ULTRASONIC PROCESSING OF ALUMINIUM ALLOY MELTS

Abstract

Grain refinement from solidification processing is an effective and inexpensive method of obtaining improved mechanical properties in light alloys. The combined effect of solute and ultrasound-assisted solidification technique can play a vital role in grain refinement of cast aluminium alloys. A systematic experimental study has been made of the combined influence of solute content (1 wt.%, 2 wt.%, 3 wt.%, and 5 wt.%) and ultrasonic intensity (0, 88, 350, 790, 1400 W/cm2) on grain refinement of Al-Si, Al-Cu and Al-Ni binary alloys. The results indicate that the grain refinement in Al-3wt.%Ni alloy is larger than Al-3wt.%Si alloy with application of ultrasonic treatment although the growth restriction factor value of silicon (Si) solute is higher than nickel (Ni) solute in pure aluminium. These observations prompted the investigation regarding the grain refinement mechanisms in aluminium based hypoeutectic alloys. For that, the grain refining efficiency of different solutes was analyzed using various factors such as the freezing range (ΔT), StJohn’s model and constitutional supercooling parameter (P). The mechanisms for ultrasonic grain refinement of aluminium alloys are discussed based on experimental findings. Ultrasonication resulted in more activated nucleants in Al-Cu alloys than in Al-Si and Al-Ni alloys by the StJohn’s model. In particular, high solute content can ensure achievement of homogeneous and consistent grain morphology in ultrasonic grain refinement. In addition, excellent grain refinement was achieved in both the Al-5wt.%Cu alloy which has an equilibrium freezing range of 100ºC and the Al-5wt.%Ni alloy which has an equilibrium freezing range of just 5ºC at the same applied ultrasonic intensity (1400 W/cm2). Constitutional supercooling parameter truly explains the grain refinement mechanism in Al-Ni alloys with ultrasonic treatment (UST), which have near-eutectic compositions as compared to the Al-Cu and Al-Si alloys studied. Evidently, such discrepancy in average grain size with content of solute cannot be completely described by using growth restriction factor (Q), ΔT, and P values alone. A combination of high solute content and high ultrasonic intensity produces significant grain refinement, including significant refinement of eutectic structures that formed in the Al-5wt.%Si alloy, Al-5wt.%Cu alloy and Al-5wt.%Ni alloy. UST enhanced the mechanical properties of USTed aluminium alloys as compared to respective as-cast aluminium alloys. ii Effect of temperatures of high power ultrasonic processing temperature (700°C, 725°C, 750°C, and 775°C) on the distribution of 1 wt.% Al2O3 nano-particles in Al6061 alloy melt is studied. Al6061 alloy composites are fabricated by varying the weight percentage (1, 2 and 3 wt.%) of Al2O3 nano-particles at ultrasonic processing temperature of 775°C. From the microstructural examination, it is observed that nano particles are better dispersed in the alloy matrix with increasing ultrasonic processing temperature. Some agglomerated reinforcement particles are seen at lower ultrasonic processing temperature of 700°C. As compared to the base alloy, mechanical properties like hardness, 0.2% offset yield strength, and ultimate tensile strength increased significantly with processing temperature. This is attributed mainly to the coefficient of thermal expansion mismatch between the alloy matrix and the Al2O3 particles, followed by Orowan strengthening, and to a lesser extent to the Hall–Petch strengthening mechanism. UST of the melt resulted in better dispersion of Al2O3 particles up to 2 wt.% additions. Al6061-2wt.%Al2O3 composite showed good mechanical properties with yield strength and ultimate tensile strength values, which are 81% and 53% higher than the base alloy. Al6061- 3wt.%Al2O3 composite showed poor yield strength and ultimate tensile strength as compared to Al6061-1wt.%Al2O3 and Al6061-2wt.%Al2O3 composites due to agglomeration of particles. A comparison is made between the experimental yield strengths of the nano composites and the theoretical yield strengths calculated by coefficients of thermal expansion mismatch, Hall-patch, load bearing effect and Orowan strengthening models. The proposed thesis consists of seven chapters: Chapter 1 contains a brief introduction to aluminium alloys and different techniques used for increasing the mechanical properties of aluminium alloys from grain refinement and other strengthening routes. Chapter 2 gives a comprehensive literature review on grain refinement of as-cast and with UST aluminium alloys. It summarizes the effect of content of solute, nano-particles and ultrasonic intensity on microstructure and mechanical properties of aluminium alloys as given in open literature. It contains brief theory of solidification, effects of ultrasound in molten metals and an overview of strengthening mechanisms. It also defines objectives of the present work based on the literature review, scope of the work and the methodology adopted in the present investigation. Chapter 3 deals with the details of experimental procedure carried out in line with the scope of the work. The procedures of specimen preparation for microstructural studies (scanning electron iii microscopy, optical microscopy and transmission electron microscopy) and mechanical testing (hardness and tensile test) are explained. Chapter 4 deals with effect of the combined influence of solute content (1 wt.%, 2 wt.%, 3 wt.%, 5 wt.%) and ultrasonic intensity (0, 88, 350, 790, 1400 W/cm2) on grain refinement of Al-Si, Al- Cu and Al-Ni binary alloys. The resulting microstructure of each aluminium alloy is characterized and the combined effect of solute content and UST is discussed. Mechanisms for ultrasonic grain refinement of aluminium alloys are discussed in Chapter 5. Grain refining efficiency of different solutes is analyzed using the StJohn’s model, ΔT, and P. All these factors are calculated from conventional technique and Scheil-Gulliver solidification simulation. Chapter 6 deals with the effect of UST on the eutectic phase and mechanical properties (hardness and tensile strength) of the Al-Si, Al-Cu and Al-Ni binary alloys of varying solute contents (1 wt.%, 2 wt.%, 3 wt.%, 5 wt.%). In Chapter 7, the effect of temperature of high power ultrasonic processing (700⁰C, 725⁰C, 750⁰C, and 775⁰C) on the distribution of 1wt.% Al2O3 nano-particles in Al6061 alloy melt is studied. Al6061 alloy is also fabricated with varying the content of nanoparticles (2 and 3 wt.% ) at ultrasonic processing temperature of 775⁰C. The microstructural features and mechanical properties of nano composites are characterized. Various strengthening mechanisms in the ultrasonically processed nano composites are analyzed. Eighth and ninth chapters deal with conclusions and scope for future work, respectively