Grain Refinement of Aluminum Alloys: Part One

Sommario:

Grain refinement is a critical process in improving key properties such as mechanical properties, formability and machinability through methodical adjustment of the microstructure.
Inoculation is an example of a common chemical method which is commonly used with aluminum alloys to achieve a fine equiaxed grain structure.

Casting is one of the oldest methods of manufacturing parts. In casting molten metal is poured into a mould cavity where, upon solidification, the metal takes on the shape of cavity. Solidification of any molten metal involves nucleation and growth. Nucleation appears in the molten metal in the form of tiny solid particles, called nuclei, when phase transformation in the metal occurs. These nuclei are created from the deposition of atoms and grow into the form of crystal and from these crystals results completely solidified grains. Grain refinement is a technique used to improve the mechanical properties of the materials by decreasing grain size.

Microstructural refining of metallic alloys has been the subject of numerous researches in the field of metallurgy. The reason lies in the fact that many of the mechanical properties, formability and machinability of the alloy largely depend on the size and distribution of grains in the microstructures. Grain refinement of Al and its alloys is particularly widely practiced in the Al industries, both in shaped casting of alloys and in the ingot casting of many wrought grades.

Methods of grain refinements used in casting processes are mainly classified into the 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

Thermal method involves rapid cooling and subsequently variation of process variables. It is well known that the appearance of a fine equiaxed structure arises when casting are cast both in cold molds and under low superheating conditions.

Grain refinement by chemical methods involves the addition of some elements promoting nucleation and hindering growth. Inoculation that is a typical example of this method is the most common practice by Al-Ti-B ternary master alloys for achieving fine equiaxed grains in Al and its alloy castings.

The mechanical refining method involves agitation of the melt during solidification by using ultrasonic, electromagnetic and mechanical stirring. Most of the techniques require elaborate processing and or special devices. They are usually applied to semisolid metal processing due to high processing cost and long process time.

Benefits of the Grain Refinement Process:

  • better distribution of Porosity
  • reduces ingot cracking
  • improves feeding to eliminate shrinkage porosity
  • improves fluidity.
  • improves surface finish and Machinability.
  • ensures uniform mechanical properties

Best Grain Refinement Practices

Al-Si Alloys: like A356 and A357 best results 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 results Best Grain Refinement Practices

Al-Cu Alloys: for Al-4.5%Cu-0.5%Mn alloys best results 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.

Al-Zn-Mg Alloys: For these alloys best results 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.

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