The effect of post welded heat treatment on mechanical properties and microstructure of friction stir welded Al 6063 E-Book

Jasveer Singh

• Herausgeber: BookRix
• Kategorie: Fachliteratur
• Sprache: Englisch

Veröffentlichungsjahr: 2018

Beschreibung

Aluminium is the third most abundant element (after oxygen and silicon), and the most abundant metal in the Earth's crust. Aluminium is remarkable for the metal's low density and for its ability to resist corrosion due to the phenomenon of passivation. Structural components made from aluminium and its alloys are vital to the aerospace industry and are important in other areas of transportation and structural materials Welding plays a crucial role or say as a back bone of manufacturing industry to join the components. Friction stir welding (FSW) is a relatively new joining process that has been demonstrated in a variety of metals such as steel, titanium, lead, copper and aluminium. The unique properties of friction stir welds make possible some completely new structural designs with significant impact to ship design and construction. Friction stir welding is especially advantageous for joining aluminium and has been exploited commercially around the world in several industries.   In the present work the effects of welding speed have been investigated on the microstructural and mechanical properties of friction stir welded aluminium alloy 6063. FSW was carried out at rotational speed of 1300 rpm (constant) and transverse speeds of 35, 50 and 65 mm/min. Mechanical performance has been investigated in terms of hardness, wear resistance and tensile strength. To study the effect of post welding heat treatment on properties of friction stir welded joint, the artificial ageing was carried out at 1600 C for a soaking period of 20 hours in the muffle furnace. The study revealed that friction stir welded joint prepared at welding speed of 35 mm/min exhibited better tensile strength, hardness and wear resistance. Better mechanical properties of the joint prepared at welding speed of 35 mm/min may be attributed due to fine, homogeneous and equaxed grain structure of stir zone. Post welding heat treatment of friction stir welded joint improved the wear resistance and microhardness of the joint. However tensile properties deteriorated with the post welding heat treatment of joint.

Leseprobe

Jasveer Singh, Dr. Chandan Deep Singh

The effect of post welded heat treatment on mechanical properties and microstructure of friction stir welded Al 6063

BookRix GmbH & Co. KG80331 Munich

Chapter 1

CHAPTER 1

INTRODUCTION

1.1 Introduction

Friction stir welding (FSW) is an innovative welding process commonly known as a solid state welding process. This opens up whole new areas in welding technology. It is particularly appropriate for the welding of high strength aluminium alloys of the 2xxx and 7xxx series which are extensively used in the aircraft industry. Mechanical fastening has long been favoured to join aerospace structures because high strength aluminium alloys are difficult to join by conventional fusion welding techniques (Pouget. et al., 2007). Its main characteristic is to join material without reaching the fusion temperature. It enables to weld almost all types of aluminium alloys, even the one classified as non-weldable by fusion welding due to hot cracking and poor solidification microstructure in the fusion zone (Sandra et al., 2009). FSW is considered to be the most significant development in metal joining in a decade and is a ‘‘green’’ technology due to its energy efficiency, environment friendliness and versatility. The key benefits of FSW are summarized in Table 1.1 (Mishra et al., 2005).

Table 1.1 Key benefits of Friction Stir Welding

Metallurgical benefits

Environmental benefits

Energy benefits

Solid phase processLow distortion of work pieceGood dimensional stability

and repeatability

No loss of alloying elementsFine microstructureNo shielding gas requiredNo surface cleaning requiredEliminate grinding wastesConsumable materials saving such as rugs, wire or any other gasesImproved materials use (e.g., joining different thickness) allows reduction in weightDecreased fuel consumption in light weight aircraft automotive and ship applications

1.2 Friction stir welding process

The working principle of Friction Stir Welding process is shown in Fig. 1. A welding tool comprised of a shank, shoulder and pin is fixed in a milling machine chuck and is rotated about its longitudinal axis. The work piece, with square mating edges, is fixed to a rigid backing plate and a clamp or anvil prevents the work piece from spreading or lifting during welding. The half-plate where the direction of rotation is the same as that of welding is called the advancing side, with the other side designated as being the retreating side (Nandan et al., 2008). The rotating welding tool is slowly plunged into the work piece until the shoulder of the welding tool forcibly contacts the upper surface of the material. By keeping the tool rotating and moving it along the seam to be joined, the softened material is literally stirred together forming a weld without melting (Rowe et al., 2005). The welding tool is then retracted, generally while the spindle continues to turn. After the tool is retracted, the pin of the welding tool leaves a hole in the work piece at the end of the weld. These welds require low energy input and are without the use of filler materials and distortion.

Fig.1.1 The principle of Friction Stir Welding (Thomas Wayne 1996)

1.3 Microstructural behavior of friction stir welding

The microstructure of a FSW weld depends on a few aspects like rotational and transverse speed, the pressure, the material and the tool design. This makes it difficult to describe the microstructure in general. However, following scheme was developed by TWI (The Welding Institute) and is accepted by the Friction Stir Welding Licensees Association. It divides the cross-section in four parts are shown in Fig1.2.

Fig. 1.2 Cross-section of weld nugget

A: Unaffected material: This is the region on a distance from the weld centre. It’s the region that is not affected by the generated heat. Although the material might have experienced a thermal cycle, it is not affected by this cycle. This means that the microstructure and the mechanical properties aren’t changed. It’s often referred to

as ‘parent material’.

B: Heat-affected zone (HAZ): This is a region a bit closer to the weld center. This has certainly experienced a thermal cycle and the mechanical properties and/or the microstructure is modified by it but it doesn’t show any plastic deformation. In the HAZ the changes in properties are comparable to those in the HAZ for other thermal processes. The shape of the HAZ is typically trapezoidal, as can be seen in above Figure

C: Thermo mechanically affected zone (TMAZ): This region has a change in microstructure and/or mechanical properties. But in contrast to the HAZ, the TMAZ has a plastic deformation. The grain size is similar to the grain size in the parent material.

D: Weld nugget: This is the part of the TMAZ that has been recrystallized. The grain sizes in this weld nugget are smaller than the grain sizes in the parent material.

1.4 Welding variables of friction stir welding

FSW involves complex material movement and plastic deformation. Welding parameters, tool geometry and joint design exert significant effect on the material flow pattern and temperature distribution, thereby influencing the microstructural evolution of material (Mishra et al., 2005). Therefore, welding speed, the tool rotational speed, the tilt angle of the tool, tool material and the tool design are the main independent variables that are used to control the FSW process. The main process parameters and their effects in friction stir welding are given below in Table 1.2

Table 1.2 Main process parameters in friction stir welding

Parameter

Effects

Rotation speed

Frictional heat, “stirring”, oxide layer breaking and mixing of material.

Tilting angle

The appearance of the weld, thinning.

Welding speed

Appearance, heat control.

Down force

Frictional heat, maintaining contact conditions.

1.4.1 Tool rotation and transverse speed

For FSW, two parameters are very important: tool rotation rate (v, rpm) in clockwise or counter-clockwise direction and tool transverse speed (n, mm/min) along the line of joint. The motion of the tool generates frictional heat within the work pieces, extruding the softened plasticized material around it and forging the same in place so as to form a solid-state seamless joint (Wang et al., 2006). As the tool rotates and moves along the butting surfaces, heat is being generated at the shoulder/work-piece and, to a lesser extent, at the pin/work-piece contact surfaces, as a result of the frictional-energy dissipation (Grujicic et al., 2010). The welding speed depends on several factors, such as alloy type, rotational speed, penetration depth and joint type (Sakthivel et al., 2009). Higher tool rotation rates generate higher temperature because of higher friction heating and result in more intense stirring and mixing of material. During transversing, softened material from the leading edge moves to the trailing edge due to the tool rotation and the transverse movement of the tool, and this transferred material, is consolidated in the trailing edge of the tool by the application of an axial force (Kumar et al., 2008)

1.4.2 Tool tilt and plunge depth