Mechanical and microstructure characterization of al 2029 by friction stir welding

Friction stir welding developed and established by the welding institute (TWI) among the all new welding technologies in 1991, and it is used commonly for welding of high strength aluminium alloy such as Al2029, which are difficult to weld by conventional fusion welding technique. Friction welding (FW) is a collection of a series of Friction- based solid state joining processes which can produce high quality of weld of different component with either similar or dissimilar material and has been attracting increasing attention. This work aim is to weld two plates of Al-2029 and to optimize the different mechanical properties of welded material and base material.

1. Introduction 

Friction stir welding (FSW) is a solid-state joining process that uses a non-consumable tool to join two facing workpieces without melting the workpiece material. Heat is generated by friction between the rotating tool and the workpiece material, which leads to a softened region near the FSW tool. While the tool is traversed along the joint line, it mechanically intermixes the two pieces of metal, and forges the hot and softened metal by the mechanical pressure, which is applied by the tool, much like joining clay, or dough. It is primarily used on wrought or extruded aluminum and particularly for structures which need very high weld strength. FSW is also found in modern shipbuilding, trains, and aerospace applications. 
Friction stir processing (FSP)/FSW is a method of changing the properties of a metal through intense localized plastic deformation. This deformation is produced by forcibly inserting a non-consumable tool into the workpiece and revolving the tool in a stirring motion as it is pushed laterally through the workpiece. The antecedent of this technique, friction stir welding is used to join multiple piece of metal without creating the heat affected zone typical of fusion welding. Efficient joints in terms of strength of aluminum matrix composite materials cannot be achieved by fusion based welding method due to the reaction between reinforcements and matrices leading to the formation of brittle secondary phase in the weld pool or decomposition of reinforcements on molten metal [1, 2]. As a versatile material, aluminum matrix composites may be selected as an alternative to high strength aluminum alloys in aero engines and aerospace structures like fins, wing and fuselage. In 2001 NASA used composite aluminum AL-Li 2195 rather than aluminum alloy Al2219 for the external fuel tank of space shuttles leading to a reduction of weight by 3400 kg. The saving in weight increases the cargo capacity of space shuttles and enables it to transport more than one components in a single flight to the international space station [3]. Titanium alloy are used extensively in the aerospace industry due to their excellent structure efficiency and good high temperature strength. Welding is an effective way to produce a structure with complex geometry and multiple components. Titanium alloys are readily fusion weld able. However, some problems associates with fusion welding of titanium alloys include porosity, distortion and formation of coarse cast grain structure [4, 5].  A large number of tool deformations occurred during the first 3 inch length welding trial, the tool configuration changed slightly due to reduced stress on the deformed pin part and tool size and weight decreased continuously. The stress induced cracks were responsible for the majority of tool weight loss. 
The defect were found in a region with bimodal microstructure of α+ transformed β phase, which indicate local processing temperature below β- transus due to low heat input [6]. The heat affected zones of a friction stir weld of aluminum alloy 7050-T651 were investigated and compare with the unaffected base metal. Composition of 7050-T651 are 5.7-6.7 Zn, 1.9-2.6 Mg, 2.0-2.6 Cu, 0.08-0.115 Zr. The rotation speed of pin was 350 rpm and travel speed was 15mm/min. Compared to parent material microstructure, the strengthening precipitates have coarsened severely and the precipitate free zone along the grain boundaries has increased by factor of five during friction stir welding, The original base metal grains structure is completely eliminated and replaced by a very fine equiaxed grain structure in the dynamic re-crystallized zone (DXZ). The DXZ consisted of re-crystallized, fine equiaxed grains on the order of 1-4 µm in diameter. Most of the DXZ grains contained a high dislocation density with various degree of recovery from grain to grain [7]. Friction stir processing has been successfully used formation of nano grains and increase the mechanical properties i.e. surface hardness, wear resistance, tensile and fatigue strength.

It was observed that when there is increment on travelling speed, hardness value will also be increased. However increased rotation speeds resulted in lower hardness value at the same travelling speed. Processing parameter including tilt angle and target depth are crucial produce sound and depth free processed region. Friction stir processing result insignificant temperature rise with in and around the weld. A temperature rise of 400-5000C has been recorded within the stir zone for aluminum alloy. The temperature rise result in significant micro structural evaluation i.e. fine re-crystallized grains of 0.1-18 mm, texture, precipitate dissolution and coarsened and residual stress with a much lower magnitude [8].

The tensile strength of the joint is lower than that of the parent metal and it is directly proportional to the travel / welding speed. Welding parameter such as tool rotation, transverse speed and axial force have a significant effect on the amount of heat generated and strength of FSW joints. Microstructure evaluation of FSW joints clearly shows the formation of new fine grains and refinement of reinforcement particles in the weld zone with different amount of heat input by controlling the welding parameter [9-11].
A mathematical model is developed to quantitatively analyze the material flow and heat transfer during RDR-FSW process. It takes into consideration the heat generated by friction and plastic deformation. The effect of reverse rotating tool pin and assisted shoulder on plastic material flow and heat transfer is numerically simulated. Because the tool pin and the assisted shoulder rotate with opposite direction independently, the material flow mode is more complex than that in conventional FSW, and different material flow patterns appear at different horizontal planes along the plate thickness direction during RDR-FSW. The effects of tool pin and reverse rotating-assisted shoulder on material flow and heat transfer are analyzed [12].
Friction stir welding technology requires a thorough understanding of the process and consequent mechanical properties of the weld in order to be used in the production of component for aerospace application. For this reason, detailed research of friction stir welding is required. Friction stir welding can be used to join a different member of material, the primary research and industrial interest has been join aluminum alloy. Defect free welds with good mechanical properties have been made in a wide variety of aluminum alloys, thickness from 1 mm to more than 35 mm will not be welded by Friction stir welding. In addition, friction stir weld can be accomplished in any position. [13-19]. The ultimate tensile strength and hardness of bimetallic weld joint increases by increasing the pre-stress, and ductility was decreases when thermal loading increases. For preventing brittle failure behavior of carbon steel the value of pre-stress and thermal stress should be low as possible [20-22].

Experimental Setup of FSW

The experiments have been carried out on the Friction stir welding machine with necessary equipment details such as tool, process parameter and safety precautions. Process parameter involved here is the tool rotation speed, welding speed, tilt angle and tool geometry. the FSP tool geometry, aluminium alloy 2029 plates, friction stir welding machine, processed zone and various tool manufactured to perform the desired experiments. The process of FSP begins with the tool design and fabrication. The main and the crucial thing of this project were the tool design for friction stir processing process, which would fix in the available Friction stir welding machine shank. Initially FSP tool designed in such a way that the tool geometry was very simple with cylindrical tool, shank dia-25 mm, shoulder dia-20 mm, pin dia-8 mm, pin lenth-4 mm.

The objectives were achieved through a selection of the experimental matrix including all material selection, friction stir processing parameters, conducting a thorough microstructure analysis, and performing representative mechanical tests.

Tool geometry

By using this tool geometry, generated forces during penetration of tool at a 778 rpm or processing were huge in the magnitude. Then we go for modification in tool geometry to reduce the huge forces at the time of penetration by using a threaded pin. This led to tunnel formation in the processed region in the form of defect and it was the observed that the heat generated was very high and in order to reduce this frictional heat the shoulder diameter was reduced to18mm and pin diameter reduced to 8mm.

This article is published in peer review journal and open access journal, International journal of research in engineering and innovation (IJREI) which have a high impact factor journal for more details regarding this article, please go through our journal website.

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