Literature Review
3.1 Processing of alloys
3.1.1 Casting process
Five alloys with different amounts of silver up to 0.1 wt.% were processed and cast to obtain 21 mm diameter 200 mm long cylindrical rods and 300 mm 100 mm 10 mm rectangular slabs. From each alloy melts, 16 numbers of cylindrical rods and 2 numbers of rectangular slabs were obtained. The casting process involves pattern making, mold making, melting and casting and is described in the following paragraphs.
3.1.1.1 Pattern making
The pattern material for the cylindrical rods was made out of wood. The pattern consists of an assembly of 8 cylindrical rods, pouring cup, down sprue, sprue base and runner as per the general principles of pattern design [111–112]. Figure 3.1 shows the schematic diagram, photograph of wooden pattern assembly and mold of the vertical split pattern assembly designed and fabricated for the present work. The mold for obtaining rectangular alloys slabs were machined from mild steel for the preparation of the rolling sample.
(a)
(b) (c)
Figure 3.1 Molding design: (a) schematic diagram of pattern assembly, (b) photograph of the wooden pattern assembly and (c) photograph of the sand mold
3.1.1.2 Preparation of sand mold
The cylindrical rods were cast into sand molds, which were prepared by No-bake process [111]. The binder used for the No-bake process was a phenol formaldehyde with paratoluene sulphuric acid as the catalyst. The sand mold preparation consisted of the following sequence:
Sand mixture used for the present work was silica sand passing through sieve of US series number 30 and retained on number 70. For each mold 22 kg of sand mixture was used.
The molding board was placed on the table. The pattern and molding flasks were properly aligned in position and clamped properly.
Initially 250 g of catalyst (paratolune salphonic acid) was thoroughly mixed with 22 kg of the silica sand for around 4 minutes.
Subsequently, 500 g phenol formaldehyde resin was added to the sand-catalyst mixture and again mixed thoroughly for another 2 minutes.
The molding sand mixture was introduced into the molding flask and rammed gently to ensure uniform filling as per the standard molding practice.
The sand mold along with the pattern was then allowed to cure for 2 hours to obtain sufficient mold strength. The pattern was then removed to obtain one part of the split mold.
The second part of the split mold was also prepared by the same procedure.
Commercially available alcohol based zircon paint was sprayed on the mold surface to ensure reasonably good surface for the castings. After 4 hours of drying the two halves of the molds were assembled and were ready for pouring of molten metal.
3.1.1.3 Casting process
2219 Al-alloys (Al-Cu-Zn-Fe-Mg-Mn-Ti) microalloyed with different amounts of silver were prepared by casting technique. Alloy processing includes preparation of the master alloys and the final alloy preparation.
The starting materials for the present investigation were aluminum–1100 alloy (99% pure) ingot, IACS grade copper rod and sterling silver (92.5% pure). Prior to the alloy preparation, Al-42 wt.% copper (Cu) and Al-5.8 wt.% silver (Ag) master alloys (1 kg melt level) were prepared by melting followed by solidification in metal ingots.
The melting temperatures of commercially pure aluminum, copper and silver are 660
°C, 1082 °C and 961.8 °C respectively. It was therefore necessary to make master alloys of Al-Cu and Al-Ag for obtaining Al-Cu-Ag melt with uniform composition.
Melting was carried out in a 25 kg capacity resistance heated melting furnace. The melt charge was 12.5 kg. The required quantities of the commercially pure aluminum (Al) ingot and Al-Cu master alloys as per the required alloy compositions were placed inside a clay graphite crucible kept inside a resistance heating melting furnace and heated up to 700 °C. The melt was then stirred thoroughly for obtaining molten metal with uniform chemical composition. The dross formed at the top of the molten metal was removed. This was followed by degassing of the molten metal using 0.5%
degasser tablet kept in a stainless steel strainer and stirred for few minutes. This was to ensure complete removal of any gases dissolved in the molten metal. The dross formed during the degassing was also removed. The required quantity of Al-Ag master alloy was added into the melt and stirred slowly by a stainless steel rod to obtain a uniform homogeneous alloy composition. Pouring of the molten metal into molds were carried out at a temperature of 700±5 °C and allowed to solidify in the molds. After cooling to room temperature, the molds were opened and the castings were taken out. Figure 3.2 shows photographs of the mold assembly and casting obtained.
Five alloy compositions of 2219 aluminum alloy containing varying amounts of silver were prepared for the present work. Table 3.1 presents the details of the raw materials used for the alloy melting. The cast alloys were subjected to different thermo-mechanical processes. Table 3.2 illustrates the details regarding different thermo-mechanical processes imparted to the alloys.
The cast alloys were homogenized at 510 °C for 10 hours to obtain homogeneous composition by reducing segregation and coring formed during the solidification process. The heat treated samples were furnace cooled. The chemical compositions of the five alloys were then determined by atomic absorption spectrophotometer (AAS).
Figure 3.3 shows the photograph of the atomic absorption spectroscopy (AAS) (Make: Varian Spectra Duo) used for the present analysis.