Science of Successfully Anodizing Die Cast Substrate
Die castings pose some of the most challenging problems in anodizing. This paper provides some explanations by tying together metallurgical science with anodizing practice.
Die castings pose some of the most challenging problems in anodizing. The finish can be too thin, non-uniform and/or have an unfavorable appearance. These are common problems with a variety of practical solutions; they are easy to recognize, but in many instances, the source for the problem remains unknown. Critical to solving the problems of anodizing die castings is understanding the die cast substrate and the impact of surface condition, alloy composition, casting quality and microstructure on the anodizing process. Substrate quality issues are just as important, maybe more so, than anodizing conditions and technique. Certain optimum anodizing conditions may be used in some cases to help overcome less than advantageous metallurgical conditions. These include well known processing tools such as various pretreatment chemistries, higher anodizing bath concentration, and higher bath temperatures. These, and other recommended solutions are not successful in every case; sometimes trial and error testing on actual production parts must be done to find the best processing techniques. Through the use of actual case studies that provide real-life solutions in terms of anodizing theory and interfacial science, this paper provides some explanations by tying together metallurgical science with anodizing practice.
Review of several questions received in Products Finishing Magazine over the years 2001 - 20091-8 determined generally consistent problems with anodizing die castings. The problems can be summarized as follows: 1) the anodic oxide finish is too thin to meet the necessary design specification or much thinner when compared to corresponding components manufactured from wrought processes; 2) the finish appearance is unfavorable: hazy, muddy, “not black enough”, and/or not uniform; and, 3) the corrosion resistance of the anodic oxide is insufficient. Review of recommended anodizing solutions to the various problems with anodizing cast alloys determined that they don’t always work, indicating that there are factors other than the anodizing process that impact the anodic oxide finish quality.
Consideration given to solve these rather easy-to-identify problems has illuminated four broad cause areas for discussion: 1) alloy selection, 2) substrate quality, 3) surface treatment, and 4) anodizing process parameters. Of the four, perhaps the metallurgical factors that impact substrate quality: alloy composition, casting quality, microstructure and surface quality are dominant in determining anodizing conditions, technique and quality.
This paper covers each of the four cause areas by discussing, in limited detail, the impact each has on the interface from which the anodic oxide originates and grows. Theoretical scientific reasons for why the problems occur and why most solutions succeed and some fail are presented. The limitations as to what can be done from an anodizing standpoint to overcome the metallurgical condition of a cast substrate are presented not as an excuse, but as a call for understanding and communication between metal finishers, component designers who would like to use die castings, and the casting houses who provide the castings in order to optimize product and process and to increase the use of anodized cast aluminum components.
Aluminum die castings have been commercially available since the beginning of the 20th century. Castings are used for a variety of applications, from decorative sculptures and jewelry to automotive pistons and engine blocks.
Die casting is a versatile process for producing engineered metal parts by forcing molten metal under high pressure into reusable steel molds. These molds, called dies, can be designed to produce complex shapes with a high degree of accuracy and repeatability. Parts can be sharply defined, with smooth or textured surfaces, and are suitable for a wide variety of attractive and serviceable finishes.9
First and foremost, cast alloys are formulated for castability, followed by mechanical properties and structural integrity. Alloys are formulated for maximum fluidity, minimum gassing when molten, and ability to remove the casting from the mold when solid. Mechanical properties are insured first by the integrity of the casting, therefore, the casting alloy and process are designed to minimize porosity and maximize surface integrity. Actual mechanical properties of strength, hardness, and resistance to wear and fatigue are produced metallurgically in aluminum castings two ways: (1) by solid solution hardening; that is: by the substitution of aluminum atoms with alloying atoms in the aluminum crystal structure and (2) by precipitation hardening: the dispersion of second phase constituents or elements in solution and precipitating them out as small intermetallic compounds, incoherent with the microstructure, which inhibit material deformation.
Cast components have limited ductility and can be brittle; therefore, castings are not usually meant for subsequent deformation processing. Other than minimal finishing processes such as machining, a casting is typically produced to function near net shape. Rarely, if ever, is a cast alloy designed for subsequent surface treatment, especially anodizing, because, alloying elements do not anodize.
Because cast components are produced to function near net shape, castings can be alloyed beyond what is typical for wrought products; that is, additions of other elements are at a higher per cent than the additions for alloys intended for extruded, rolled or deep drawn product (up to 16% total alloy content for castings vs. up to 8% for wrought alloys). As such, cast alloys are metallurgically more complex than their wrought counterparts; increased alloy additions produce correspondingly higher levels of solution phases, intermetallic compounds and precipitates. Castings, therefore, in addition to their strength and fatigue resistance, exhibit more complex surfaces, with less free aluminum, which make them more difficult to anodize.
Cast Alloy Designations
Table I presents the compositions of some commonly specified aluminum die cast alloys according to their American, European and Japanese designations10. The alloy chemistry and the casting process affect the level of microstructural homogeneity, the defect population and therefore the variation of chemical potential across a cast component surface. It is important to understand the nature of the surface and therefore the interface between the component surface and the anodizing electrolyte such that anodizing process parameters can be modified to effect optimum oxide growth.
Silicon is the alloying element that essentially makes the commercial viability of the high volume aluminum casting industry possible. Silicon content between 4% to the eutectic level of 12% reduces scrap losses, permits production of much more intricate designs with greater variations in section thickness and yield castings with higher surface and internal quality (forms sound outer surface layer), because of this, aluminum-silicon cast alloys can be used in applications where pressure tightness is a requirement. These benefits derive from the effects of silicon and aluminum molten mixtures which exhibit increased fluidity, reduced cracking and improved feeding to minimize shrinkage porosity. Alloys with the eutectic composition (Al-12%Si) exhibit highest fluidity during casting.
Copper, Magnesium and Zinc are the most important secondary alloying elements which impart fluidity during casting and various phases which impart mechanical properties such as strength, corrosion resistance and fatigue resistance. Alloying elements have varying characteristic solubility in aluminum, and how they mix, stay in solution or precipitate as intermetallic compounds impact the cast microstructure. All die cast components contain some level of both phenomena, to varying levels depending upon alloy chemistry, casting process and any post-cast heat treatment (whether accidental or actually imposed).
Table 1: Compositions of Some Commonly Specified Aluminum Die Cast Alloys*