Aging Time Effects on the Mechanical Properties of Al 6061-T 6 Alloy

This work investigates the influence of artificial aging and solution heat treatment on the hardness and tensile strength (mechanical properties) of Al 6061-T6 alloy. For this investigation, several aluminum 6061-T6 alloy specimens were prepared following the ASTM 176000 recommendations. The prepared specimens were heated for 1 hour at 500oC before being water-quenched. The procedure for artificial aging was performed for 1, 2, 3, and 4 hours at 190oC before being slowly cooled in air. Several mechanical and characterization studies were performed on the treated specimens, including an investigation on their microstructure, tensile strength, hardness, and X-ray diffraction pattern. From the results, the strength and hardness properties of the specimens were found to be generally improved, even as the best features were obtained after 2 hours of artificial aging. Keywords-heat treatment; Al alloy 6061 T6; mechanical properties


I. INTRODUCTION
The 6xxx group of Al alloys has received much research attention due to their great technological potential, as well as their exceptional strength enhancement following precipitation hardening.These alloys are commonly employed in the construction and automotive sector.The metastable precursors of the equilibrium phase are precipitated in a process that involves one or more complex sequences.Their chemical content, heat treatment parameters, and casting conditions greatly influence their extrudability and determine their microstructure.This implies that the properties of several aluminum alloys can be manipulated through specific heat treatment processes.The heat treatment can be executed either through solution heating (SH) or through artificial aging (AA).In the SH process, the alloy is heated to a temperature range of 400 ºC-530ºC, before water quenching at ambient temperature, while AA or simply "hardening" is performed at about 200ºC (in case of the group 6xxx of Al alloys).The temperature for age hardening usually ranges from 160°C to 200°C [1-3].Aluminum alloys subjected to SH treatment are believed to have varying mechanical properties which account for their varying machinability.The stability and corrosion resistance of these materials can be improved by the addition of magnesium and silicon.Several mechanical properties exhibited by the 6xxx group of aluminum alloys are attributed to the type of treatment (solution, aging time and temperature) they are exposed to [4,5].
The impact of heat treatment on the mechanical and microstructural features of alloys has been extensively studied.Authors in [1] investigated and reported the influence of precipitation hardening on the microstructural and mechanical attributes of 6xxx-group, 6061, 6063 and 6082 Al alloys.The alloys were subjected to a 12-h heating in a furnace at a temperature of 565°C before being water quenched.The heated and cooled specimens were further exposed to AA for 98h at 175°C.The microstructural and mechanical properties of the studied samples were noted to be unaffected during AA as a result of the prior precipitation strengthening process.In [6], authors investigated the effect of combining different specified cold working and precipitation hardening procedures on the tensile strength of 6061 Al alloy.The results showed that the application of a single AA process at 180°C for 4h improved the strength of the studied specimens, while a double aging process had no influence on the mechanical features of the specimens.Additionally, the pre-aging process had a counteractive effect on the subsequent precipitation process on the material.These changes in the mechanical features of the specimens were attributed to the microstructural developments due to the conflicting influence of hardening by precipitation, work softening, and strain hardening processes on the samples.Authors in [7] studied the mechanical properties of the 6061-T6 Al alloy at ambient temperature under various pre-thermal histories that represent an electron beam welding process.During the study, a fast-heating device was fabricated to deliver a controlled application of thermal loadings on the tensile samples.From the tensile tests conducted, it was observed that the yield stress decreased at RT when the optimum temperature limit was reached and when there was decrease in the heating rate.These differences in the mechanical properties were attributed to the microstructural changes that were evidenced by Transmission Electron Microscopy.Furthermore, authors in [8] studied the quench sensitivity of the mechanical properties of 6061 and 6069 Al alloys by determining the relationship between quench delay time and the mechanical properties at varying temperatures in the range of 200°C to 500°C.From the study, it was reported that the quench sensitivity of 6069-T6 was higher compared to that of 6061, and this was attributed to the differences in their composition.The study also provided more information on the quench sensitivity of the conventional 6061 alloys.In the present study, our aim is to investigate the influence of AA time on the mechanical properties of Al 6061 T6 alloy.

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