INFLUENCE OF FRICTION STIR WELDING PARAMETERS ON THE MICRO STRUCTURAL AND MECHANICAL PROPERTIES OF

ALUMINUM– METAL THIN WELDS

Seminar Report

Submitted in partial fulfillment of the requirements for the award of the degree of

Bachelor of Technologyin

Mechanical Engineering

by

BIBIN P (Roll No: B100005ME)

Department of Mechanical Engineering

NATIONAL INSTITUTE OF TECHNOLOGY CALICUT

JANUARY 2014

CERTIFICATE

This is to certify that the report entitled “Influence of friction stir welding

parameters on the micro structural and mechanical properties of Aluminum–

metal thin welds” is a bonafide record of the Seminar presented by BIBIN P (Roll

No : B100005ME)’ in partial fulfillment of the requirements for the award of the

degree of Bachelor of Technology in Mechanical Engineering from National

Institute of Technology Calicut.

Dr. M D Narayanan

(ME443S- Seminar)

Dept. of Mechanical Engineering

Dr. Allesu Kanjirathingal

Professor & Head

Dept. of Mechanical Engineering

Place: NIT CalicutDate: 3/02/2014

ABSTRACTAl–Li alloys are characterized by strong anisotropy. 2198 Al–Li sheets were joined via Friction Stir Welding (FSW) in parallel and orthogonal direction with respect to the rolling one. The material microstructure and the different phases were individuated by means of TEM observations in different sections ofthe produced joints; in addition, the mechanical properties were evaluated by means of tensile and fatigue tests at room temperature; the fatigue tests were conducted in axial control mode with R=r min/r max= 0.33 for different welding conditions. The crack initiation and propagation in the welded zone was also studied by applying thermo elastic stress analysis (TSA) during cyclic fatigue tests, employing single edge notched specimens. Thermo elastic data were used to measure the principal stresses and principal strains on the specimens surface around the crack tip, according to growth rate; all the results were validated by employing finite element analysis (FEM) to model the crack evolution.The prediction of residual stresses is a relevant and, under many points of view, still open issue for a proper welding process design. In the present paper a 3D FE model, with general validity for different joint configurations, was used to simulate the Friction Stir Welding (FSW) process of butt joints through asingle block approach. The model is able to predict the residual stresses by considering thermal actionsonly, thanks to a new time efficient approach. A good agreement between calculated and experimentallymeasured data was found; the effectiveness of the presented numerical procedure was evaluated bycomparing the calculation times of the proposed method with the ones of already known FE approaches.

CONTENTS

LIST OF ABBREVIATIONS

LIST OF SYMBOLS

LIST OF FIGURES

LIST OF TABLES

CHAPTER 1

INTRODUCTION

1.1 INTRODUCTION

Friction-stir welding (FSW) is a solid-state joining process (the metal is not melted) that uses a third body tool to join two faying surfaces. Heat is generated between the tool and material which leads to a very soft region near the FSW tool. It then mechanically intermixes the two pieces of metal at the place of the join, then the softened metal (due to the elevated temperature) can be joined using mechanical pressure (which is applied by the tool), much like joining clay, or dough. It is primarily used on aluminium, and most often on extruded aluminium (non-heat treatable alloys), and on structures which need superior weld strength without a post weld heat treatment. It was invented and experimentally proven at The Welding Institute UK in December 1991. TWI holds patents on the process, the first being the most descriptive.

1.1.1 Principle of operation

A constantly rotated non consumable cylindrical-shouldered tool with a profiled nib is transversely fed at a constant rate into a butt joint between two clamped pieces of butted material. The nib is slightly shorter than the weld depth required, with the tool shoulder riding atop the work surface.

Frictional heat is generated between the wear-resistant welding components and the work pieces. This heat, along with that generated by the mechanical mixing process and the adiabatic heat within the material, cause the stirred materials to soften without melting. As the pin is moved forward, a special profile on its leading face forces plasticised material to the rear where clamping force assists in a forged consolidation of the weld.

This process of the tool traversing along the weld line in a plasticised tubular shaft of metal results in severe solid state deformation involving dynamic recrystallization of the base material.

1.1.2 Micro structural features

The solid-state nature of the FSW process, combined with its unusual tool and asymmetric nature, results in a highly characteristic microstructure. The microstructure can be broken up into the following zones:

The stir zone (also nugget, dynamically recrystallised zone) is a region of heavily deformed material that roughly corresponds to the location of the pin during welding. The grains within the stir zone are roughly equiaxed and often an order of magnitude smaller than the grains in the parent material. A unique feature of the stir zone is the common occurrence of several concentric rings which has been referred to as an "onion-ring" structure. The precise origin of these rings has not been firmly established, although variations in particle number density, grain size and texture have all been suggested.

The flow arm zone is on the upper surface of the weld and consists of material that is dragged by the shoulder from the retreating side of the weld, around the rear of the tool, and deposited on the advancing side.

The thermo-mechanically affected zone (TMAZ) occurs on either side of the stir zone. In this region the strain and temperature are lower and the effect of welding on the microstructure is correspondingly smaller. Unlike the stir zone the microstructure is recognizably that of the parent material, albeit significantly deformed and rotated. Although the term TMAZ technically refers to the entire deformed region it is often used to describe any region not already covered by the terms stir zone and flow arm.

The heat-affected zone (HAZ) is common to all welding processes. As indicated by the name, this region is subjected to a thermal cycle but is not deformed during welding. The temperatures are lower than those in the TMAZ but may still have a significant effect if the microstructure is thermally unstable. In fact, in age-hardened aluminium alloys this region commonly exhibits the poorest mechanical properties.

1. 2 PROBLEM DEFINITIONThe application of FSW technology is in particular dependence on mechanical performances affected by the processing parameters, since fatigue is the principal cause of failure for welded joints; as James et al. showed the different fatigue behaviour of two Al–Mg alloys as a function of welding speed. In addition, Ericsson and Sandtrom and Dickerson and Przydatek showed the variation of fatigue life of AA6082 joints with the welding speed and Al–Mg and Al–Mg–Si alloys plates of different thickness, comparing the results with conventional fusion welding techniques. Recent papers were published regarding the micro structural and

mechanical properties of friction stir welded Al–Li alloys; in particular Hao et al. presented interesting results of tensile and bending mechanical properties as a function of processing parameters in 1420 alloys In the present work, 2198 Al–Li sheets were joined via Friction Stir Welding (FSW) in parallel and orthogonal direction. The mechanical properties were evaluated by means of tensile and fatigue tests at room temperature for different welding conditions; in addition the material microstructure and the different phases were individuated by means of optical and TEM observations in different sections of the produced joints. The crack propagation in edge-cracked specimens beside the welded zone was also studied by applying thermo elastic stress analysis (TSA).

1.3 OUTLINE

CHAPTER 2

REVIEW OF LITERATURE

2.1 SIGNIFICANCE OF MODEL

2.1.1 Materials

The material under investigation was a 2198-T851 aluminium–lithium alloy produced by ALCAN (Toronto, Canada) under the form of rolled sheets of 5 mm thickness with the following composition (wt%): Si 0.03, Fe 0.04, Cu 3.3, Mn 0.01, Mg 0.32, Cr 0.01, Ni 0.01, Zn 0.02, Ti 0.03, Zr 0.11, Pb 0.01, Li 1.0, Al bal. Rectangular plates 200 mm length _ 80 mm width were welded in perpendicular and parallel direction with respect to the rolling one. The employed rotating velocities (in clockwise direction) of the cylindrical threaded tool was 1000 rpm, while the advancing selected speed was 80 mm/min with tilt angle set equal to 2°.

2.1.2 Principles of TSA

The use of TSA is aimed at tracking the crack initiation and growth in relation to the different areas in which it could take place; areas which are affected in various ways by the weld presence. In particular, the stress intensity factor was numerically evaluated taking into account a term including the residual stresses effect (which was calculated apart using a weight function) and subsequently compared to that inferred from TSA.

2.2 K ɪ NUMERICAL EVALUATION FOR CRACK ANALYSES

The J integral is used for the evaluated for the analysis of material response. It is related to the energy release associated with the crack growth and gives the measure of the deformation at the crack tip. In the case of linear materials it can be related to the stress intensity factor.

J= limΓ → 0

∫Γ

❑

n . H . qdΓ

Γ is a contour starting on the bottom surface of the crack and ending at the top surface in anti clockwise direction. The limit Γ→0 indicates that Γ dimension decreases at the crack tip, q is a vector in the direction of the crack growth and ‘n’ is the vector perpendicular to the Γ contour.

The factor H is described by

H=WI−σ δuδx

I is the identity matrix σ is the stress tensoru is the displacement in the x direction.For an elastic behavior of the material, W is the strain elastic energy, while for an elasto-plastic behaviour W is the strain elastic energy plus the plastic dissipation.J-integral can be expressed as:

J= limΓ → 0

∫Γ

❑

¿¿¿

Where Si = σ ij n j is the tensile vector perpendicular to Γ.It was demonstrated that the J-integral defined along a contour surrounding the crack tip is the variation of the potential energy for a virtual extension of the crack:

J=∂ V∂ a

Where V is the potential energy and a is the crack length.

The materials behavior was considered to be governed by the rate-independent theory of plasticity with isotropic hardening. The loads were applied in an incremental way, and the solution to the non linear boundary value problem at each increment wasobtained by a fully implicit update using the Newton–Rapson method.

CHAPTER 3

RESULTS AND DISCUSSIONS

3.1 RESULTS

3.1.1 Thermo-mechanical joints behaviour

The temperature variation as a function of the distance from the welds centreline for both the joining configurations is shown in Fig. 1.

Fig. 1 Temperature profiles, in the T and L configurations, on both the advancing and the retreating sides of the tool

For both the cases, the maximum measured temperature (at a distance of 10 mm from the centreline) was close to 250 ˚C and then decreasing by increasing the distance; for both the welding

configurations the temperature behaviour is similar. In all the measurements, the temperatures in the retreating sides resulted lower with respect to the advancing one, by increasing the distance from the welds centreline. For the L welding configuration, the absolute temperatures resulted higher (in the same zone) than the T configuration. Such different thermal history for the two welding configurations led to different profiles in the micro hardness measurements. In the case of L configuration it seems that the material is less dissipating the thermal energy developed by the tool movement; in fact, for both the value is very close (90–95 HV) for both the welds. By increasing the distance from the centreline in the case of T configuration, the temperature starts to decrease faster than L one and in this case the softening zone appears broader.

Fig. 2. Temperature profiles coupled with the micro hardness values, in the T and L configurations, on both the advancing and the retreating sides of the tool.

3.1.2 Friction Stir Welding microstructure analysis

Fig. 3 Optical micrographs of the cross sections of the studied joints in L and T configuration3.1.3 Mechanical properties

The mechanical strength of the base material results much more higher in the L direction, accompanied with a reduction in ductility and an increase in the necking value, demonstrating the strong anisotropy of such kind of Al–Li alloys. The fatigue specimens have been cut from the rolled plates in a number of 25 for each condition. Both the materials show similar fatigue behaviour with higher number of cycles to failure exhibited by the material in L direction at the same stress level. Such behaviour produces a higher fatigue limit (estimated in 220 MPa for the L direction, 180 MPa for the T direction in terms of stress amplitude). The total strain fatigue behaviour for low cycle numbers was also studied for the rolled sheets in L and T configuration. The alloy exhibits two characteristic strain amplitude ranges, having different slopes in the cyclic stress–plastic strain curve; in the L direction the material fractures at a higher number of cycles with respect to the plastic strain amplitude. The fatigue specimens have been cut from the welded plates for each condition and polished, according to standard procedures and avoiding superficial defects. The bead material in all the welding configurations shows similar fatigue behavior in the low cycle regime, since extremely narrow data differences are produced in terms of stresses, especially in the intermediate region. It can be observed decreasing stressamplitude limits as function of life endurance up to the fatigue limit(4×106 cycles to failure) and such value is similar for both the configuration at a value of the maximum stress equal to 200 MPa. In different configuration and with the same welding parameters it was reached a good combination between the material plasticization during tool rotation and the mixing affect produced by the advancing movement leading to the disappearance of the effects due to the rolling process. The tool advancing action (inclined at a certain angle) is extremely similar to an extrusion process which requires optimal temperature conditions for the better quality of the material in terms of microstructure; consequently, since the resultant fatigue behavior for butt welded joints is directly related to the microstructure, provided that porosities occurrence is avoided and micro-cracks formation is absent, and considering thatstress concentrators are missing, the studied FS Welded joints offer the best fatigue performances only when optimal microstructure configuration are reached. With a revolutionary pitch in the range of 0.07–0.1, the process is in the optimal temperature and strain rates conditions to produce good microstructure quality without defects for butt joints and therefore sound welds are achieved. The longitudinal residual stresses were measured in the cross sections of the welds by X-ray diffraction using the sin2ψ method. It must be observed that the residual stresses can be relaxed for the cut of the mechanical testing specimens, for this reason the residual stresses showed in the present work are related to the measurements performed on the cut material. As a general behaviour, the residual stresses have compressive character by approaching the weld line, changing to a tensile character in the weld zone from the heat affected one. It can be observed that the higher values of residual stresses are achieved in the advancing side of the tool, the profiles show a very similar behaviour in both the configurations with higher values experienced in the case of T joints. The residual stresses values differences depend on the asymmetry of the FSW process; it is demonstrated by several finite element calculations that the higher deformation across the weld line are achieved in the retreating side of the tool when a clockwise direction

is employed for the rotation. Such a higher deformation produces an increase in the temperature respect to the advancing side leading to a softening during the process. The small difference observed in the T and L configuration can be attributed to the fact that in the T configuration the main deformation acts perpendicularly to the main resistant direction of the sheets leading to a hardening respect to the L configuration.

3.2 ANALYSIS

3.2.1 Crack behaviourWhen the material is loaded at high frequency in the elastic region, the thermo elastic data relative to the conduction are linearly proportional to the sum of principal stresses. Crack formation and propagation can be followed in real-time providing continuously the real stress fields around the crack tip, including closure effects. All the stress maps were obtained using the scale factor K calculated following Eq. (2) together with the output signal V from the measurement system in each measurement point and considering the material as isotropic and homogeneous. Eq. (2) requires also adiabatic conditions to hold and it is therefore important to find a way to verify that these are fulfilled, examining carefully the thermal images around the area of crack initiation. In the surroundings of this zone, adiabatic conditions may fail to verify. There are two basic phenomena that lead to a lack of adiabatic conditions and hence a change in phase: heat generation due to plastic work and the presence of high stress gradients. Both are conditions occurring near the crack tip area. The effect of these phenomena on the thermo elastic images is a blurring around the crack tip areawhich makes it difficult to determine crack tip only from observation. The exact location of the crack tip is important when, for example, the stress intensity factor is inferred from thermo elastic measured data. The loss of adiabaticity can be identified from the phase maps and from the examination of the different phase profiles. When adiabatic conditions are achieved the phase is constant and equal to zero. Observing the phase profile three different zones can be identified approaching the crack tip area (I) a region where the phase is constant and therefore adiabatic conditionsare achieved.(II) a region where the phase starts assuming positive values indicating a loss of adiabaticity due to plasticity and high stress gradients.(III) a region where the phase changes to negative until the crack tip is reached, due probably to reverse plasticity effects. From the crack tip onwards the phase sign changes continuously, this fact being attributed probably to contact between the crack faces and background reflections coming from inside the crack. In this way it is possible to calculate the position of the end of region III during the fatigue tests in all the loading conditions relating it to the number of cycles. This data is used to infer the crack length and rate.

The change in the shape of the curves related to the stress fields is therefore due to the lack of adiabatic conditions and then to the different heat generation and conduction due to the plastic deformation of the material in the crack zone subjected to various cyclic loadings. The principal stresses distribution directly measured and normalized

around the crack site for different number of loading cycles. An increase of the stress values around the crack site was observed as increasing the number of cycles as expected. The broadening of stress profiles around the crack by increasing the cyclic loading reveals that the stress concentration zone increase also with stress. The results of ABAQUS simulations were used to calculate the theoretical K ɪ values at different crack lengths. stress intensity factor (SIF) values as a function of the crack lengths were obtained. In such calculations it was not considered the effect on residual stresses on the stress-field maps and consequently on the effective values of the stress intensity factors. To take into account such residual stresses effects it was calculated their contribution to the K ɪ as follow:

K ɪ=∫0

a

S0 ( x )m(x , a)dx

Where

m (x , a )=[1+m1(1− x

a )+m2(1− xa )

2

]

√2 π (a−x)

m1=0.6147+17.1844 ( aW

)2

+8.7822( aW

)6

m2=0.2502+3.2899( aW

)2

+70.0444 ( aW

)6

S0 is the residual stresses profile along the crack path,m(x, a) is a weight function and m1 and m2 its coefficients.

CHAPTER 4

CONCLUSIONS

6.1 CONCLUSIONS

The effect of rolling direction respect to the welding one on the mechanical behavior of FSW Al–Li plates was studied and the results reported in the present paper. The thermal profiles of the material during welds was very similar for both the configurations with a temperature of 250 _C at 10 mm from the centreline to320 _C in the centre of the welds. The TEM observations in different zones of the weld allowed the identification of the different reinforcing phases coupled with the grain size variation, the temperature evolution and the micro hardness profile. The mechanical strength of the base material results much more higher in the L direction respect to the T one accompanied with a reduction in ductility and an increase in the necking value, demonstrating the strong anisotropy of such kind of Al–Li alloys with respect to the rolling direction. It was observed decreasing stress amplitude limits as function of life endurance up to the fatigue limit ¿cycles to failure) and such value was very similar for both the configurations at a value of the cyclic maximumstress of 200 MPa.

Both the L and T welds show a similar behavior in the crack initiation and growth leading to the conclusion that the FSW process eliminates the effect of rolling direction on the fatigue properties of the welds.

REFERENCES

MAIN REFERENCEP. Cavaliere, M. Cabibbo, F. Panella, A. Squillace2198 Al–Li plates joined by Friction Stir Welding: Mechanicaland microstructural behaviorMaterials and Design 30 (2009) 3622–3631ADDITIONAL REFERENCESD.M. Rodrigues, A. Loureiro, C. Leitao, R.M. Leal, B.M. Chaparro, P. VilaçaInfluence of friction stir welding parameters on the microstructuraland mechanical properties of AA 6016-T4 thin welds[ Materials and Design 30 (2009) 1913–1921]G. Buffa, A. Ducato, L. FratiniNumerical procedure for residual stresses prediction in friction stir weldingFinite Elements in Analysis and Design 47 (2011) 470–476