Effect of preheat and post heat on mechanical properties of 6061 aluminium tungsten inert gas welded joints

Tungsten inert gas (TIG) welded joints of 6061 aluminium are often used in the fuel tanks of large launch vehicles, because of the massive loads these vehicles carry, dealing with weld reinforcement on TIG joints represents an important issue in their preheat and post heat on mechanical properties. Experimental and calculation methods were used to investigate the effect of weld preheat and post heat on the mechanical properties of these joints. The purpose of this study was to determine the effect of preheat and post heat on mechanical properties of 6061 aluminium. The experimental result indicated that mechanical properties of Al 6061 of the welding changed. The hardness of weld metal increase in temperature 130°C = 68,72 BHN, in 160°C = 70,51 BHN, and in 190°C = 76,26 BHN. Tensile strength also increase in temperature 130°C = 158,57 N/mm2, in 160°C = 169,85 N/mm2, in 190°C = 174,50 N/mm2.


Introduction
Aluminium 6061 is a nonferrous metal with excellent cryogenic properties, high fracture toughness, and stress corrosion resistance. It is often used in manufacturing fuel tanks for large launch vehicles such as the Thor and Delta rockets [1][2] [3]. Under heat treatment. It has excellent stress corrosion cracking resistance and can be applied as covering for supersonic aircraft [4]. Variable polarity tungsten inert gas (VPTIG) arc welding is an appropriate process for welding aluminium and it has wide application in various industries, especially aviation and aerospace [5]. The weld metal generally has less hardness and strength than other parts of the joint due to its solidification microstructure. In practical welding structures, the strength shortage in the weld metal is often offset by weld reinforcement. However, the weld reinforcement can induce stress concentration around the weld toe, impairing its reinforcing effects. Furthermore, a partially melted zone exists adjacent to the weld [6][7][8] [9], in which there in near-continuous  phase along the (Al) grain boundary that cause serious deterioration of the PMZ's plasticity. In addition to these two regions of weakness, an overaged zone (OAZ) exists in joints because of the effects of the thermal welding cycle, and the most serious OAZ. Consequently, the joint consists of different zones with different mechanical properties and geometric shapes, In other words. The joint is heterogeneous in both properties and geometry, such that the joint's tensile behavior becomes very complicated. Researchers investigated the geometrical and microstructural effect on tensile behavior of 2219 aluminium alloy fusion welded joints [10] [11]. However, insufficient research has been conducted on the effect of the reinforcement's shape on the welded joint's tensile behavior. In this study, the effect of preheat dan post heat TIG welded joint on mechanical properties were investigated through experimental methods and calculation. Various tensile test and hardness test were conducted on joints with preheat and post heat on welded joint.

Numerical Method
The formula used in the experimental Tensile Test. The formulation for determine tensile strength as: (1) And the formulation for determine percentage of strain as: (2) Modulus of elasticity formulation as:

Hardness test
Calculating the hardness number, the formulation as:  After testing the hardness it shows that hardness increase because of preheat and post heat of 6061 aluminium joint welded. The diagram shown in figure 2.

Tensile test
Tensile test was conducted on the joint according to the standard procedure (6B/T 228.1-2010) with a cross head speed of 2mm/min. The dimensions of the tensile sample shown fig.3 the sample length was 50 mm.  The result diagram of tensile test was shown in figure 4.

Microstructure
Microstructure test conducted by using Reflected Metallurgical Microscopic Type Rax Vision No 545491,MM-10A, 230V, 230Hz, The specimen is 6061 Aluminium alloy which has joint welded by TIG and the figure of micro structure as shown in Figure 5.

Conclusions
Used heat treatment as preheat and post heat on 6061 aluminium used TIG joints increase the hardness and increase tensile strength. The highest in 190 o C preheat and post heat, and the strain increase but decrease elasticity modulus, then decrease the toughness of material. Preheat and post heat treatment increase solubility of Mg2Si, as a result increase hardness of specimen. The microstructure of weld point and HAZ look different due to weld point influenced by ER5356 as a filler.