TESTING 1-2-3 Optimizing Aluminum’s Tensile Properties
A new test bar casting method for aluminum alloys yields more accurate data. MODERN CASTING STAFF REPORT
density of steel and have less strength, improving their mechanical properties is an important task. Prior research has demonstrated how
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to strengthen aluminum alloys with chemical modification, heat treatment and reduced porosity. Te presence of microporosity is well known to have a key effect on the mechanical proper- ties of A356 and 319 alloys. Degassing treatments for A356 and 319 alloy melts generally are required to reduce hydro- gen content, minimizing porosity in the casting. Hot isostatic pressing (HIP) also can reduce microporosity and increase mechanical properties.
ADDING IT ALL UP Breaking down the latest research is as easy as 1-2-3.
“Optimizing Mechanical Properties in Aluminum Alloys With Separately Cast Test Bars,” Chia-Jung Chen, David Schwam and David Neff, Case Western Reserve University, Cleveland, Ohio
Background—The influence of metal cleaning practices (degassing, filtration) on tensile properties of A356 alloy was studied previously. The present work focused on optimizing tensile properties by grain refining, microstructure modification and heat treating.
Procedure—A sand test bar mold and standard permanent mold test bar mold (Stahl mold ASTM B-108) are widely used in metalcasting facilities, but the latter still suffers from microporosity and less than optimum mechanical properties. To improve the tensile properties, a modified mold involving a knife ingate between the feeder and test-bar cavity, in-mold filter cavity and heater (Case-H mold) was evaluated. In addition, a Step mold was used to evaluate how porosity and SDAS affect the mechanical properties. A356 and 319 aluminum alloy test bars were cast.
Results and Conclusions—The Case-H mold test bar exhibited better tensile properties and quality index than the Stahl mold test bar due to better feed- ing. HIPping on a specimen cut from step mold coupons showed improved tensile and fatigue poperties over non-HIPped samples in the heat treated condition (T6 for A356, T7 for 319) for both alloys.
luminum alloys are used widely to replace cast steel, especially in the automobile indus- try. Because they are around one-third the
Te 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. Te 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 pro- ducing 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.
Question 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?
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. Te mechanical properties in the test bar will be a reasonable measure of the melt quality, especially with regard to the presence or absence of impuri- ties—inclusions, porosity or microporos- ity—which will exert a strong influence on the resultant mechanical properties. A modification was proposed to
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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 microstruc- tures of test bar castings. A new mold was fabricated based on the modified design, and its performance was com- pared 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. Te 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 treat- ment was 1,000F (538C) for 5 hours;
November 2014 MODERN CASTING | 45
Background Mechanical properties
derived from separately cast test bars do not replicate the prop- erties in a full casting except,
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