Author ORCID Identifier

https://orcid.org/0000-0002-5981-4821

Date of Award

25-2-2025

Document Type

Thesis

School

School of Mechanical Engineering

Programme

Ph.D.-Doctoral of Philosophy

First Advisor

Dr.S.Ramesh Kumar

Keywords

Aluminium 6063 Alloy, Fatigue Behaviour, Heat Treatment, Cryogenic Treatment, Severe Plastic Deformation

Abstract

This research focuses on improving the Low Cycle Fatigue (LCF) life of Al6063 using three different processing techniques – Heat Treatment (HT), Deep Cryogenic Treatment (DCT), and Equal Channel Angular Pressing (ECAP). Al 6063, widely used in structural applications, has limited fatigue performance, necessitating the exploration of advanced processing techniques to enhance its LCF life.

In HT phase, Al 6063 T5 samples were subjected to solid solution HT followed by air and water quenching. The air-quenched (T6A) and water-quenched (T6W) samples were subjected to precipitation hardening. The Ultimate Tensile Strength (UTS) of the T6A sample increased to 188 MPa from 170 MPa of the T5 sample. The LCF life of T5 is 836 cycles, and T6A exhibited an enhanced LCF life of up to 1286 cycles due to the uniform distribution of Mg2Si precipitate and reduced residual stress.

The LCF life of the T6W sample was 708 cycles, which is lower than the T5 sample, indicating that water quenching led to coarser grains, compromising fatigue resistance In the DCT phase, samples are deep cryogenic treated between 2-24 hours with increments of 2 hrs, and its effect on the LCF life of Al 6063 was tested. UTS increased for the DCT samples up to 230 MPa, and elongation reduced to 17% for CT24, which refers to samples subjected to deep cryogenic treatment for 24 hours. LCF life increased up to 6 hr about 2216 cycles. Beyond 6 hr and for a longer duration of up to 24 hours, LCF life is gradually reduced due to prolonged exposure to DCT.

The influence of grain refinement achieved by ECAP at 90° and 120° angled dies and LCF life is studied. It was found that for the ECAP 90° sample, grain size reduced to 12 μm from 90 μm. UTS increased by about 312 MPa, and LCF life increased by 481% with 7489 cycles. For ECAP 120°, UTS is 265 MPa and 95 HV with an average grain size of 50 μm. It exhibited a higher LCF of 5860 cycles to failure than T6. ECAP 90° and 120° samples showed cyclic stabilization with a minimal plastic strain accumulation range of 0.001%.

In conclusion, the study demonstrates that ECAP 90° significantly enhances the mechanical properties with UTS of 312 MPa and 115 HV and LCF life of Al 6063. ECAP 90° provides enhanced Al 6063 LCF life with 7489 cycles and makes ECAP a capable Al 6063 processing technique for aerospace applications (hoop frame and stinger) where fatigue performance is critical.

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