Description / Abstract:
1. Scope--Automotive parts can be fabricated from either coiled sheet,
flat sheet or extruded shapes. Alloy selection is governed by finish
requirements, forming characteristics, and mechanical properties.
Bright anodizing alloys 5657 and 5252 sheet provide a high luster and
are preferred for trim which can be formed from an intermediate temper,
such as H25. Bright anodizing alloy 5457 is used for parts which
require high elongation and a fully annealed ("0") temper. Alloy 6463
is a medium strength bright anodizing extrusion alloy; Alloy X7016 is a
high strength bright anodizing extrusion alloy primarily suited for
bumper applications.
To satisfy anti-glare requirements for certain trim applications, sheet
alloy 5205 and extrusion alloy 6063 are capable of providing the
desired low-gloss anodized finish.
Bright anodizing alloys require control of the chemical composition of
the alloy to enhance response to chemical brightening and to result in
the formation of anodic coatings that are essentially transparent.
Additionally, aluminum producers employ fabricating practices to
minimize other metallurgical factors that adversely affect response to
bright anodizing procedures. For non-heat-treatable alloys, a highly
fragmented grain structure is preferred. Fully annealed, recrystallized
grain structures are not optimum for bright anodizing. Where high
elongations are required with intermediate tempers, fabricating
practices are selected to minimize grain recrystallization.
Another factor to be considered for trim application is the type of
mill surface finish that is required. When the metal working treatments
do not mar the mill produced surface appreciably, the smooth, bright
rolled, "automotive trim" surface is desirable since it often
eliminates the need for expensive mechanical buffing operations. Where
trim fabricating procedures might be expected to damage a bright-rolled
surface, duller mill finishes can be used and parts are buffed after
forming. Bright rolled mill surfaces occasionally are protected with a
removable tape or water soluble film.
Selection of anodic coating required to protect aluminum parts is
influenced by the required corrosion performance and appearance
characteristics. Generally, anodic coatings 0.0003-0.0005 in
(0.0076-0.0127 mm) thick are used for exterior trim application.
Thinner anodic coatings 0.0001-0.0003 in (0.0025-0.0076 mm) are
sufficient for interior trim components. Anodic coatings can be dyed to
impart color, painted, or inlaid with vinyl or other plastics for
aesthetic and/or functional purposes.
The Aluminum Association's "Designation System for Finishes" is a
recommended guide to assist in specifying anodic coatings for
automotive trim.
The American Society for Testing and Materials (ASTM) offers several
test methods which are commonly used as the basis for many user
specifications. These are:
ASTM B110--Dielectric Strength of Anodically coated Aluminum
ASTM B457--Measuring Impedance of Anodic Coatings of Aluminum
ASTM B244--Measuring Thickness of Anodic Coatings on Aluminum with Eddy
Current Instruments
ASTM B136--Resistance of Anodically Coated Aluminum to Staining by Dyes
ASTM B137--Weight of Coating on Anodically Coated Aluminum
ASTM B368--Copper-Accelerated Acetic Acid Salt Spray (Fog) Testing
(CASS Test)
ASTM B538--Fact (Ford Anodized Aluminum Corrosion Test) Testing
ASTM B580--Guide to the Specification of Anodic Oxide Coatings on
Aluminum
ASTM 429--Measurement and Calculation of Reflecting Characteristics of
Metallic Surfaces Using Integrating Sphere Instruments
ASTM E430--Measurement of Gloss of High Gloss Metal Surfaces Using
Abridged Goniophotometer or Goniophotometer