant alternative to steel, also recognized the potential of aluminum following which its use increased progressively since early 70's (EAA, 1996) presently positioning it as the 3rd most favored material for automotive applications after steel and iron. Its use in the automobile sector is destined to increase in the coming years, and by 2005 aluminum use is projected to increase to approximately 140 kg per car from the 1995 value of 75 kg per car (EAA, 1996). Aluminum finds use in vehicles in different forms and components like cast components, extruded forms and rolled products. From the standpoint of engineering and design, aluminum and its alloys stand out to be a very promising alternative for application in various automotive casting components and products and presently more than 80% of aluminum applications in automobiles are directed towards cast components e.g. engine blocks, gear blocks, cylinder heads, etc. (EAA, report). The selection of any automotive casting material is essentially attributable to a host of parameters broadly falling under the domains of - material characteristics & compatibility, vehicle design & engineering, availability & supply, vehicle safety & performance, fuel economy & environmental standards, cost factors, etc.

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This essay aims to appraise the potential of aluminum alloys for use in automotive casting applications specifically considering the engineering & design aspects of automotive cast components. Some of the important determinants and attributes of aluminum and its alloys like present and emerging areas of applications, design-casting and manufacturing technology considerations, supply-availability & environmental issues and costs & other factors have been assessed and reviewed in this paper with a view to see how it fits into the scheme of things for automotive design engineers in accomplishing the ultimate goal of designing affordable and reliable modern vehicles. As a whole this paper is expected to extend our knowledge on the real status and potential of use of aluminum alloys for automotive applications.

2.      Drivers & Automotive Applications of Aluminum

The automobile industry is one of the most dynamic sectors continuously confronting various technological challenges and steep market competitions. Vehicles are now required to comply with increasingly stringent exhaust emission regulations and fuel economy mandates like the Corporate Average Fuel Economy (CAF̉ۡ) standards along with other emerging requirements like performance improvement, quality enhancements, needs for better safety & weight-to-strength ratio, consumer styling demands, etc. (The Aluminum Association Inc, AT7, 2001). Primarily this situation compelled the automotive designers to explore options for reducing total vehicle weight along with innovations in engine technologies. Steel and iron, the conventional automotive application materials having higher density and weight do not leave much room for any further benefits in terms of vehicle weight reduction whereas aluminum having a density of only 2.7 g/cm3 which is just 1/3 rd of steel, easily offers itself to be a better option in this case and this eventually opened the gates for aluminum use in automobile applications. Thus, stringent regulatory and fuel efficiency requirements leading to preference for light weight materials are the important drivers encouraging use of aluminum alloys for automobile applications. The weight of a vehicle has direct impact on its fuel economy and any saving in fuel consumption in a way translates into overall vehicle emission reductions. This is illustrated in Figures: a & b which reveal that reducing vehicle weight by 100 kg using aluminum components allow fuel saving of approximately 0.6 liters/100 km and throughout vehicle lifespan fuel saving is about 3000 liters.

A variety of automotive components have made successful transitions from cast iron and steel fabrication designs to aluminum. At present, as much as 80% of the total aluminum requirements of a vehicle are met by cast aluminum (EAA). Aluminum castings are increasingly finding use in different vehicle performance critical components such as power train, transmission, steering system, brakes, suspension and wheels. Other prominent aluminum alloy cast components are intake manifolds, cylinder heads and engine blocks. As per projections of the European Aluminum Association (EAA), total aluminum content in a car is expected to rise to 130-150 kg by 2005. Along with the obvious rise in use of cast aluminum products in vehicles, there will be further requirements for other extruded and rolled aluminum products mostly for vehicle body structure applications. Aluminum is currently making smooth progress towards the goal of substituting conventional automotive steel however; initially it faced minor resistances which have ceased to exist now with the advances in design and development of optimized aluminum casting procedures. Aluminum is already making inroads in various automobile applications and holds the promise for extended use in the near future. To seize this opportunity, major companies like Ford, Chrysler, etc. have already worked out a road-map with comprehensive plans for a greater uptake of aluminum based components for vehicle production. Ford, the world's largest user of automotive aluminum, has also experimented with a concept called Aluminum Intensive Vehicle, AIV (Mark Phillips, 2001) which is aimed to reduce vehicle weight further by 21% to 688 pounds/car. Also, there is a promise for aluminum in the field of electric vehicles which are sure to become popular very soon. Thus, with its unique attributes and advantages, aluminum is all set to emerge as the most viable substitute to automotive steel in the coming years.

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3.      Characteristics, Design & Manufacturing Considerations

Material characteristics, design compatible properties and availability of efficient and affordable manufacturing technology are very important attributes that need to be considered before any material is put to use in automotive applications. Steel is known to posses all such characteristics and attributes and as such went on to become the most favored material for automotive applications. However, steel having a higher density or weight has practical limitations for application in modern light weight vehicles, and hence this has openedakhtarauk.ibm.com up the possibility for the introduction of other favorable materials. Aluminum has a wide range of characteristics and properties that can be engineered precisely to the demands of specific automotive applications through the choice of alloy, temper and fabrication processes (The Aluminum Association Inc., AT7-2001). A comparative account of important characteristics of aluminum and steel from the standpoint of automobile engineering and design requirements is given below:

Yield Strength: In general steel has higher yield strength compared to aluminum when both are of same material thickness. However, aluminum has the advantage of being lighter (2/3 less heavy) when compared to steel and this gives a scope for increasing aluminum's yield strength to levels comparable to that of steel by doubling the thickness of aluminum. Selection of better aluminum alloys also prove effective in this case. Moreover, the combination of the strength of aluminum with its low density results in a higher strength to weight ratio than steel (Aluminum Manual, 2001) and thus enables significant weight reduction where strength is the design limiting criterion.

Impact: Aluminum vehicle structures absorb energy exactly the same as steel by the deformation, folding and concertinaing of the front longitudinal−box−structural beam members. The amount of energy absorbed is related to the yield strength of the material, its thickness and the rate at which the material work hardens as it is deformed. The aluminum can be in the form of sheet structural assemblies, extruded beams or even as ductile castings. Comparative tests with steel show that a spot welded and bonded aluminum box beam will absorb as much energy as a similar steel beam at 55% of steel's weight. This same relationship applies for bending collapse. Also, just as with steel, the geometric design and dimensioning of the energy absorption members are critical in ensuring that folding collapse develops and premature buckling does not occur at the base of these units (Aluminum Association; www://aluminum.org).

Corrosion Resistance: Aluminum does not rust away on exposure to the environment like steel; its natural oxide coating blocks further oxidation. The risk of galvanic corrosion can be minimized by the appropriate choice of alloy, component design, and protective measures.

Forming, Fabricating & Joining: Aluminum can be formed and fabricated & joined by all common metalworking methods including casting, stamping, forging, bending, extruding, cutting, drilling, punching, machining, finishing, and slide-on, snap together or interlocking joints.

The main focus of an automotive design engineer is very much directed towards three material properties - strength (ultimate tensile and yield, as well as elongation), stiffness (modulus of elasticity) and fatigue life. A good designer overall strives for integration of functions and components, maintaining stiffness and strength along with improved performance and reducing total scraps or wastes. While designing components and structures in aluminum, it is important to take advantage of the available product forms to reduce the number of individual parts that have to be made and assembled. This will reduce costs, both by reducing tooling and assembly operations, and will also improve dimensional accuracy, fatigue durability and structural stiffness. The combination of the strength of aluminum with its low density results in a higher strength to weight ratio than steel, and thus enables significant weight reduction where strength is the design limiting criterion. In closure panels for example, aluminum automotive closure sheet can replace steel with a 40-50% weight saving. However, when aluminum is being substituted for steel in an existing component or structural design, there is generally less scope for accommodating larger beams. Aluminum also has a particular advantage over steel for body structure protection in crash situations. Its lower modulus allows for greater elastic deflection and higher energy absorption at weight savings of up to 64%. Even when the beam is designed to match the steel stiffness, energy absorption is higher and 40-50% weight is saved. The utility of aluminum under certain specific design considerations have been illustrated below to see where it stands compared to conventional steel.

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Case-A: Aluminum cross-section's response to the main criteria of bending stiffness - Considering two beams of steel and aluminum with somewhat equal or comparable yield strength, following are the observations for the aluminum section (Figure-c):

         Stiffness: More than 1/3 of the steel beam

         Max elastic displacement:  more than 3X that of the steel beam

         Energy; More than 3x elastic energy absorbed

         Energy: More elastic + plastic energy absorbed

         Weight saved: 64% - however, stiffness is still lower than for a steel beam.

Case-B: Response when the main criterion is Stiffness - Considering two beams, one of steel and one of Al, material yield strength is equal, followings are the observations (Figure- d, below):

         Stiffness: equal to steel beam

         Displacement: approx. 2.5x that of the steel beam

         Energy: More than 6x elastic energy absorbed

         Energy: Elastic + plastic energy absorbed: Much more

         Weight saved: 40 - 50 %

Thus, we observe that aluminum and its alloy have good acceptability to the modern automotive design engineers owing to the fact that along with reduction in total vehicle weight, it also offers other benefits pertaining to vehicle safety, corrosion resistance, etc. The attribute of being comparatively lighter makes aluminum not only preferable to designers but it offers maneuverings and handling advantages to all sections of the product-business chain including design benefits, shop floor handling ease and other end user benefits. There is no doubt that aluminum alloys in all forms offer better design flexibility compared to other automotive application materials.

Besides the design aspects, other important issues which need to be considered for a prospective automotive material like aluminum alloy is the availability and efficiency of its manufacturing and processing technology. In general, Aluminum is one of the few metals that can be cast by any of the standard metal forming methods - die, sand, permanent mold, investment, plaster, continuous, lost foam, squeeze, and hot isostatic pressing. With the advances & innovations in manufacturing technology & processing procedures for quality castings, aluminum alloy is now poised to make further inroads in automotive applications. Component quality can now be enhanced easily by following approaches like - vacuum and pressure casting processes including permanent mould and semi-solid/squeeze casting and adopting automated inspection and non-destructive testing techniques (Bujalski, ). These diverse casting options along with the favorable design aspects and material characteristics are expected to have synergistic effects in augmenting the overall potential of aluminum alloy thereby making it a very viable and attractive substitute to steel for automotive applications.

4.      Supply- Availability & Environmental Issues

There were some concerns regarding the availability and the industry's ability to keep on supplying and meeting the ever growing demands for aluminum materials. However, careful scrutiny reveals that there is no cause for concern, either with regard to the availability of materials required to produce aluminum, or the industry's ability to expand production capacity for meeting growing demands (EAA-Report). Aluminum is one of the most abundant elements on earth and the raw material used for production and processing of aluminum metal is predominantly bauxite and currently availability of bauxite ore is estimated to be over 40 thousand million tons, which is enough to last for more than 400 years at current rates of consumption (EAA, 1996). Presently, use of aluminum based products is very minimal and even considering a steady growth in consumption of automotive aluminum over the next 10 years the total percentage demand works out to be only 20%. Thus with the advances in production technology and manufacturing base and with such reserves of raw materials, availability and supply of aluminum will not be a barrier to wide-spread use of aluminum for automotive applications.

Aluminum use also offers a wide range of environmental benefits the most significant of which is reduction in automobile fuel consumption consequential to reduced vehicle weight resulting from its use. It also has superior corrosion resistance properties. Aluminum does not rust away on exposure to the environment like steel and its natural oxide coating blocks further oxidation. The risk of galvanic corrosion can be minimized by appropriate choice of alloy, component design, and protective measures. Another good property of aluminum is recycleability. It has substantial scrap value providing both economic and environmental benefits. More than 70% of automotive aluminum can be sourced from recycled metal (Aluminum Association Inc.) and importantly there is virtually no cycling limit as it doesn't loose its properties on recycling. Thus on these aspects too, aluminum stands out to be a very attractive alternative for automotive applications.

5.      Costs & Other Factors

Initially, aluminum was not very cost effective but with technology maturing and infrastructures improving, cost issues can no longer remain a negative for aluminum compared to steel. For many automotive applications, aluminum is already in use and cost-wise it proves to be a better option. However for some components it doesn't enjoy a cost advantage over steel. But, there is reason to believe that with growing use, aluminum may find itself very cost competitive to steel and other materials. Another area of concern is the uncompetitive repair and maintenance cost of aluminum applications but this too should fade away with time and the development of better repair techniques and facilities.

There also are other factors which augment the prospects of aluminum for extended use in near future. Aluminum is found to offer a better quality finish and enables styling features that make it more acceptable to modern designers and new-age customers. Aluminum's lighter weight and stiffness can also enhance vehicle acceleration, handling, and reduce its noise, vibration and harshness characteristics.

6.      Conclusions

The review of the above significant parameters and attributes gives us a good impression on the overall potential, prospects and efficacy of aluminum alloy as a befitting and competitive substitute to conventional steel for producing environment friendly, reliable and affordable automobiles. We understand that with increasingly stringent vehicle emission and fuel efficiency regulations and concepts like the 1 Liter cars (mini cars) and other alternative NZEVs (Near Zero Emissions Vehicles, EPA) coming up, aluminum is an inevitable choice. It is important to note that even after having the qualities to surpass steel on all possible fronts; aluminum could only make very limited headway (only 8-10% use) into the automotive industry at present and a projected 12-20% rate of use in future. This is substantially lower compared to the 80% and above rate of use for steel and this situation demands immediate efforts from the vehicle manufacturers, aluminum producers and suppliers and other stakeholders. Nevertheless, we are now confident that this present status of aluminum in the automotive sector is destined to change for the better in the coming years and it is possible that a complete role reversal between steel and aluminum, at least in the automobile sector will occur in the not too distant future.

LIST OF REFERENCES

1.       European Aluminum Association, 1996, 'Aluminum in the automotive industry'.

2.       The Aluminum Association Inc, 2001, AT7, May-2001, 'Aluminum-The Corrosion Resistant Automotive Material'.

3.       Spada, Alfred. T, Engineered Castings Solutions, 2002, 'In Search of Light-Weight Components: Automotive's Cast Aluminum Conversion'.

4.       'Design Considerations for Automotive Castings, Proceedings # SP-1684 from the 2002 Society of Automotive Engineers World Congress, Dearborn, Michigan.

5.       'Aluminum Now', Vol. 5, No. 5, September/October, 2003.

6.       Phillips Mark, My Recycling Today, 2001, 'Automotive Aluminum Challenges Steel'.

7.       'Materials World', May 1999, (P-261).

8.       'http://aluminio-en-coches.html', Archived on 18/11/2005.

9.       Final Report, European Aluminum Association,' Lightweight Potential of an Aluminum Intensive Vehicle', Project No. 24020.

10.   'http://aluminum.org' & http://eaa.org/ama Archived on 17/11/2005.