96 CHAPTER 3 Materials for consideration and use
3.2 MATERIAL CANDIDATES AND SELECTION CRITERIA
The range of body materials that may be considered for volume car body construction is shown inTable 3.3. It will be apparent that the criteria used by major manufacturers when considering a new design extend beyond the range of physical and mechanical properties on which selection was once based. Not all factors are shown and it is easy to subdivide any of the columns shown. The legislative requirements concerning emissions and end-of-life (ELV) disposal, for example, are now influencing the initial choice of material. Increasingly the process chain or 98 CHAPTER 3
Materials for consideration and use
Criteria
YS (MPa)
UTS (MPa)
A80 (min%)
E. Mod (GPa)
D
(g/cm3) Forming Joining Paint
CO2þ Emissions
Disposal (ELV)
Forming Steel¼1 Forming grade
steel EN 10130 DCO4þZ
140 min
270 min
40 210 7.87 8 9 9 7 9 1.0
HSS EN 10292 H300YDþZ
300 min
400 min
26 210 7.87 6 8 9 8 8.5 1.1
UHSS – martensitic
1050–
1250
1350–
1550
5 210 7.87 4 7 9 8 8.5 1.5
Aluminum 5xxx 110 min
240 min
23 69 2.69 6 5 8 9 9 4.0
Aluminum 6xxx 120 min
250 min
24 69 2.69 6 5 8 9 9 5.0
Magnesium sheet
160 min
240 min
7 45 1.75 4 4 7 9.5 6 4.0
Titanium sheet 880 min
924 min
5 110 4.50 6 5 7 9 6 60.0
GRP 950 400–
1800
<2.0 40 1.95 8 7 8 8 5 8.0
Carbon fiber composite
1100 1200–
2250
<2.0 120–250 1.60–1.90 8 7 8 9 5 50.0þ
*Based on range: 1¼difficult to process; 10¼production without difficulty
**Ease with which prevailing legislation can be met: 1¼extensive development required; 10¼without difficulty
3.2Materialcandidatesandselectioncriteria99
successive stages of manufacture are also considered to ensure that there is minimum disruption, which could have consequences on productivity and quality.
Any allowances for new materials will have been thoroughly proven at the prede- velopment stage of production.
Steel is still the predominant material used for manufacture1,2and the generally high ratings levels shown under the ‘ease of manufacture’ column reflect the provision already made by the industry for compatible facilities. The lower ratings for the processing of aluminum do not necessarily indicate that these newer materials are inferior; rather, they are indicative of the need to introduce new practices and of the size of change needed to accommodate them. However, changes are inevitable to ensure the different legislative requirements are met and perhaps Table 3.3indicates the ‘pain’ necessary to implement these lighter, but sometimes problematic, alternative materials. Figures are also included for polymeric materials, frequently used in specialist car manufacture, and carbon fiber composites used in competition vehicles.
3.2.1 Consistency:a prime req uirement
Regardless of a material’s physical and mechanical properties, a key requirement for maintaining manufacturing productivity is consistency. Once a piece of equipment has been set to operate within a given range of compositional, mechanical and dimensional specifications, to ensure maximum output it must run continuously without disruption or the need to persistently reset process variables such as press and welding settings. Of course, the tightest tolerances must be held with regard to tooling and machine efficiency, but uniformity of material characteristics is essential if the full benefits of these facilities are to be realized. This is particularly true of state-of-the-art automated tri-axis progression presses and multi-station robotic welding equipment. Due to the momentum of these systems and parts in process any delay resulting from defective processing can quickly lead to large quantities of scrap (or parts needing rework) being generated before the fault is detected. Rigid monitoring of the feedstock production process is, therefore, required with regard to both constituents and process. As for automotive assembly manufacture, statistical process control should be rigorously implemented, with all contributors to the supply chain demonstrating compliance with the requirements of BSEN ISO 9002, QS9000 and individual company quality approval procedures.
Uniformity and the highest possible quality are the twin aims. Continuous pro- cessing must, therefore, be encouraged as batch manufacture of primary material, even with the best available controls, is, by definition, liable to local inhomogene- ities in composition, temperature and other aspects. Continuous processing, due to the scale of operation and improved operating efficiencies, must, therefore, be beneficial, and although the material might deviate slightly from the original specification it can be utilized efficiently. With regard to steel manufacture, continuous casting and annealing processes have already been widely introduced (the advantages of which are described later in this chapter), and casting to thin 100 CHAPTER 3
Materials for consideration and use
section dimensions is well on the way to becoming reality. Another interesting trend is the coupling together of increasingly complex major operations, such as pickling and cold rolling. More ambitious attempts have been made at the amalgamation of pickling, rolling and annealing processes, but were hampered by capacity imbal- ances. More widely, similar processing is being introduced in the aluminum industry, where continuous annealing using rapid induction heating has been developed and continuous casting has been accepted practice for many years.
Uniformity of processing and product are, therefore, as significant as property characteristics and this is the underlying theme of this chapter. However, before considering materials in the context of design, manufacturing and service, it is essential that the manufacturing processes associated with each of the main mate- rials are understood so that the implications of various grades, treatments and finishes can be appreciated.
In the following sections, the materials are presented in order of their degree of utilization within the automotive industry.