July 8, 2008
10 Presentations That Can Help You Now
CastExpo'08 featured 66 worthy technical presentations. The following is a look at 10 that offer information immediately useful to metalcasters. 1 Cleaner Aluminum Melts in Foundries
2 Aging Effect on Gist Iron Machinability
3 Reducing Gas From Cores in Magnesium Melts
4 Desirable Sand Properties With Aeration Filling
5 Effect of Ti and N on Iron Age Strengthening
6 Improving B206 Aluminum-Copper Lost Foam Casting
7 Modification of Ai-Si Casting Alloys
8 Optimizing Strontium Additions in Aluminum
9 New Method of Waste Green Sand Reclamation
10 Reducing Hot-Tears in Mag Permanent Mold
The Practical Ten
Technical presentations walk a fine line between pure and practical. Pure research is intended to advance the science behind metalcasting, but often doesn't have an immediate impact on the industry. Practical research, on the other hand can affect the way metalcasters operate right here and now, making plants more efficient and profitable.
Following is a look at several presentations given at CastExpo'08 that offered practical takeaways for the metalcasters in the audience.
Aluminum in Flux
1 In a study that gathered data from a variety of sources over four years, researchers determined that the rotary flux injection process of aluminum treatment produced cleaner metal than other processes commonly used in metalcasting facilities.
Presenting his paper "Cleaner Aluminum Melts in Foundries: A Critical Review and Update" (08-094), Rafael Gallo, Foseco Metallurgical Inc., Cleveland, said that the rotary flux injection process was most efficient at removing inclusions from the aluminum melt while at the same time producing dross with the least amount of aluminum. However, Gallo did not advocate the use of the process for all metalcasting facilities.
"It is up to each aluminum [metalcasting facility] to define the molten metal cleanliness level they can live with," he said. "Each has a choice between implementing discipline to use rotary flux injection and using a different technology for metal cleaning."
Through his experimental inquiry, Gallo also examined several different rotor and flux configurations, each of which offered different benefits and drawbacks.
Age Takes Effect
2 Previous research has shown that natural iron nitride precipitation over a short period of time at room or elevated temperatures induces elevated mechanical properties in cast iron. New research took this discovery a step further and has shown that it also improves its machinability.
Following up on previous studies, Jared Teague, Von Richards, Simon Lekakh and Kent Peaslee, Missouri Univ. of Science and Technology, Rolla, Mo., further explored the results of natural age strengthening of cast iron in their presentation, "Aging Effect on Cast Iron Machinability" (08-134). By using CNC tools to machine cast iron before and after a 10-day natural aging period, the researchers determined the differences in required force between the control and test groups. The aged samples required significantly less force to achieve the desired shape. Repeated industrial testing revealed that tool wear also was lessened when machining the aged samples.
Removing Magnesium's Gas
3 A water-based insulating coating and a dash of additive could make magnesium castings less gassy and susceptible to blow defects, according to researchers.
Leonard Winardi, Preston Scarber Jr. and Robin Griffin, Univ. of Alabama, Birmingham, Ala., and Dave Weiss, Eck Industries, Manitowoc, Wis., dipped a variety of cores into baths of molten aluminum and magnesium and presented the results in a session titled "Comparison of Gas Evolution Results From Chemically Bonded Cores in Contact With Magnesium and Aluminum Melts" (08-048). After confirming magnesium is more prone to producing enough gas to create a blow defect in metal castings-the metal is more reactive in the presence of oxygen than aluminum-the researchers set about reducing that production.
Because the oxygen that reacts with magnesium is found in the cores themselves, the researchers were able to limit the oxidation effects by adding 1% potassium fluoroborate to certain sand mixtures. In a core with 0.9% PUCB resin, the additive reduced the gas evolution rate by 25% and volume by 6%.
Aeration Filled Green Sand
4 A new series of tests have shown that a recently developed aeration method of filling flaskless green sand molding systems isn't full of hot air.
For green sand metalcasters utilizing flask-free molding lines, aeration allows complex shapes to be evenly filled by fluidizing the molding material before blowing it onto a matchplate at a pressure of approximately 0.1 MPa. However, before Sam Ramrattan and Ananda Paudel, Western Michigan Univ., Kalamazoo, Mich., and Hiroyasu Makino and Minoru Hirata, Sintokogio Ltd., Aichi, Japan, put the method through production testing, not much was known about the quality of the molds created in the process.
The results of the researchers' study, released in their presentation, "Desirable Green Sand Properties via Aeration Sand Filling" (08-128), indicated that castable molds could be produced with compactability between 30 and 40%. When held in that range, the molds showed increased resistance to molten metal erosion with strength and hardness equal to that of molds produced in more traditional methods. The tests further showed that silica sands of nearly all levels of grain fineness could be used to make quality molds in that compactability range.
Mitrogen, Titanium Face Off on Aging
5 Nitrogen in cast gray iron makes the material age quickly, but titanium can make it act its age, according to researchers from the Missouri Univ. of Science and Technology, Rolla, Mo.
Drawing on past research that indicated cast iron goes through a natural aging process in the initial few days after production-its tensile strength increasing while the material is held at or above room temperature-Thottathil Anish, Simon Lekakh and Von Richards dissected the causes of iron aging. In their studies, gray iron with plenty of nitrogen and little titanium showed marked tensile strength increases over 30 days of natural aging. Samples that were treated with titanium additions, however, showed less strengthening.
"The critical concentration of titanium necessary to eliminate age strengthening depends on nitrogen content," the researchers said in their presentation, "Effect of Ti and N on Iron Age Strengthening" (08-063). "Reduction of the iron nitride formation is the main reason for suppression of aging in iron micro-alloyed by titanium."
New Alloy for Autos
6 B206 aluminum-copper has mechanical properties superior to those of aluminum-silicon, and a study on its use in lost foam casting could get the material revved up for automotive applications.
Aluminum-silicon alloys hold the pole position in automotive lost foam casting, with many of the existing production shops using the material. But Nilesh Dahata, Comondore Ravindran, Centre for Near- Net-Shape Processing of Materials, Toronto, Canada, Fawzy Samuel, Univ. of Quebec at Chicoutimi, Chicoutimi, Quebec, Canada, and Thomas Gustafson, GM Powertrain, Pontiac, Mich., wanted to optimize the lost foam process for aluminum-copper. To do so, they tested three variables in "A Study on Fillability, Microstructure and Porosity in Lost Foam Casting of B206 Aluminum-Copper Alloy" (08- 023): copper vs. titanium levels, types of expanded polystyrene beads and type of coatings. Their work concluded that titanium levels at 0.25% of the metal weight reduced porosity, and T175 EPS beads with a mica-base, high-permeability, high-absorption coating worked best with the alloy.
Bettering Aluminum Silicon
7/8 Aluminum-silicon metalcasters have to strike a balance when modifying their alloy with strontium for increased ductility; use too much, and porosity is likely to develop.
But after an extensive study tided "Modification of Al-Si Casting Alloys: Important Practical and Theoretical Aspects" (08-019), Geoffrey Sigworth, Alcoa Primary Metals, Rockdale, Texas, offered the following practical tips that can help casters modify for improved microstructure without inducing scrap:
1. Use longer degassing times and lower metal temperatures to limit the amount of gas in your molten metal.
2. Insert chills in sand castings and conductive die coatings in permanent molds to induce rapid solidification, which helps reduce porosity.
3. When designing a mold for a modified aluminum-silicon alloy, keep sources of heat away from critical areas where porosity would be particularly detrimental.
4. Filter the metal to remove inclusions, which can give gas pores a place to nucleate during solidification. According to Sigworth, for modification of A356 aluminum, the best strontium additions occur at 0.012%, or 120 ppm. "In practice, however, good modification can be obtained at significantly lower concentrations, depending on the purity of the alloy," he said.
In a separate study, researchers from the Institute for Technological Research-IPT, Sao Paulo, Brazil, determined that strontium is best used to modify A356 aluminum at levels between 0.005% and 0.0075%. Tarita Furlan and Ricardo Fuoco said in a presentation titled, "Optimization of the Sr Addition in Permanent Mold A356 Alloys" (08-142), that "overmodification was found to occur before the formation of a third strontium-rich phase." Saving More Sands
9 A new method of reclaiming used metalcasting sand could keep molding material in your cores for longer periods of time.
In "Waste Green Sand to Core Sand Reclamation, Demonstration via Casting Study With Simultaneous Clay Recovery via a Novel Ultrasonic Cavitation System" (08-140) John Fox, Fred Cannon and Robert Voigt, Pennsylvania State Univ., University Park, Pa., James Furness, Furness Newburge Inc., Versailles, Ky., Frank Headington, David Coan and Steve Lewallen, Neenah Foundry, Neenah, Wis., described the effects of using cavitation, advanced oxidants, sonication and settling to segregate silica sand from clay, coal and carbonaceous products. Their combination of previously known methods for reclaiming sands yielded quality cores containing 40-100% reclaimed sand.
To test the adequacy of the cores, the researchers used them to produce castings identical to castings produced with cores of 100% new sand. The metal components were then inspected by experts unaware of the production method used on each group. The inspectors were unable to determine the difference between the groups and deemed the castings produced with reclaimed sand defect free.
Improving Mag Perm Mold Casting
10 Blazing hot dies can alleviate hot tears when casting magnesium in permanent molds, according to researchers from the Centre for Near-Net-Shape Processing of Materials, Toronto, Canada.
In "Reduction of Hot-Tears in Permanent Mold Casting of AZ91D Magnesium Alloy. Effect of Mold Temperature" (08-109), Lukas Bichler, Francesco D'Elia and Comondore Ravindran said that mold temperatures above 644F (340C) were particularly effective at stopping the common casting defect. At that temperature, the molds induce a slow rate of solidification that contributes to the lack of tearing.
The researchers' experimental trials showed that 428F (220C) molds induced a critical casting cooling rate where "healing of incipient hot tears became possible." Elevating the molds above that temperature resulted in a decreasing number of hot tears.
Geoffrey Sigworth offered a presentation at CastExpo'08 on how to modify alloys with strontium for increased ductility without inducing porosity.
Copyright American Foundrymen's Society Jun 2008
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