Staying ahead of the curve with a network: five AFS development projects that utilized a sponsor to exchange technical solutions and develop cost savings ideas are profiled.
It has been proven that operations that utilize these strategies outperformed the industry at-large as measured by profit margin both as a percentage of sales and sales per employee. Five recently completed AFS projects that harnessed this approach are highlighted below. The research discussed is funded by AFS, the Defense Logistics Agency (DLA) through the American Metalcasting Consortium (AMC), the U.S. Dept. of Energy Industrial Technology Program (DOE ITP) through the Cast Metals Coalition (CMC) and the U.S. Council for Automotive Research (USCAR).
Gray Iron Age Strengthening
Background--The occurrence of age-strengthening in gray iron is believed to depend on chemistry and melting practice. Controlling the aging process gives metalcasters another tool to optimize the strength or the yield of cast iron. Understanding and controlling this phenomenon may enable the meeting of SAE standard J431 (tensile to-Brinell ratio) without significant changes in metal chemistry.
Goal--The primary objective of this research was to identify the age strengthening mechanism in gray and ductile iron and to quantify the con trolling parameters in order to develop a predictive model for gray and ductile iron strength and hardness.
Approach--Gray and ductile iron production castings by sponsor foundries were used as a cost share with the DOE-ITP "Cast Metals Industries of the Future." The aging process is characterized to provide a basis to develop a physics-based model to understand and control the aging of gray and ductile irons. The results include analyzing the evolution of strength and hardness due to aging and the impact of aging on machinability.
Results--To a statistically significant extent, age strengthening occurs on a logarithmic scale of time for both gray and ductile cast iron. Most of the improvement occurs in 6-10 days rather than the 30 days previously assumed in some specifications. This finding has reduced inventory in casting and machining facilities. It is possible to accelerate the aging by modestly raising the temperature. A correlation exists between machinability and aged iron. The increased strength has allowed some metalcasting facilities to move their compositions to higher carbon equivalent and improve casting yield. The age hardening phenomenon occurs in both cupola-melted iron and induction-melted iron. However, it does not happen in all compositions. Nitrogen content should be monitored periodically for cast irons that exhibit aging. The effects of age strengthening on machinability of this alloy system remains to be determined.
Natural Aging Alloys
Background--The 7xx aluminum casting alloys are based on the ternary aluminum-zinc-magnesium system. These alloys age naturally to high strength at room temperature without a high temperature solution and aging treatment. Consequently, these alloys have the potential to deliver properties nearly equivalent to conventional A356-T6 (Al Si-Mg) castings, with a cost saving of $0.20/lb. In spite of their excellent properties, the 7xx casting alloys are seldom used because of their propensity for hot cracking.
Goal--A new method to grain re fine these alloys was developed to significantly reduce this problem. Besides the hot cracking issue, several other technological barriers exist that make it difficult to use natural aging Al-Zn-Mg alloys commercially.
Approach--A DOE-ITP "Cast Metals Industries of the Future" program with AFS-sponsored plant trials evaluated hot cracking for several M-Zn-Mg alloys that were given different grain refinement treatments. The mechanical properties also were measured. These alloys present a number of interesting and potentially valuable idiosyncrasies. The most important is that mechanical properties are relatively insensitive to cooling rate.
Results--The 705 alloy appears to be a good candidate for permanent mold casting, but the other 7xx alloy compositions are not suitable. The 705, 706 and 707 alloys appear to be good candidates for sand and lost foam casting. They offer improved castability over the traditionally-used 7xx alloys, like 772. Careful casting and mold design is necessary for crack-free parts but even more important is good feeding. It is likely that the high-pressure lost foam casting process would be especially suitable for the 7xx alloys. Casting processes that naturally produce high thermal gradients, like some semi-permanent mold processes, also are good candidates for these alloys. However, good melt treatment and runner design is needed because the high magnesium contents in the 7xx alloys make them susceptible to oxidation.
The mechanical properties of natural aging alloys are relatively insensitive to cooling rate, making 7xx alloys good candidates for sand, lost foam or investment casting. The reasons for this behavior need to be understood before this phenomenon can be applied to large, complex engineered components for metalcasting.
Grain Refinement of Permanent Mold Cast Copper Base Alloys
Background--Grain refinement is a well-established process (especially in aluminum) to enhance casting characteristics and improve mechanical properties. Before grain refinement is used to improve permanent mold hot tearing resistance of copper alloys, various aspects (e.g., effectiveness of various refiners, loss of grain refinement due to overheating, prolonged holding and remelting) must be understood.
Goal--This project investigated the role of processing variables on the grain refinement of four permanent mold cast copper alloys--yellow brass (C85800), EnviroBrass III (C89550), silicon brass (C87500) and silicon bronze (C87600)--and developed thermal analysis techniques to control grain refinement, fading and hard spot formation.
Approach--An evaluation of the effectiveness of various commercial grain refiners containing boron or zirconium was conducted. The work was completed with DOE-ITP "Cast Metals Industries of the Future" program support in laboratory and AFS-sponsored plant trials. The effect of grain refinement with various alloying elements such as iron, tin and silicon on the hard spot formation also was done. The corrosion behavior of these alloys was then characterized.
Results--The thermal analysis can be used to not only detect the magnitude of expected grain refinement but also fading. The presence of any undercooling will result in a relatively large grain size. Some typical results are shown in Table 1. The grain size is comparable to CANMET/ MTL standards, which consist of eight grain sizes ranging from 1C (0.118 in.) to 8C (0.039 in.).
Instead of expensive analytical equipment to analyze boron content, information from inexpensive thermal analysis is available within a minute.
Improved Processing of Austenitic Manganese Steels
Background--Manganese steels that have a high toughness could still have thin carbide grain boundary delineations. However, if these carbides grow in thickness, they could cause embrittlement. It is desirable to predict the effectiveness of heat treatment from microstructures across section sizes by understanding the microstructure-impact property relationship in thick sections. This includes investigating microsegregation associated with industrial practices when high phosphide levels are present.
Goal--The heterogeneous distribution and variety of embrittling phases makes it difficult to verify if heat-treatment objectives are achieved in austenitic manganese steels by microstructure interpretation. The primary goal is to improve understanding of carbide, micro-porosity and phosphide eutectic distribution to improve the prediction of mechanical properties and to develop insights into phosphide eutectic embrittlement and its recovery.
Approach The AFS Steel Div. (9) solicited AFS research funds to quantifying macro and microsegregation for different section sizes and their effect on embrittlement from industrial practices.
Results--A procedure to estimate in, pact toughness from metallographic observations was developed along with quench time determination from quench water temperature measurements. It was determined that only the "thick" carbide films are harmful for impact toughness. They are distinguishable from the "thin" carbides by the appearance of austenite-cementite phase boundary on both sides of the carbide film. Macrosegregation is not a factor for embrittlement in thick sections but microsegregation increases with section thickness. Phosphide eutectic embrittlement is reversible with heat treatment.
Grain Refining Magnesium Alloys
Background--The inherent difficulty producing consistent, high-quality sand and permanent mold magnesium castings has limited market penetration of these products in the automotive industry. Significant breakthroughs could develop if the same quality and property enhancements through effective grain refining for aluminum alloys are duplicated in magnesium alloys.
Goal The purpose of the study was to evaluate several potential grain refiners including Sr, [A1.sub.4][C.sub.3], AIN, SiC, TiC, Sc, [C.sub.2][Cl.sub.6] and Ca[C.sub.2]. Hot tearing behavior is evaluated through tensile bar molds and through sand-cast step block molds. Tensile property behavior is surveyed for each grain refiner in the as-cast (F), natural age (T4) and peak aged (T6) condition.
Approach--Financial support from AFS was used by the AFS Magnesium Div. (6) to develop the best means of grain refining magnesium alloys AZ91E and AM60. Grain size was determined in castings before and after grain refining for the permanent mold and sand casting for eight grain refiners at two levels.
Results--The inoculants were successful to different degrees with variations in properties determined by the holding temperature and holding time. The most successful of the grain refinements was the addition of 0.5% by weight of [C.sub.2][Cl.sub.6] with some tendency for this inoculant to fade with low holding temperature. The addition of strontium was effective for long holding times and may well be desirable for heavy section magnesium castings. Grain refinement of AZ91E and AM50A, especially with hexachloroethane, provided more resistance to hot tearing.
Aging Iron Conclusions
* Statistically significant age strengthening of gray and ductile iron does occur.
* Brinell hardness also increases with age strengthening.
* Industrial machinability improves at the same rate as age strengthening.
* Holding for short times (1-10 hours) at modestly elevated temperatures (83-250C) accelerates the age strengthening process.
* Age strengthening increases with the amount of nitrogen in ferrite as a solid solution.
7xx Composition Recommended Practices
* The upper limit for magnesium in 705 alloy should be increased with some benefit.
* The minimum now established for manganese (0,4%) should be eliminated for both 705 and 707 alloys because lower manganese content improves feedability, hot crack resistance and elongation.
* Chromium in the 7xx alloys improves strength and reduces quench sensitivity There also is evidence from the literature that chromium improves stress corrosion resistance. Thus, the 0.2-0.4% range for chromium in 705 and 707 alloys appears to be well founded.
* Grain refinement is accomplished best by an addition of Ti-B grain refiner a few minutes to half an hour before casting. The recommended addition level is 10-20 ppm of boron
* A residual (dissolved) titanium level of 0.02-0.04% is recommended for best grain refinement.
Table 1. Solidification behavior and grain size ratings of a Cu-Zn melt adding alloying elements. Alloy Liquidus(C) Solidus(C) Cu-35%Zn 908 863 Cu-35%Zn-0.3%Sn 904 858 Cu-35%Zn-0.3%Sn-0.3%Al 906 875 Cu-35%Zn-0.3% Sn-0.3% 901 842 Al-1%Pb (C85800) Cu-35%Zn-0.3%Sn-0.3% 902 835 Al-1%Pb, 35 ppm B Cu-35%Zn-0.3%Sn-0.3% 902 841 Al-1%Pb, 35 ppm B, 300 ppm Fe Solidification Grain Time Size Alloy (in sec.) Rating Cu-35%Zn 62.31 <1C Cu-35%Zn-0.3%Sn 41.21 <1C Cu-35%Zn-0.3%Sn-0.3%Al 45.82 <1C Cu-35%Zn-0.3% Sn-0.3% 67.58 2.5C Al-1%Pb (C85800) Cu-35%Zn-0.3%Sn-0.3% 65.60 4C Al-1%Pb, 35 ppm B Cu-35%Zn-0.3%Sn-0.3% 48.47 7C Al-1%Pb, 35 ppm B, 300 ppm Fe
For More Information
"The Influence of Potential Grain Refiners on Magnesium Foundry Alloys," J.F. Wallace, D. Schwam and Y. Zhu, 2003 AFS Transactions, paper No. 03-142.
"Age Strengthening of Pray Cast Iron: Kinetics, Mechanical Property Effects," V.L. Richards and D.C. Van Aken, 2003 AFS Transactions, paper No. 03-037.
A comprehensive list of on-going developmental projects can be found at www.afslibrary.com
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|Date:||Mar 1, 2004|
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