Grain Refinement of Aluminum Alloys Presented By SUBRATA SAHA
Objectives Whats and Whys? Grain Refinement is a process used for grain boundary strengthening of casting material. Aluminum alloys with ultra fine grain structure has received tremendous increase in research interest in various disciplines specially in automotive and aerospace industries for properties like low density and good castability. Grain refinement of Al and its alloys improve the mechanical properties of casting along with surface finish, soundness etc.
Grain Refining Effect a. Grain refiner added: fine, equiaxed structure; b. no grain refiner: coarse, columnar structure; c. no grain refiner: predominantly twinned columnar grains (feather grains) Source: Quested, 2004
Methods of Grain Refinement Methods of grain refinements used in casting process are mainly classified into following three groups Thermal method i.e. Cooling rate control Chemical method i.e. Grain size refiners addition Mechanical method i.e. agitation of melt during solidification
Grain Refinement Technology TiAl 3 nucleates aluminum grain Source: Backerud, 1983
Cooling Curve Cooling curve during solidification Source: Sigworth and Kuhn, 2008
Comparison of Effect of Grain Refinement Grain refining of A356 alloy by three master alloys Source: Lu, Wang and Kung, 1981
What s the Figure Says? TiAl 3 crystals are poor nuclei. TiB 2 particles are excellent nuclei. Mostly used AlB 2 is the best nucleus. Not Suitable for Industry.
Best Grain Refinement Practices Al-Si Alloys : like A356 and A357 best result by adding 10-20 ppm of boron in the form of Al-5Ti-1B or Al-3Ti-1B rod. Al-Cu Alloys: for Al-4.5%Cu-0.5%Mn alloys best result by titanium less than 0.05% and adding 10-20 ppm of boron as Al-5Ti-1B or Al-3Ti-B rod. Al-Si-Cu Alloys : like A319 alloys (Al-3%Cu-5.5%Si) best 10-20 ppm of boron in the form of Al-5Ti-1B or Al-3Ti- 1B rod.
Best Grain Refinement Practices (Continued) Al-Zn-Mg Alloy: For these alloys best result found with 0.02 to 0.05% of titanium and addition of 10-20 ppm of boron as Al-5Ti- 1B or Al-3Ti-1B rod. Al-Mg Alloys: like 535 alloys (Al-7%Mg) best result found with addition of 30 ppm B in the form of Al-3%Ti-1%B master alloy.
Benefit of Grain Refinement Process Major benefits: better distribution of Porosity. improved Feeding. improved Fluidity. improved Surface finish & Machinability. better mechanical properties
Future Work Development of a model to predict grain size in the presence of significant thermal gradient. Development of an economical viable method allowing tight control of the size distribution of inoculants particles present in the melt. This might be achievable via filtering of the molten maser alloys or by direct inoculation of ceramic Application of grain refinement theories to non-aluminum based system
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