Chapter 5
The rolled and homogenized alloys exhibited higher strength and ductility than the cast and homogenized alloys. Hence, rolling is suggested for obtaining these alloys with better mechanical properties.
Peak hardness in all the alloys was attained when aged at 170 °C for 24 h. Addition of trace amounts of Sn had no influence on the peak ageing time.
Precipitation hardening of rolled alloys yielded the highest strength for these alloys.
The reduction in ductility observed in the peak aged alloys can be improved by optimizing the ageing conditions.
DSC studies showed two exothermic peaks in the temperature region of 200 ºC to 300 ºC in the case of the base alloy (Alloy-A) and the alloy with 0.06 wt.% Sn (Alloy-D).
For all the other alloys, only one exothermic peak could be observed in this temperature range. Activation energy analysis indicated that Sn addition up to 0.06 wt.% had no adverse effect on the precipitation process.
XRD patterns of the heat treated alloys showed evidence of CuAl2 precipitation in the Al matrix.
Flow softening after attaining the peak stress was observed in all the alloys at low strain rate of 0.001 s-1.
Undulations were observed in the flow curves in all alloys microalloyed with Sn at low strain rates and lower temperatures.
The solved constitutive equations for high temperature deformation yielded good prediction of peak flow stress for all the alloys within the range of temperatures and strain rates investigated. The peak flow stress during deformation were predicted using the above lay within an RMS error of 8.53, 9.33, 4.84, 3.24, 10.03 and 7.52 for alloys A-F, respectively.
The activation energy for hot deformation for alloys with Sn wt.% > 0.04 was higher than that of the base alloy. The high activation energy of the alloys with Sn wt.% > 0.04 shows that these alloys are relatively difficult to deform, in spite of the fact that these alloys exhibit better mechanical properties.
Flow stresses at various strains, strain rates and temperatures during hot deformation were predicted for all these alloys using MLR analysis and ANN modeling for the first
time. While the former gave reasonably good predictions, the latter’s excellent predictions revealed its superiority over the former in the current predictions.
Processing maps were generated for all the investigated alloys. To the best of the investigator’s knowledge, this is the first attempt made to develop such maps for microalloyed Al alloys.
The power dissipation efficiency maps of these alloys revealed that the maximum efficiency was between 60 % and 70 % for all the alloys except for the alloy with 0.06 wt.% Sn, which showed a low value of 40 %.
The instability maps generated for the alloys showed (i) only one instability regime in the alloy with 0.06 wt.% Sn and (ii) very narrow processing regimes for the alloy with 0.1 wt.% Sn.
Instability during deformation is mainly driven by shear band formation and/or inter- crystalline cracking. The safe processing zones are regions characterized by DRX.
The investigations carried out show the influence of microalloying Al–5.9wt.%Cu–
0.5wt.%Mg alloy with Sn on its microstructure, mechanical properties and hot deformation behavior. It is evident that heat-treatment and/or mechanical working of the as-cast alloy is necessary in order to obtain alloys with improved microstructure and mechanical properties. Microalloying the base alloy with 0.06 wt.% Sn resulted in an alloy exhibiting the best combination of mechanical properties among all the alloys investigated.
Strengthening occurs in these alloys by the precipitation of the CuAl2 phase.
Microstructural investigation of the hot worked samples in conjunction with modeling studies provided a deep insight on the metallurgical changes occurring during hot deformation in these alloys. The safe processing zone for these alloys has a strong dependence on Sn content. The safe processing zone narrowed for alloys with Sn wt.%
higher than 0.06. Excellent prediction of flow stress as a function of temperature, strain and strain rate by the ANN modeling showed the superiority of this technique for flow stress prediction of these alloys.
5.2 Scope for future work
The present investigations were focused on understanding the influence of trace additions of Sn to a commercially available Al-Cu-Mg alloy which is considered to be a high strength light weight alloy. The considerable improvement achieved in the microstructure and mechanical properties shows the advantage of microalloying of Al alloys with Sn. These investigations have not only yielded many interesting results but have also pointed towards several possible directions of further research. Some of these are enumerated below:
1. For high temperature deformation behavior of the investigated alloy system, flow stress can be modeled in terms of strain, strain rate, deformation temperature and percentage of Sn additions i.e. σ = f( , , ,ε ε& T composition). The superiority of the ANN modeling over MLR method shows that the former is the appropriate technique for such investigation.
2. The present high temperature deformation studies are limited to high temperature compression tests. It would be interesting to extend this work to high strain rates (~
10 to 100 s-1) and over a wider temperature range.
3. Further investigation of these alloys under high resolution TEM would throw more light on the nature of the various phases present in these alloys under various heat treatment conditions.
4. Determination of other properties such as fracture toughness, fatigue behavior, corrosion resistance and elevated temperature creep properties would help in assessing the potential of these alloys for light weight structural applications.
5. Effect of microalloying Al alloys with elements other than Sn is worthy of investigation, due to the immense potential to tailor the mechanical properties of these materials.