Understanding inclusions in aluminum castings.
The growing use of aluminum for castings over the past decade has brought with it the increased scrutiny of component properties. One area that has received much attention is the effects of inclusions--or impure particles held in the metal--on casting properties.
Inclusions in the melt can lead to various problems, ranging from the reduction of mechanical properties, excessive tool wear, increased porosity, poor surface finish and lack of pressure tightness.
Foundrymen have traditionally been most concerned with quality detractors such as hydrogen and volumetric shrinkage porosity, misruns and dimensional issues, according to C. Edward Eckert, Metallurgical Products and Technologies.
"During the last 15 years, however, metal cleanliness--specifically inclusions--emerged as the preeminent quality detractor, particularly for premium and commercial grade castings in which high strength to weight ratios, machined surface finish, fracture toughness and low cycle fatigue performance are primary design considerations," he said.
Diran Apelian, Worcester Polytechnic Institute, echoed the growing pressure to solve inclusion problems. "Inclusion removal from the melt is a critical step in producing components with added value and high performance," he said.
Reflecting this concern, inclusion "problem-solving" was one of the primary topics examined at the AFS International Conference on Molten Aluminum Processing held November 9-10 in Orlando, Florida.
In most cast components, inclusions with particle sizes greater than 10-20 |mu~, can have a drastic effect on the quality of the casting. Even if the volume fraction of inclusions is small, Apelian said, an enormous number of inclusions may be present in the melt. If the average inclusion size is about 40 |mu~, then at an inclusion concentration of 1 ppm, 1 lb of metal will contain about 5000 particles. Thus, from a statistical point of view, the number of inclusions in the melt is significant.
"Our customers are demanding castings that are inclusion-free, yet it is ironic and perplexing that we as an industry believe that we can achieve such an outcome," Apelian said. "It's impossible to control a processing step if you can't define and measure it."
The assessment of metal cleanliness is an integral part of any casting operation. The industry desperately needs an evaluation test on the factory floor (on an on-line basis), yielding information to the operator and allowing him/her to make adjustments to the molten metal stream prior to the casting.
Because of the relatively low concentrations of inclusions present in the metal, an estimate of inclusion level is a complicated task. And because of the nonuniform distribution in particle size, additional complexities are introduced. Several qualitative and semi-quantitative tests are being used by aluminum metalcasters to estimate and control TABULAR DATA OMITTED the inclusion concentrations in the end product. Apelian classified assessment techniques into five categories:
Chemical analysis--These techniques, based on wet chemical or instrumental analysis, haven't been successful primarily because of the nonuniform distribution of inclusions in samples.
Quantitative metallography--This method uses computers and image analysis to differentiate between 100 shades of gray. Although this technique is now automated, its major shortcoming is sampling bias and not knowing whether the sample being analyzed is representative of the melt.
Volumetric tests--These tests differ from the previous two in that they don't depend on sampling methods and sectioning techniques. These include electron beam melting, centrifugal separation and filtration.
Electron beam melting involves drip melting of a sample under a vacuum using an electron beam and collecting the molten aluminum in a hemispherical, water-cooled mold. During melting and solidification, the nonmetallic particles rise to the surface and are subsequently concentrated in a central floating raft, where they can be removed chemically for quantitative analysis or observed using SEM.
Centrifugal separation heats about 100 g of aluminum to 1300-1380F and transfers it to a well-insulated, closed crucible. During centrifuging at high speeds, the inclusions with a higher density are sedimented to the bottom of the crucible.
Filtration is another common approach to preconcentration of inclusions for analysis. After melting, samples are passed through filters. The residue on the filter is analyzed to obtain quantitative information on inclusion concentrations.
Nondestructive techniques--Two prominent techniques include the ultrasonic test and the liquid metal cleanliness analyzer (LIMCA). The ultrasonic tests transmit an ultrasonic wave through the metal sample. Inclusions are identified through sound damping. The LIMCA establishes an electrical sensing zone through which inclusions pass for counting and sizing.
Shop floor tests--Straube-Pfifer vacuum tests are used to obtain qualitative estimations of the inclusion level. Visual observations during solidification of the test sample provide estimates of the inclusion concentration. Cross sections and fracture surfaces of test castings also provide ways to judge melt cleanliness.
Practical Inclusion Control
"Proper metal handling and treating practices are capable of separating oxides, carbides and salts from the metal stream before they become inclusion manifestations in castings," Eckert said.
Inclusions generally fall into two classes, exogenous and insitu. Table 1 includes observations on these inclusions and where they come from.
Exogenous inclusions are imported to the molten metal stream from external sources, such as occluded particles on and within primary and secondary ingot, major alloying elements and master alloys, as well as containment refractories.
The more prevalent inclusions are insitu, which form as a result of a chemical reaction within the metal handling system. Insitu inclusions are formed by oxidation, aluminum carbide and halide salts. These formation mechanisms, however, can be avoided by paying attention to practical rules.
Oxide particles and films are the most common sources of inclusion defects in the industry. Although aluminum oxidation is practically unavoidable, the character of the oxide formed is determined by the prevailing oxidation mechanism.
* Because increasing the melt temperature also increases the oxidation rate exponentially, a "rule of thumb" states that for every 10-12C increase in melt temperature, the rate of oxidation will double.
* Direct flame impingement on the aluminum melt develops high melt surface temperatures, while water vapor increases the potential of oxygen over the melt and induced metal turbulence enhances surface renewal. Radiant heating panels, immersion heaters and induction heating are desirable alternatives to direct flame impingement.
* Turbulence creates a high melt surface area-to-volume ratio and also intimately mixes the surface oxide films with metal to become inclusions.
* Reactive elements in pure form, such as zinc and magnesium, must be carefully added to minimize the impact of oxidation on metal quality.
* Where possible, beryllium additions to aluminum-magnesium and aluminum-lithium alloys significantly reduce the formation of oxides.
* Although aluminum oxidation cannot be prevented, the oxidation rate can be reduced through protective cover gases, such as dry nitrogen, argon and carbon dioxide, over the melt.
Avoiding Aluminum Carbide
The principal sources of aluminum carbide inclusions are primarily ingot, and the reaction of paints, oils and other carbon-containing compounds with aluminum. Higher purity grades of aluminum are more likely to form aluminum carbide from reactants than highly alloyed materials.
To reduce the likelihood of inclusions formed from aluminum carbide, several issues should be kept in mind:
* Scrap containing oils and paints should be processed off-line prior to furnace charging.
* Higher temperatures cause melt contact with carbon sources due to improved wetting. Carbon and graphite oxidation can also result, causing porous surfaces and particulate carbon, which promote reaction with aluminum.
* Molten halide salts, such as magnesium chloride and eutectic mixtures thereof, promote contact and ion exchange with carbon sources. Chlorine in flux should be minimized to under 10 vol.% of total fluxing gas. Efficient phase contactors for chlorine fluxing operations are preferred over open-ended wands.
* If hexachlorethane is employed as degassing tablets, a quiescent settling period of usually 15-30 minutes permits particles to separate. Careful metal withdraw is also desirable.
Avoiding Halide Salts
Halide salts constitute a class of hydrogen-containing compounds that may be generated when halides react with aluminum. The dominant halogen source is chlorine, used for fluxing purposes, which is unparalleled in its value as a reactive gas for inclusion removal, skim control and in some cases, degassing. The potential exists, however, to form molten and solid chloride salts that are particularly difficult to remove from the melt.
Halide salt problems can be controlled by following these tips:
* Avoid using excessive chlorine for the fluxing of magnesium containing alloys. Generally, less than 10 vol.% and preferably 3-5 vol.% chlorine are appropriate for enhancing inclusion floatation and providing an acceptably dry skim.
* Maintain the minimal practical melt superheat to discourage refractory wetting and chemical deterioration by molten halide salts. Wetting characteristics and reactivity of molten halides increase significantly above 1325-1350F.
* If casting alloys are produced from high scrap or secondary ingot, residual sodium, calcium, potassium and lithium concentrations should be maintained at the lowest economical levels (if chlorination is used). Sodium additions, for the purpose of modification, should be made after the chlorination process. Also, salt carryover from certain recycling operations and "bath" carryover from low-grade primary ingot can represent an exogenous inclusion type that is imported into the system.
"Practical inclusion control is best facilitated through prudent selection of capable melting, metal handling and treatment processes," Eckert said. "The careful control of the meaningful process variables of these operations generally results in acceptably clean metal."
Removal by Floatation
Molten aluminum has a very high surface energy and low density, making it difficult for inclusions to separate from the liquid metal. Three methods are used to remove inclusions from molten aluminum: sedimentation, floatation and filtration. The production of high-quality aluminum requires the use of all three methods.
"The removal of inclusions from molten aluminum by floatation is a commercially accepted technique for improving metal cleanness," said Luiz Martins, RPT Technologies, Inc.
The rotary impeller degasser, used initially to remove hydrogen from liquid aluminum, has enabled foundrymen to enhance the floatation process.
Lab experiments show the correlation between the presence of halogens and the production of a dry dross lead to good separation of inclusions from the liquid metal. The use of halogens can also improve melt cleanness substantially.
Martins said dross can indicate whether floatation is properly removing inclusions. If the dross is dry, floatation is working properly. He also said aluminum nitride may play a role in the wetting of inclusions by molten aluminum. In addition, wet dross indicates a lack of halogens, caused by not adding them or by poor absorption due to slow kinetics.
"The removal of inclusions from molten aluminum is essential for the generation of high-quality aluminum products," Martins said.