Microstructural evolution and fatigue crack growth characteristics through the HAZ of welded AA 6061
Akhusama, Eliakim Niva
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A comprehensive study of the microstructure and mechanical properties of the regions of the heat affected zone (HAZ) of the aluminium alloy AA 6061 in three temper conditions; TO, T 4 and T6 was carried out. This was achieved through the influence of the HAZ thermal cycle simulation using seven specimens representing the base metal (BM) and the six regions of the HAZ. The specimens were placed in a muffie furnace and heated to predetermined thermal cycle peak temperatures and left to cool in open air. Emphasis was placed on the effects of the HAZ peak temperatures. Optical micrography of the BM and the six HAZ regions was carried out on a universal microscope (OPTIKA B-353 MET) and micrographs were taken at a magnification of XlOO. Scanning electron microscopy (SEM) was carried out on a scanning electron microscope (JEOL JSM 200) at a magnification of X 2000. Fatigue crack growth characteristics of the HAZ regions were achieved through cyclic loading at a loading ratio, R of 0.1 and 0.6 and driving force, M>, of 964N for constant load and successive reduction of 10010 of M> for fatigue crack growth threshold, M<.th, test. Hardness surveys across the BM and the HAZ regions were conducted using a digital macrovickers hardness tester, LV 800. During thermal simulation, the temperature quickly rises from room temperature to the peak temperature.The attained peak temperature then quickly falls back to the room temperature upon exposure to open air. Fatigue crack growth is high at the point with a thermal cycle peak temperature of 5900C for all the temper conditions TO, T4 and T6. This point is 5mm from the fusion line (FL) and is designated as region 'F'. This point also has low hardness values (HV) of 37.6, 48.4 and 51.8 for specimens AA 6061-TO, AA 6061-T4 and AA 6061-T6 respectively. The grain number, G, for this region is 9.089 ﾱ 0.1619 for TO temper condition, 10.229 ﾱ 0.1671 for T4 temper condition and 11.096 ﾱ 0.1671 for T6 temper condition. This region 'F', for all the temper conditions, had the largest grain size as compared to all other HAZ regions. SEM micrography images show most surface cracks originate from this region. The use of AA 6061 specimen in three temper conditions TO, T4 and T6 to determine the weakest link in the heat affected zone (HAZ) through fatigue crack growth and hardness tests is shown to qualitatively predict the weakest link as being 5 mm (region 'F') from the weld centerline. This is caused by the dissolution of strengthening phases. At 590ﾰC, degradation in strength occurs due to dissolution of the major metastable strengthening precipitate Pl. This results in the softening of this region of the HAZ and hence resulting in hardness degradation and least resistance to fatigue crack growth. Upon thermal simulation, the specimens show PI precipitate coarsening coupled with formation of other plate like precipitates, P and spherical precipitates al at particular HAZ points. Grain growth is high in grains of the specimens whose thermal simulation resembled those adjacent to the 5mm point from the fusion line (FL). Thermal simulations having haracteristically high heat input generate coarse grains in the HAZ. The hardness distribution in the HAZ of precipitation hardened AA 6061 depend on the interplay between precipitate dissolution and reprecipitation. Thermal simulation is an effective tool in the study ofHAZ-FCG characteristics by making it able to determine the weakest link in the HAZ on the overall welding joint and can also be used as a tool for weld design optimization in a wide range of circumstances.