Optimizing Aluminum’s Tensile Properties
A new test bar casting method for aluminum alloys yields more accurate data.
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Aluminum alloys are used widely to replace cast steel, especially in the automobile industry. Because they are around one-third the density of steel and have less strength, improving their mechanical properties is an important task.
Prior research has demonstrated how to strengthen aluminum alloys with chemical modification, heat treatment and reduced porosity. The presence of microporosity is well known to have a key effect on the mechanical properties of A356 and 319 alloys. Degassing treatments for A356 and 319 alloy melts generally are required to reduce hydrogen content, minimizing porosity in the casting. Hot isostatic pressing (HIP) also can reduce microporosity and increase mechanical properties.
The Stahl mold permanent test bar is used widely to evaluate melt quality, but the test bar itself has porosity that makes the evaluation imprecise. In the Stahl mold design, the sprue is narrow to control the flow into the mold and reduce turbulence. The feeder and test bar sections are designed to obtain good filling and reduce shrinkage porosity. However, metalcasting practice indicates the Stahl mold still has difficulty in producing the best mechanical properties, due to microporosity.
Chia-Jung Chen, David Schwam and David Neff, Case Western Reserve University, Cleveland, Ohio, conducted a study on cast test bars for these alloys.
Can improved gating for aluminum alloy test bars cast separately using the permanent mold process provide better feeding and change the solidification path to prevent microporosity?
Mechanical properties derived from separately cast test bars do not replicate the properties in a full casting except, perhaps, for those sections of a casting with similar secondary dendrite arm spacing (SDAS). But metalcasters can derive the potential for near-optimum properties by pouring test bars from their melt. The mechanical properties in the test bar will be a reasonable measure of the melt quality, especially with regard to the presence or absence of impurities—inclusions, porosity or microporosity—which will exert a strong influence on the resultant mechanical properties.
A modification was proposed to improve mechanical properties by applying a thin knife ingate, named for its knife-blade shape, between the feeder and test bar to improve filling capability and reduce microporosity attributed to shrinkage. Computer simulation predicted its effect on the solidification pattern and microstructures of test bar castings. A new mold was fabricated based on the modified design, and its performance was compared to standard Stahl mold cast A356 aluminum test bars.
Prior research found the Stahl mold could not produce the same high mechanical properties as the Aluminum Association step mold. The specific reason was microporosity due to shrinkage in the gage section of the Stahl mold. But, the AA step mold also is not porosity free, especially in the 2 and 3-inch sections.
A study on the effect of hot isostatic pressing (HIP) on the microstructure and tensile properties of A356-T6 cast aluminum alloy cast into a plate and machined to a test bar according to ASTM E8M found the solution treatment was 1,000F (538C) for 5 hours; artificial aging was 320F (160C) for 4 hours. The authors compared different SDAS with ultimate tensile strength (UTS) and elongation, showing if SDAS is lower than 80 µm, the HIP sample has higher elongation then the non-HIP sample at the same UTS level.
In the current research, A356 and 319 alloys were cast to determine further beneficial effects of microstructural enhancements, i.e., grain refining, modification, SDAS with melt fluxing and degassing. Several separately cast test bar molds from a commercial metalcaster as well as laboratory melts were evaluated for resultant mechanical properties, particularly in the heat treated condition.
The Case Western Reserve University (CWRU) foundry melted A356 alloy in an electric resistance melting furnace. Virgin and recycled (10% clean) alloy was used and the melt temperature was held at 1,300, 1,350 and 1,450F (704, 732 and 788C).
In the CWRU Foundry study, virgin versus recycled metal was evaluated. Degassing was performed by bubbling argon through a degassing unit for 30 minutes. A continuous hydrogen measurement system controlled the hydrogen level to 0.1 ml/100g Al and mold pours were performed at this level. In addition, the reduced pressure test (RPT) was employed to ascertain qualitative hydrogen, i.e., porosity levels in the melts solidified under reduced pressure. Generally, the RPT results were held at 2.6 specific gravity (s.g.) at 1,300F and 2.5 s.g. at 1,450F. To evaluate the effect of microstructural enhancement on the mechanical properties and build upon the results of prior research, Tibor and strontium were added to the melt for grain refinement and modification.
The chemical compositions of alloys used at CWRU and two commercial metalcasting facilities are listed in Tables 1 and 2. The alloys nominally called 319 by Foundries A and B are special automotive grade compositions (similar to alloy 320) suitable for their cast component requirements. Foundry A evaluated commercial melt A356 alloy, and both commercial foundries evaluated melts of 319 alloy.
At Foundry A, the metal was continuously rotor degassed with nitrogen and filtered inline with a bonded particle filter prior to casting. A rod grain refiner was employed. For the A356 alloy, casting temperature was 1,350F (732C); for 319, 1,320F (716C). Specific gravity was held to 2.55 for A356; to 2.67-2.7 for 319 alloy. In both instances, the established practices were commensurate with customer requirements for those castings and yielded acceptable results.
At Foundry B, the 319 melt was continuously rotor-degassed with argon, treated with strontium rod and filtered inline with a bonded particle filter prior to casting. Specific gravity was held to 2.71.
Each facility poured at a mold temperature at or near 625F (329C), or slightly above in most instances. The step mold was poured at 400F (204C) mold temperature.
Four types of molds were used: the Stahl test-bar permanent mold (Fig.1a), Case-H mold (Fig.1b), a sand mold (Fig.1c) and a step mold (Fig.1d). The Case-H mold contains the knife-ingate into the gage section and embedded heating elements for consistent thermal control. The Stahl and Case-H mold were coated with Dycote 34ESS on the sprue and runner sections and graphite on the gage section surface, the latter to create best solidification conditions. The Stahl mold and Case-H mold were preheated to 400F (204C) for mold coating application, then poured at 625F (329C) mold temperature in all foundries; the step mold was preheated and poured at 400F (204C) mold temperature.
The step mold produces sections of different thickness, forming a step-like shape. The variation in thickness allows one to examine the mechanical properties of castings solidified under different cooling rates in one pour. However, because the mold does not produce a test bar shaped casting, significant machining labor is required to prepare test bars. Tensile test bars were machined from the 2-inch section and fatigue samples were machined from the 1-inch section (Fig.1d).
The A356 test bars poured at CWRU and Foundry A were heat treated with T6 condition: solution treatment at 1,000F for 12 hours, quench, artificial aging at 320F (160C) for 6 hours. The 319 test bars poured at Foundry A were heat treated with T6 and T7 conditions. T6: solution treatment at 950F (510C) for 8 hours, quench, artificial aging at 320F for 6 hours. Both commercial operations applied T7 to the 319 alloy test bars: solution treatment at 925F (496C) for 8 hours, quench, artificial aging at 462F (239C) for 4.75 hours.
The gauge section diameter of Stahl mold and Case-H mold is 0.5 inch. The test bars in the as-cast, T6 and T7 condition were pulled to fracture with a tensile test machine at room temperature at a strain rate of 10-3s-1. In addition to the UTS and elongation, the Quality Index was used to evaluate the overall mechanical properties of the A356 test bars. The Quality Index is defined by the equation:
QI = UTS+150log10 (elongation)
The UTS and elongation values were determined from an average of six bars for the Stahl mold and the Case-H mold and at least three bars for the step mold and the sand mold. In the latter stages of this work, the yield stress also was determined.
Fatigue test bars were cut and machined from the step mold’s 1-inch section as shown in Figure 2. Testing was conducted at C-T-C at 125 Mpa in fully reversed sinusoidal loading at 60 Hz.
3. Results and Conclusions
The mechanical properties of heat-treated, separately-cast test bars using the four molds were evaluated to discern the effects of microstructural enhancement, i.e., modification and grain refinement and the beneficial effects of reduced microporosity.
In the separately cast test bars poured at CWRU and Foundry A, the metal quality is principally virgin with just 10% recycled. All melts were fluxed, degassed, grain refined and modified, with inline furnace filtration. The Case-H mold improved UTS by about 2ksi in Foundry A and CWRU. It exhibited improved elongation almost twice that of the Stahl mold at CWRU, but less in Foundry A. This may relate to the iron content of A356 in CWRU being lower than in Foundry A. The Case-H mold test bar result at CWRU had a much higher quality index than any other test bars principally because of the high elongation. Despite the high strontium content (0.024) and possible over-modification (with visible microporosity), the achieved mechanical properties obtained in the test bars easily exceed the 40-30-10 (tensile strength-yield strength-elastic limit) results most casting designers call for. This also suggests further confirmation that the enhanced feeding afforded by the knife ingate in the Case-H mold overcomes the inherent loss of properties such microporosity would suggest.
Naturally, the permanent mold test bar results showed improvement over the sand mold test bar results due to faster solidification resulting in smaller SDAS.
To evaluate the influence of high melting and/or holding temperatures on the possible deterioration of mechanical properties, the melt at CWRU was evaluated at 1,300F (704C) and 1,450F (788C).
For A356 alloy test bars poured with the Stahl and Case-H molds, the results reveal higher pouring temperature reduces both UTS and elongation of the two molds. The higher melt and pouring temperature also resulted in a much higher hydrogen content as measured by Alspek readings and evaluated by reduced pressure test.
In addition, the higher pouring temperature created a larger difference between the mold temperature and pouring temperature. Thus, the solidification rate at 1,450F (788C) pouring temperature would be slower than at 1,300F (704C), and more and larger porosity is expected.
Despite these points, only a minimum deterioration of mechanical properties was observed due to higher temperature exposure.
In Foundry A, 319 alloy was cast from melts that were cleaned, degassed, grain refined and modified per that foundry’s normal commercial practice.
The results of test bars poured at Foundry A were well within agreement of their own test results.
For 319 alloy cast in Foundry A, T7 heat treatment is known to increase elongation by sacrificing a small amount of UTS. With T6 or T7 heat treatment, the results showed no great difference between the Stahl and Case molds.
For the 319 alloy cast in Foundry B, the Case-H mold showed improvement in UTS and elongation in T6 and T7 conditions versus the Stahl mold.
The Step mold has four steps: 0.5, 1, 2 and 3-inch thickness sections. In this study, the 2-inch section was chosen to compare with Stahl and Case-H mold test bars. It is easy to imagine this part of the step mold sample has large SDAS and more porosity than the Stahl and Case-H mold test bars. Figure 3 shows the porosity of Stahl/Case-H mold test bars and step mold 2-inch section. Figure 4 presents the SDAS of Stahl/Case-H mold test bars and the step mold 2-inch section sample where the SDAS was determined to be 18μm and 40μm, respectively.
Hot isostatic pressing (HIP) should be able to remove most porosity in the step mold 2-inch section sample. It was shown to improve the mechanical properties of A356 alloy better than 319 alloy. Although HIP improves UTS and elongation for 319 alloy, the tensile properties of the step mold samples are much lower than the separately cast Stahl and Case-H mold samples. The reason is the step mold’s higher SDAS. The UTS of 319 alloy has been established at 42ksi and 26ksi with respect to 30μm and 70μm SDAS. In this study, the 2-inch step mold section 319-T7 sample with about 40μm SDAS having 32ksi UTS is reasonable. The UTS of A356-T6 alloy decreases from 35.4ksi to 34.5ksi when the SDAS increases from 82μm to 96μm, based on prior research. In this study, the 2-inch step mold section A356-T6 sample with about 40μm SDAS having 37ksi UTS also is reasonable.
The fatigue properties of a 1-inch step mold sample with and without HIP were compared to reference data with 125 Mpa fully reversed sinusoidal loading at 60 Hz. The no-HIP samples had worse results than the reference curve, and the HIP samples had better results than the reference curve.
HIP also improved 319 alloy fatigue properties by reducing microporosity, which improved the fatigue properties dramatically. While certainly not an original result, the improvement with HIPping does confirm that microporosity in a separately cast test bar, or tensile specimens harvested from a test mold, can be counteracted with this technique to improve desired mechanical properties.
In conclusion, clean metal practices (fluxing, degassing, flux injection, filtration) and appropriate application of grain refining and modification result in acceptable separately-cast test bar results in both lab and commercial foundry melts in A356 and 319 alloys in their respective T6 and T7 heat treat conditions. The Case test-bar mold with knife ingate consistently produces measurable mechanical property increases over the standard Stahl ASTM B108 test bar mold and should be given further consideration for use as a standard methodology.
Despite over-modification with high strontium and apparent porosity, separately-cast Case test bar mold tensile properties yielded a high Quality Index number aided by high elongation results (13-15%). This is due to low iron content (less than 0.10%) and the effect of the knife-ingate configuration on the Case testbar mold, which provides better feeding despite some microporosity experienced with the high strontium.
HIPping improves the tensile and fatigue properties of both alloys by closing off any microporosity. And Computed Tomography (CT Scan) techniques may be useful in ascertaining microposity distribution and quantification as influenced by testing or processing variables.
This article summarizes a paper that was presented at the 2014 AFS Metalcasting Congress. See www.moderncasting.com.