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?

1. Background

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.

2. Procedure

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.


ith gridlock in the 113th Congress and continued to resistance to the administration’s legislative agenda, President Obama is utilizing federal agencies to achieve his policy agenda. Dozens of federal regulations, directives and policies are being pushed through via government agencies such as the U.S. Environmental Protection Agency (EPA), National Labor Relations Board, U.S. Department of Labor and the Occupational Safety and Health Administration (OSHA). The wave of regulations is reflected by the fact that over the last five years, 157 new major regulations have been released, many of which have direct impact on U.S. manufacturing and the metalcasting industry.
A major regulation is defined as a rule with $100 million or more in expected economic impact. Since President Obama was elected, a record 3,659 final rules and 2,594 proposed rules have been issued. Those that are not deemed significant are not required to include a cost-benefit analysis, even as their layered implementation has a cumulative economic impact on businesses. More regulations are on the docket for 2014, 2015 and 2016 addressing issues like health care, crystalline silica, power plant emissions and ozone protection.
EPA Regulations
Among the several EPA regulations recently introduced, the power plant rule, ozone legislation and the proposed waters of the U.S. rule may have the most impact on metalcasters.
Power Plant Regulation
EPA’s Clean Power Plan Rule, released June 2, proposes emission guidelines for states to follow in developing plans to address greenhouse gas emissions from existing fossil fuel-fired electric generating units. Specifically, EPA is proposing state-specific rate-based goals for carbon dioxide emissions from the power sector, as well as guidelines for states to follow in developing plans to achieve the state-specific goals.  
Manufacturers will be hit twice by greenhouse gas regulations, both as users of the energy being regulated and as industries considered next in line to receive similar regulations from EPA.
The current proposed power plant rule would substantially increase electricity and natural gas costs and create reliability problems, all for a relatively small global climate impact. For example, EPA says the proposed rule would eliminate 730 million metric tons of carbon by 2030. From 2010 to 2011, China’s carbon dioxide emissions rose by 705 million tons. The rule would substantially reduce use of coal-fired generation. Coal-fired power is a low cost and reliable source of electricity. Importantly, coal competes with natural gas on a Btu basis and helps keep electricity prices from rising. Many in the business community believe the EPA power plant rule would increase dependency on natural gas for power generation.
A lawsuit against EPA challenging the agency’s failure to assess the job-loss impact of its power plant rules has been allowed into federal court (Murray Energy Corporation v. U.S. Environmental Protection Agency, No. 14-1112). A three-judge panel was expected to hear the case this month.
In September, EPA extended the comment period for its “Clean Power Plan Rule” 45 days, until December 1. The American Foundry Society (AFS) is in the process of receiving input from its members and drafting comments.  Based on comments from EPA staff, the agency still intends to finalize the rule by June 2015.
Ozone Regulation
This summer EPA’s Advisory Panel and staff recommended to the EPA Administrator that the national air standards for ozone be lowered to 60-70 parts per billion (ppb) from the current 75-ppb standard, which was set in 2008. The agency cites scientific data and exposure information that “provide strong support” for revising the health-based national ambient air quality standard for ozone of 75 ppb.
The EPA Office of Air Quality Planning and Standards, which prepared the assessment, said a revised standard set within that range “could reasonably be judged to provide an appropriate degree of public health protection, including for at-risk populations and life stages.”  
The regulation could become the most costly in U.S. history if the new standard is implemented. In 2010, EPA estimated the annual compliance costs for a 60-ppb standard would be $90 billion in 2020. The National Association of Manufacturers (NAM) released a report in July that estimated a revised ozone standard of 60 ppb could cost the U.S. economy up to $270 billion per year and result in the closure of one-third of the nation’s coal-fired power plants. The lower standard will require large reductions in NOx and volatile organic compound (VOC) emissions from power plants, manufacturing facilities and mobile sources such as cars, trucks and off-road vehicles. Requiring a reduction to 60 ppb would leave nearly all of the U.S. in a so-called “nonattainment zone.” Metalcasting facilities of all sizes in nonattainment areas would not be able to make investments and expand operations without other businesses reducing their emissions or, worse yet, shuttering their operations. EPA has until December 1 to decide whether to keep or change current national air quality standards for ozone. President Obama delayed EPA’s previous attempt to promulgate a lower ozone air quality standard in 2011. A final rule is expected to be made by October 2015.
Waters of the U.S. Rule
In March, EPA and the Army Corps of Engineers proposed a new rule to redefine the term “waters of the United States” and the agencies’ jurisdiction over waters they can regulate under the Clean Water Act. The rule extends federal jurisdiction well beyond traditional navigable waters to tributaries, adjacent waters (such as ponds) and vaguely defined “other waters.” EPA’s proposal exposes new facilities and expansion projects to additional federal permitting, triggering new upfront costs, project delays and threats of litigation. Permitting requirements could cost metalcasters nearly $200,000 in some cases.
On September 10, the U.S. House of Representatives passed the Waters of the United States Regulatory Overreach Protection Act (H.R. 5078), which requires EPA and the Corps to revisit the proposed rule with direct consultation with state and local officials to determine which bodies of water should be covered under the Clean Water Act. The White House has issued a veto threat against this legislation, and the Senate is unlikely to take up the Overreach Protection Act this year.
EPA has received more than 500,000 comments to date on the proposal, and the comment period was extended to Nov. 14. The AFS Environmental Health and Safety 10-F Committee assembled comments from the industry on how the new permitting requirements will impact metalcasters. Information about the proposed rule can be found at www.epa.gov/uswaters.
OSHA Initiatives
OSHA’s top priorities in President Obama’s second term have included increased injury reporting requirements, crystalline silica, combustible dust and increased enforcement. 2014 has shown a continued focus on high hazard industries such as metalcasting through the use of national and local emphasis programs, such as the silica and primary metals national emphasis programs.
Crystalline Silica Standard
Of perhaps largest concern to the metalcasting industry is the proposed standard on occupational exposure to respirable crystalline silica. OSHA formally unveiled the comprehensive regulation to control crystalline silica in September 2013. It is one of the safety agency’s most far-reaching regulatory initiatives ever proposed for the metalcasting industry and a number of other key sectors. In addition to the 50% reduction in the permissible exposure limit (PEL), OSHA is proposing requirements including, but not limited to, medical surveillance, record keeping and prohibitions on certain work practices, including compressed air and dry sweeping.
AFS believes the current PEL is adequate to protect the health of exposed workers from silica-related disease when it is fully complied with and enforced. OSHA estimates the rule will result in approximately $44 million in annual costs to the industry. This stands in sharp contrast to the industry analyses whereby AFS estimates a conservative cost to the industry of $2.2 billion per year, or 276% of profits. OSHA expects to issue a final rule by 2016.
For the past year, AFS has been working on gathering and submitting detailed comments and background materials for OSHA, including: prehearing comments submitted Feb. 11; testimony at a March 28 public hearing in Washington, D.C.; post-hearing comments responsive to OSHA’s request for additional information June 3; and the post-hearing brief filed on August 18.
With the docket closed, metalcasters can continue to educate their lawmakers about the impact that rule will have on their metalcasting facility and our industry. The metalcasting association in its meetings with lawmakers is focusing on the regulatory overreach, feasibility and cost of the proposed standard.
Temporary Workers
Metalcasters also should be aware of an initiative launched last year to better train and protect the safety of temporary workers. The OSHA Temporary Worker Initiative includes outreach, training and enforcement.
At least 14 temp workers died during their first day at a new worksite in 2013 across all industries. In recent months, OSHA has investigated reports of temporary workers suffering serious or fatal injuries and cited a number of businesses. The agency and the National Institute for Occupational Safety and Health (NIOSH) has released recommended practices for staffing agencies and host employers to protect temporary workers from hazards on the job. The new publication highlights the joint responsibility of the staffing agency and host employer to ensure temporary workers are provided a safe work environment.
The new guidance recommends that staff agency/host employer contracts clearly define the temporary worker’s tasks and the safety and health responsibilities of each employer.  The new Recommended Practices publication is available at: www.osha.gov/Publications/OSHA3735.pdf.
Injury and Illness Reporting
Two proposals on injury and illness reporting were reclassified as long-term action in May, including the Injury and Illness Prevention Program (I2P2), which would require employers to establish formal written plans to find and fix real and potential workplace hazards, and the MSD Column to OSHA’s Form 300 Injury and Illness Log, which would be used to enforce ergonomics.
Combustible Dust Standard
The Chemical Safety Board documented hundreds of fatalities and serious injuries resulting from combustible dust explosions in a 2006 study. As a result, OSHA began working on a potential rule in 2009 that would require industries, including metalcasting, to better control combustible dust hazards. A number of OSHA standards address aspects of this hazard, but the agency does not have a comprehensive standard. According to the agency’s current regulatory agenda, OSHA intends to initiate a review panel on the proposed rule in December, as required under the Small Business Regulatory Enforcement Fairness Act (SBREFA).  If OSHA does commence a review panel, AFS plans to have a member company be part of the discussions. While a proposed standard is not expected soon, OSHA is gathering information and currently regulating combustible dust through a national emphasis program.