Investigation of Micro Hardness, Cooling Rate and Microstructure of ATIG Welded samples of Al-SiC composite

Activated TIG welding has been performed on Al – 8% SiC composite 5mm plate with various fluxes such as Al 2 O 3 , MnO 2 , CaO, MgO, SiO 2 & TiO 2 , to study & analyze the Microstructure, Micro hardness and cooling rate. Correlation study between micro hardness, microstructure and cooling rate for Constant Current TIG welding and Activated TIG welding on Al-SiC composite are also carried out to analyze the relation between the effect of cooling rate on microstructure & the effect of microstructure on micro hardness. The experimental results of ATIG welding on Al-SiC composite shows fine grain weld microstructure on ATIG – SiO 2 & ATIG – TiO 2 , which results in higher micro hardness. Micro hardness values are taken in different locations of weld surface at 1mm, 2mm & 3mm below the weld surface and the same is also observed along the weld zone to heat affected zone upto 12mm for the center of the weldment. Minimum micro hardness values found in ATIG – MnO 2 , ATIG – CaO & ATIG – MgO are due to intermediate micro structure between coarse and fine in heat affected zone. ATIG – Al 2 O 3 weld zone & heat affected zone and heat affected zone of ATIG – MnO 2 , ATIG – CaO & ATIG – MgO shows coarse microstructure leading to reduction in micro hardness value. Cooling rate for the different CCTIG & ATIG welding are recorded and correlation between the micro structures are studied. Coarse micro structure in weld zone and heat affected zone have least cooling rate whereas fine micro structure in weld zone resulted at higher cooling rate. Heat affected zone strongly depends on temperature gradient between the weld center and weldment’s heat affected zone.


Introduction
Gas Tungsten Arc Welding (GTAW) also known as Tungsten Inert Gas (TIG) welding showed high quality weld than other arc welding process. Since non ferrous metals requires quality weld with reduced defects and improved mechanical properties of the weld, TIG welding is considered to be more suitable and appropriate welding process. However, its shallow weld penetration causes lesser production rate. Increase in weld passes for thick sections lead to higher input in the weld which reduces the weld strength [1] due to the formation of coarse grain microstructure [2].
To reduce the heat input into weld the Activated TIG welding (ATIG) a new variant was found out. ATIG welding process was developed in early 1960's by the Paton Welding Institute, Ukraine to improve the weld penetration [3,4]. By improving the weld penetration, number weld passes get reduced by reducing the weld pass the heat input given the weld material is also get reduced [5]. The activated flux TIG welding may have two types of mechanisms that are mostly accepted, first one is based on the reverse Marangoni convection effect [6][7][8], and the other one is based on arc construction effect [9,10].
Active flux coated on the weld increases the penetration by following the theory of reverse Marangonic effect proposed by Helipe & Roper [11][12][13] and arc constriction effect as stated by Lucas & Howse [14,15]. Due to the constricted arc & reverse Marangonic effect the weld penetration is observed [16].

Experiments
Autogenous welding was performed on Al -8% SiC composite material with a plate thickness of 5mm using ADOR CHAMPTIG 300AD welding machine [17]. Other welding parameters considered during welding are provided in Table 1. Welded samples are shown in Figure  1  During welding, cooling rate at 10mm away weld zone was calculated from time -temperature profile acquired using data acquisition system (DAQ) through LabView software coupled with National Instrument NI cDAQ 9174 kit which employs a temperature acquiring module -NI 9211 with K type thermocouple.
Using standard metallographic procedure the welded samples were prepared for microstructure observation from microscopic image analyzer.
Vickers micro hardness of different region such as weld zone, heat affect zone and base material were measured with load of 500g with 10 seconds as indentation time performed using Shimadzu micro hardness tester.   Table 2 shows the average micro hardness value of CCTIG & ATIG weldment on weld centre, 3mm away from weld centre & 7mm away from weld centre. Standard deviation is also tabulated. Figure 3, shows the micro hardness 1mm below the weld surface. Figure 4 shows the micro hardness value 2mm below the weld surface. Figure 5 shows the micro hardness values of 3mm below weld surface. Here micro hardness values are observed in different locations from weld centre to 12mm away from the weld centre. Micro hardness value did not have much change after 10mm away from the weld centre which is more or less equal to the base material micro hardness.      Figure 8 shows the micro structure of ATIG -MnO 2 , ATIG -SiO 2 & ATIG -TiO 2 weldments. From Figure 6, the following results are observed, very fine grain microstructure is observed in base material and coarse grain size microstructure in weld zone and in HAZ. Figure 7 shows ATIG -CaO & ATIG -MgO is also showed coarse grain structure which is similar to CCTIG welded microstructure. Intermediate size between fine and coarse microstructure is found in weld zone as well as in heat affected zone in ATIG -MnO 2 & ATIG -Al 2 O 3 . Figure-8 shows ATIG -SiO 2 & ATIG -TiO 2 is having fine grain microstructure in both weld zone and also in heat affected zone. Base material showed very fine grain microstructure. Table 3 shows the cooling rate CCTIG & ATIG weldments of various active fluxes are recorded and tabulated. Highest cooling rate is observed in ATIG-SiO2 followed by ATIG -TiO2 of 295K/s & 290K/s. Lowest cooling rate is observed in CCTIG & ATIG -Al2O3 of 286K/s & 283K/s. Higher cooling rate produces the fine grain microstructure and lower cooling rate produces the coarse grain microstructure in weld zone and heat affected zone. This is due to the temperature gradient observed in weld zone and between weld zone & heat affected zone.

Conclusion
Correlation between micro hardness, microstructure and cooling rate of Constant Current TIG welding and Activated TIG welding on Al-SiC composite has been studied and validated. Micro hardness are observed in three different locations of weld surface at 1mm, 2mm & 3mm locations below the weld surface and it also observed along the weld zone to heat affected zone upto 12mm for the plotting the micro hardness value around the weldment.
Microstructure observed in weld centre of the weldment. The interaction of weld zone & heat affected zone occurs 3mm away from the weld centre line and heat affected zone formation is 7mm away from the weld centre line.
Minimum micro hardness value is found in ATIG - Cooling rate for the different CCTIG & ATIG welding are recorded and correlation between the micro structures are studied. Coarse micro structure in weld zone and heat affected zone have least cooling rate. Fine micro structure in weld zone should have higher cooling rate. It is also inferred that heat affected zone directly depends on temperature gradient between the weld centre and weldment's heat affected zone.