Materials, Design and Manufacturing for Lightweight Vehicles

P.K. Mallick

• Publisher: Woodhead Publishing
• Year: 2010
• ISBN:
  9781845697822
• Topic: Technology & Engineering
• Subtopic: Mechanical Engineering
• Index: Technology & Engineering

Research into the manufacture of lightweight automobiles is driven by the need to reduce fuel consumption to preserve dwindling hydrocarbon resources without compromising other attributes such as safety, performance, recyclability and cost. Materials, design and manufacturing for lightweight vehicles will make it easier for engineers to not only learn about the materials being considered for lightweight automobiles, but also to compare their characteristics and properties.Part one discusses materials for lightweight automotive structures with chapters on advanced steels for lightweight automotive structures, aluminium alloys, magnesium alloys for lightweight powertrains and automotive structures, thermoplastics and thermoplastic matrix composites and thermoset matrix composites for lightweight automotive structures. Part two reviews manufacturing and design of lightweight automotive structures covering topics such as manufacturing processes for light alloys, joining for lightweight vehicles, recycling and lifecycle issues and crashworthiness design for lightweight vehicles.With its distinguished editor and renowned team of contributors, Materials, design and manufacturing for lightweight vehicles is a standard reference for practicing engineers involved in the design and material selection for motor vehicle bodies and components as well as material scientists, environmental scientists, policy makers, car companies and automotive component manufacturers.

• Provides a comprehensive analysis of the materials being used for the manufacture of lightweight vehicles whilst comparing characteristics and properties
• Examines crashworthiness design issues for lightweight vehicles and further emphasises the development of lightweight vehicles without compromising safety considerations and performance
• Explores the manufacturing process for light alloys including metal forming processes for automotive applications

1

Overview

P.K. Mallick, University of Michigan-Dearborn, USA

Abstract:

With increasing demand for fuel economy improvement and emission control, there is a great deal of interest in finding alternative solutions to producing lighter vehicles through material substitution, design modification and efficient manufacturing. The materials available today include high-strength steels, light alloys, such as aluminum and magnesium alloys, and a variety of composites. This chapter gives an overview of these materials and their current and future applications in automobiles. It also provides general guidelines that need to be considered in selecting these materials for lightweight vehicles.

Key words

lightweight vehicles

steel

light alloys

composites

design

manufacturing

1.1 Introduction

Fuel economy improvement and emission control are the two most important challenges the automotive industry faces today. in the USA, the average fuel economy is regulated by the government-mandated CAFE (corporate Average Fuel Economy) standard, which is a sales-weighted average fuel economy (expressed in miles per US gallon or mpg, 1 mpg = 0.43 km/liter) of a vehicle manufacturer’s fleet of passenger cars or light trucks. For passenger cars, the CAFE standard increased from the initial 18 mpg in 1978 to the current 27.5 mpg. it is destined to increase to 35 mpg in 2020. The fuel economy standards in other countries are either directly or indirectly regulated by their governments or voluntarily maintained by the car companies. These standards are different from the US standard and many of them are much more stringent, but they all have the same dual goals of improving fuel efficiency and reducing environmental pollution and CO₂ emission, the major cause of the greenhouse effect.

Fuel economy of a vehicle is measured using a specific driving cycle (the EPA driving cycle in USA) and depends on many factors, which include vehicle power requirement, vehicle speed, engine and transmission efficiencies, and fuel type. The vehicle power requirement is the sum of the power requirements for vehicle acceleration, driving on a grade, overcoming the rolling resistance at the tire-road interface, overcoming the aerodynamic drag and operating the accessories, such as air conditioner, heaters and entertainment modules. The first three of these power requirements are directly proportional to the vehicle weight. Thus, reducing the vehicle weight can cause a significant reduction in vehicle power requirement, and consequently increase the fuel economy. Aerodynamic drag, tire rolling resistance coefficient and accessory/standby power requirement have much smaller effects on fuel economy. Studies have shown that every 10% reduction in vehicle weight can result in 5 to 8% greater fuel efficiency (Brooke and Evans, 2009). In terms of the greenhouse effect, reducing vehicle weight by 100 kg results in CO₂ reduction up to 12.5 g/km.

In addition to the primary benefit on fuel economy, weight saving has several secondary benefits. For example, when the vehicle weight is reduced, the power needed for acceleration and braking is also reduced, which creates an opportunity to design smaller engine, transmission and braking systems. From the standpoint of vehicle dynamics, vehicle weight reduction does not affect the vehicle stability and control. On the other hand, weight saving in selected components can be utilized to equalize the vehicle mass distribution between the axles and lower the vehicle’s center of gravity, both of which improve vehicle handling. Two vehicle attributes that may be negatively affected by vehicle weight reduction are ride comfort and safety. However, they are also influenced by vehicle design and material selection.

Historically, vehicle weight reduction started in the USA in the 1970s, when the vehicle manufacturers started downsizing their vehicles to meet the 1978 CAFE requirement of 18 mpg (Brooke and Evans, 2009). Downsizing was achieved primarily by introducing cars with smaller wheelbases and by shifting from body-on-frame structures to unibody or body-in-white (B-I-W) structures (see Section 1.3.1). Other changes that contributed to vehicle weight reduction include smaller engines (4-cylinder engines instead of 6-and 8-cylinder engines) and front-wheel drive transmission instead of rear-wheel drive transmission. As a result of these design changes, the average vehicle weight of US cars decreased from 1839 kg in 1976 to 1385 kg in 1986. However, as Table 1.1 shows, the average vehicle weight started to creep up in the 1990s due to the addition of several new features, such as safety equipment, emission-control devices and entertainment modules. In the US market, bigger vehicles, such as sports utility vehicles (SUVs) and pick-up trucks, became more popular than the sedans, which also contributed to the increase in vehicle weight. Starting in 2007, as the gasoline price began to increase in the USA, sales of SUVs started to decline and, in response, the vehicle manufacturers started to produce crossover vehicles that were smaller than the full-size SUVs, but bigger than the mid-size sedans. This started a declining trend for the average vehicle weight, which is expected to continue and may even become more critical for future vehicles.

Table 1.1

Average vehicle weights of US cars

^*Forecast

Source: Brooke and Evans, 2009

With increasing fuel price, unpredictable long-term availability of petroleum and greater realization that auto emission is hazardous to both the environment and public health, fuel economy improvement has become the top priority for the vehicle manufacturers around the world. Vehicle weight reduction is considered one of the key elements in the fuel economy improvement strategies. While vehicle downsizing is still an option for achieving significant vehicle weight reduction, at least in the near future, there will be a mix of small and large size vehicles to satisfy the customer base. The other option is to reduce component weights using material substitution, parts consolidation and design optimization. This chapter gives a broad overview of the materials that are being considered for making lightweight components. Many of these materials are covered in much greater details in the remaining chapters of this book.

1.2 Materials scenario

Low carbon steel and cast iron were the workhorse materials in the automotive industry prior to 1970s. As Table 1.2 indicates, even today steel is used in much larger quantities than any other material, although instead of only low carbon steels, there is now a mix of low carbon steels and high strength steels. However, with greater emphasis on vehicle weight reduction, the materials scenario is changing rapidly to include other materials, such as aluminum alloys, magnesium alloys and polymer matrix composites (Powers, 2000). Table 1.3 lists the tensile properties of a few selected materials that are in competition with steels for future vehicle construction. They are all lighter than steels and many of them also provide a good opportunity for parts consolidation, but they are at present not cost-competitive with steels, particularly for large production volumes. Nevertheless, their technical viability and weight saving potential have been demonstrated in many concept vehicles, and now they are appearing in increasing quantities in many production vehicles.

Table 1.2

Material distributions in typical automobiles

Material	Percentage of vehicle weight	Major areas of application
Steel	55	Body structure, body panels, engine and transmission components, suspension components, driveline components
Cast iron	9	Engine components, brakes, suspension
Aluminum	8.5	engine block, wheel
Copper	1.5	Wiring, electrical components
Polymers (plastics) and polymer matrix composites	9	interior components, electrical and electronic components, under-the hood components, fuel line components
Elastomers	4	tires, trims, gaskets
Glass	3	Glazing
Other	10	carpes, fluids, lubricants, etc.

Table 1.3

Material property comparisons

L is the longitudinal direction and T is the transverse direction.

CFRE is carbon fiber reinforced epoxy, GFRE is glass fiber reinforced epoxy.

The greatest opportunity for component weight reduction exists in the body and chassis components, which comprise 60% of a vehicle’s weight. Many new materials and manufacturing processes have been developed in the last 20 years to decrease the weight of the body structure, body panels and suspension components. Powertrain weight, which includes both engine and transmission, is between 25 to 30% of the vehicle weight. Several new materials and manufacturing process developments have also occurred to reduce the powertrain weight. This section reviews the new materials and manufacturing processes that either are being used, or have the future potential for use, in achieving the vehicle weight reduction necessary for fuel economy improvement.

1.2.1 Steel

Approximately 55% of a typical US car’s weight is due to steel. The greatest advantages of steel over other materials that are either being used or are under consideration for use in automotive structures are as follows:

• Low cost and high modulus (207 GPa, which is higher than all other competitive structural metals)

• Wide range of strength and ductility, which can be achieved by a variety of means, such as alloying, work hardening (for low and medium carbon steels) and heat treatment (for medium carbon, high carbon and alloy steels)

• Excellent formability for low carbon steels and many newly developed high strength steels, such as high strength low alloy (HSLA) steels and dual phase (DP) steels, which make them highly suitable for high production rate forming operations, such as stamping and roll forming

The automotive steel scenario has changed tremendously in the last 25 years, principally because of the challenges posed by lighter weight materials, such as aluminum and plastics. New steel making processes (e.g. vacuum degassing) have made it possible to produce steel more cost effectively with much lower impurity levels (only about 10–20 ppm compared to 200–400 ppm by the traditional processes). Combination of new alloying techniques and improved heat treatment procedures, such as continuous annealing, are now used to produce not only a broad spectrum of strength and ductility, but also better surface qualities and more uniform properties in sheet steels. Better corrosion resistance is achieved b...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Preface
• Chapter 1: Overview
• Part I: Materials for lightweight automotive structures
• Part II: Manufacturing and design of lightweight automotive structures
• Index