INVESTIGATION ON FRICTION STIR WELDING OF DISSIMILAR ALUMINIUM ALLOYS AA 7075 AND AA 1200 BUTT JOINTS

ABSTRACT

Dissimilar aluminium alloys of AA1200 and AA7075 have significant applications in automobile sector, aerospace, defence and shipping industries. These alloys faces problems like hydrogen solubility, formation of aluminium oxides, solidification shrinkage etc when welded with fusion welding process. Friction stir welding is the process which is able to successfully weld these dissimilar alloys of aluminium. Process parameters like rotational speed, welding speed, tool design, vertical force plays important role in determining the joint properties. Rotational speed and welding speed has its major role in producing the necessary frictional heat to plasticise the material which on solidification produces the weld joint. After developing workable process parameters (process window) from preliminary welds, the welds are taken on rotational speed of 900 rpm to 2100 rpm and welding speed for 30 mm/min to 90 mm/min using the response surface methodology approach. The tool geometries were varied as tapered tool (TT) and tapered threaded tools (TTT). The tensile test performed shows that the ultimate tensile strength for the TT ranges from 118-152.48 MPa translating to a joint efficiency range of 65.78-84.71%, microhardness ranges from 77-99.72 HV, while for TTT the ultimate tensile strength ranges from 130.52-161.84% translating to a joint efficiency of 72.52-90.32 %, microhardness ranges from 77.8-105.75 HV . Rotational speed of 1500 rpm, Traverse speed of 60 mm/min and Tilt Angle of 2o were obtained as optimal welding parameters for the tapered (TT) and tapered threaded tools (TTT) and Tapered threaded tool weldment gave better mechanical properties. The result of the corrosion test performed revealed that TTT weldment performed better at optimal level in terms of corrosion resistance with a corrosion rate of 0.497 mm/year as against 0.698 mm/year obtained using the tapered tool (TT). The microstructures obtained using both tool geometries revealed presence of onion ring formation which is an indication of mixing or material coalescence at the optimal level of parameters. It was recommended that  compressive  and  bending  strengths  of  the  weldment  of  the  two  dissimilar aluminium alloys should be evaluated, friction stir welding in lap configuration arrangement should be conducted on the two dissimilar alloys for other application purposes, further research should be performed on the effect of residual stress on the weldment and that the effect of the axial load on the mechanical properties of the weldments be investigated.

CHAPTER ONE

1.0       INTRODUCTION

1.1       Background to the Study

Friction Stir Welding (FSW) is a solid state welding process invented and patented by The Welding Institute (TWI) in the United Kingdom in 1991 for butt and lap welding of metals and plastics. It is a joining technique that employs plastic deformation to create solid state joints between wider ranges of materials which are used in the manufacturing industries and can be used for materials that are difficult to weld using fusion welding (Thomas et al., 1991). They have been used in production of vehicles bonnets, wheel rims (Smith et al., 2012) and vessels bulkheads and decks (Gesto et al., 2008) and freezing plants (Midling et al., 1999). The mechanical properties of the friction stir welded materials are higher than the conventional welding joints. Friction stir welding process has been used to successfully weld both similar and dissimilar alloys.  The friction stir welding technology is considered to be the most significant metal joining process due to its environmental friendliness, energy efficiency and its broadness and the process is currently used for many applications and employed in many industries such as aerospace, marine, railway and electrical. The benefits of friction stir welding being that it generates no harmful fumes, no solidification cracking, results in reduced distortion  and  improved  weld  quality  for  the  proper  parameters,  adaptable  to  all positions and are a relatively quiet process (Hussain & Quardri, 2010).

The joining of two dissimilar materials such as aluminium (Al) and copper (Cu) is of great demand for industrial applications. The need to join these materials is due to the thermal and mechanical properties they possess, such as a high corrosion resistance and a high electric conductivity. However, aluminium and copper are difficult to weld using the conventional welding processes due to the thermal properties of both materials. The current conventional welding methods result in the formation of hard and brittle intermetallic phases at the interface of the joint (Akinlabi, 2012). These phases will eventually result in cracks.

The use of friction stir welding to join these two materials result in improved contact surface, improved current flow and less resistance. Friction Stir Welding consumes little energy  and  no  gas  or  flux  is  used,  therefore  making  the  process  environmentally friendly. The improvements lead to energy savings; this will lead to a global energy consumption decrease if the method is implemented on a global scale (Akinlabi, 2012). The friction stir welding technology produces high quality welds but to achieve all these, there are several parameters that need to be addressed during the welding process of materials. The welding process parameters, tool geometry, joint design and heat generation  exert  a  significant  effect  on  the  material  flow  pattern  and  temperature distribution thus influencing the microstructural evolution and the properties  of the materials being welded (Leal et al., 2010). Several researchers have successfully joined aluminium to copper using the friction stir welding process (Akinlabi et al., 2011; Esmaeili et al., 2011; Akinlabi et al., 2012). Tool shoulders are designed in a way such that frictional heat is generated on the surface and subsurface of the specimens being welded. The shoulder and pin combination work hand in hand. In situations where thin sheets are to be welded, the shoulder produces the most deformational and frictional heat. During the welding of thick specimens, the main heating is produced by the pin. The most important parameter of the shoulder is the diameter because it has significant effect on the amount of frictional heat generated (Byung-Wook et al., 2010). The larger the shoulder diameter, the larger the pressure force which causes changes in the weld shape.

The response surface methodology is a collection of mathematical and statistical techniques for experimental model building.  By careful design  of experiments, the objective is to optimise a response (output variables) which is influenced by several independent variables (input variables).

Response surface methodology is a collection of mathematical technique useful for developing and optimizing processes (Myers & Montgomery, 2002). It is extensively applicable in situations where several input variables potentially influence some performance measures or quality characteristics of the process. It implies that performance  measure  or  quality  characteristics  are  called  responses  while  input variables are sometimes called independent variables and are subject to the control of the Scientist or Engineer.

Response surface methodology consists of experimental strategy for exploring the space of the process or independent variables, empirical statistical modeling to develop an appropriate approximating relationship between the yield and the process variables, and optimization methods for finding the values of the process variables that produce desirable values of the response. (Myers & Montgomery, 2002).

1.2       Statement of the Research Problem

The difficulties faced in welding aluminium and other non-ferrous metals are more severe in fusion (conventional welding process) resulting in defects such as cracks and porosity as well as emission of toxic fumes and smokes than in the solid state welding. The friction stir welding process is more suitable for welding aluminium alloys especially in aerospace vehicles.  It has been established that a great number of fasteners and rivets are used in aerospace vehicles, apart from the weight which these fasteners and rivets (1000, 000 rivets on each wing of the aircraft) adds to the aircraft they are equally susceptible to crevice corrosion (Caio-Palumbo et al., 2017). The application of friction stir welding eliminates this additional weight imposed on the aircraft. Also, toxic fumes and smoke is generated with conventional welding techniques but friction stir welding is a green and environmental friendly technology. Lastly, the mechanical properties of the weldment are dependent upon the process parameters and if not properly selected can lead to production of defective weldment. The optimisation of these parameters becomes imperative in order to produce defect- free weldment.

1.3       Aim and Objectives of the Study

The aim of this work is to investigate friction stir welding of dissimilar aluminium alloy butt joints and determine the effect of the welding parameters on the mechanical properties of the weldment, establish a suitable joining parameters for producing defect- free weldment. The objectives are to: –

i.      determine  the  effect  of  friction  stir  welding  parameters  (rotational  speed, welding speed and tool tilt angle) on the mechanical properties of weldment of two dissimilar aluminium alloys.

ii.      determine   single   response   optimal   welding   parameters   and   percentage contributions to mechanical properties of the weldment using (S/N) signal to noise ratio and analysis of variance (ANOVA), respectively

iii.      develop empirical models for the mechanical properties of the weldment based on experimental results.

iv.      determine  multi-  response  optimal  welding  parameters  using  grey  relational analysis (GRA).

v.      evaluate   the    corrosion   behaviour    of   the    optimised    weldment    using potentiodynamic polarisation (PDP).

vi.      examine  microstructure  of  the  optimised  weldment  using  scanning  electron microscopy and optical microscopy.

1.4       Justification of the Study

Light weight and good surface finish are desirable qualities in welding for specific applications especially in aerospace vehicles (João-pedro, 2014). Riveting and conventional (fusion) welding technique applied in aerospace joints are associated with heavy weight and crevice corrosion therefore, resulting in poor joint strength and eventually damage under service condition over time. Also, aluminium alloys are difficult to weld conventionally due to high reflectivity and thermal conductivity, the inter-metallic phase in the conventional (fusion) welding lowers the toughness of the weldment and causes cracks during and after the welding. With friction stir welding, all these defects are reduced in the weldment.

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