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TitleIntegrated modeling of friction stir welding of 6xxx series Al alloys
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Total Pages89
Table of Contents
                            1 Introduction
	2 The friction stir welding process
		2.1 Principle of the friction stir welding process and its main features
		2.2 Microstructural zones in a friction stir weld
	3 Individual model description and validation
		3.1 Thermal model of the friction stir welding process
			3.1.1 Overview of literature on thermo-mechanical process models Thermal models neglecting material flow Models accounting for the convective heat flow Models accounting for material flow with coupling Contact conditions at the workpiece/backing plate interface
			3.1.2 Thermal model for an integrated process model Heat dissipated by friction at the tool/workpiece interface – surface heat sources Heat dissipated by plastic deformation – volume heat sources Distribution of mechanical power between volume and surface heat sources Material convection around the friction stir welding tool Contact condition at the workpiece/backing plate interface Model parameters
		3.2 Microstructure evolution in 6xxx series Al alloys
			3.2.1 Hardening precipitation in 6xxx series Al alloys
			3.2.2 Literature review on microstructure evolution models Internal variable approach Size class approach
			3.2.3 Microstructure evolution model Rate law Nucleation law Continuity equation and numerical discretization Extension to elongated precipitates Application to 6xxx series Al alloys
		3.3 Strength and strain hardening of precipitation hardened Al alloys
			3.3.1 Yield strength model Literature survey on yield strength models Yield strength model Natural aging model Model parameters
			3.3.2 Strain hardening model Strain hardening behavior of Al alloys Modeling strain hardening in age-hardenable Al alloys Strain hardening model - Dislocation sto Strain hardening model – dynamic recover Final expressions for θ and β Stage IV hardening
		3.4 Damage model for Al alloys
			3.4.1 Non-hardening precipitates in Al alloys
			3.4.2 Damage mechanisms in Al alloys
			3.4.3 Model description Main geometrical void parameters Void nucleation Void growth Void coalescence Influence of a second population of voids on coalescence
			3.4.4 Application to 6xxx series Al alloys
		3.5 Conclusions
	4 Integrated modeling applied to friction stir welding
		4.1 Principles of the integrated chain modeling
		4.2 Validation on friction stir welds in 6xxx series Al alloys
			4.2.1 Base materials
			4.2.2 Validation of the thermal model Measurement of the mechanical power Use of the vertical force to infer the mechanical power Assessment of the thermal model
			4.2.3 Validation of the microstructure evolution model
			4.2.4 Validation of the strength and strain hardening model
			4.2.5 Validation of the finite element modeling of the tensile test on welded samples
			4.2.6 Validation of the damage model
	5 Process optimization
		5.1 Optimization of the welding parameters
			5.1.1 Effects of the advancing and rotational speeds
			5.1.2 Effects of the preheating
			5.1.3 Effects of the cooling rate
			5.1.4 Effects of the material of the backing plate
		5.2 Alloy design
			5.2.1 Effect of the proportion of alloying elements
			5.2.2 Effect of the intermetallics content
			5.2.3 Effect of the alloy state before welding
		5.3 Effects of a post-welding heat treatment
		5.4 Summary
	6 Conclusions
Document Text Contents
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Integrated modeling of friction stir welding of 6xxx
series Al alloys: Process, microstructure and properties

A. Simar, Y. Brechet, B. de Meester, A. Denquin, C. Gallais, T. Pardoen

To cite this version:
A. Simar, Y. Brechet, B. de Meester, A. Denquin, C. Gallais, et al.. Integrated modeling of friction
stir welding of 6xxx series Al alloys: Process, microstructure and properties. Progress in Materials
Science, Elsevier, 2012, 57 (1), pp.95-183. �10.1016/j.pmatsci.2011.05.003�. �hal-00664802�

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a w

Integrated modeling of friction stir welding of
6xxx series Al alloys: Process, microstructure
and properties
A. Simar a, Y. Bréchet b, B. de Meester a, A. Denquin c, C. Gallais c, T. Pardoen a

a Institute of Mechanics, Materials and Civil Engineering, Université catholique de Louvain, B-1348 Louvain-la-Neuve, Belgium

aP/INP Grenoble, Domaine Universitaire, 1130 rue de la Piscine, B.P. 75, 38 402 Saint Martin d’Heres Cedex, France

c ONERA, BP72 – 29 avenue de la Division Leclerc, 92322 Chatillon Cedex, France
Compared to most thermomechanical processing methods, friction stir welding (FSW) is a recent technique
which has not yet reached full maturity. Nevertheless, owing to multiple intrinsic advantages, FSW has
already replaced conventional welding methods in a variety of industrial applications especially for Al alloys.
This provides the impetus for developing a methodology towards optimization, from process to
performances, using the most advanced approach avail-able in materials science and thermomechanics. The
aim is to obtain a guidance both for process fine tuning and for alloy design. Inte-grated modeling constitutes
ay to accelerate the insertion of the process, especially regarding difficult applications where for instance

ductility, fracture toughness, fatigue and/or stress corro-sion cracking are key issues. Hence, an integrated
modeling frame-work devoted to the FSW of 6xxx series Al alloys has been established and applied to the
6005A and 6056 alloys. The suite of models involves an in-process temperature evolution model, a
microstructure evolution model with an extension to heterogeneous precipitation, a microstructure based
ngth and strain hardening model, and a micro-mechanics based damage model. The presenta-tion of each
model is supplemented by the coverage of relevant recent literature. The ‘‘model chain’’ is assessed towards a
wide range of experimental data. The final objective is to present routes for the optimization of the FSW
process using both experiments and models. Now, this strategy goes well beyond the case of FSW, illustrating
potential of chain models to support a ‘‘material by design approach’’ from process to performances.
1. Introduction

Light metallic alloys are facing a fierce competition against structural polymer-based composite
materials in many structural applications, the most obvious being in aeronautical design. The search
for lighter structure keeps motivating investigations towards the enhancement of specific material
properties. If the mainstream effort in the case of Al alloys has long been based on the control of
the microstructure to improve the yield stress, toughness, fatigue resistance and corrosion resistance

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Fig. 18. Evolution of the friction coefficient with the welding parameters for the 6005A-T6 welds.
Table 9 presents the uniaxial tensile properties of the 6005A and the 6056 Al alloys for various heat
treatments states. The T78 treatment is an overaged condition consisting of two successive isothermal
holdings of 6 h at 175 �C and 5 h at 210 �C. The 6005A-T6 base material used in the welds is 10%
weaker than the 6056-T78.

4.2.2. Validation of the thermal model Measurement of the mechanical power. The thermal model presented in Section 3.1.2 has been
applied to welds produced for a variety of conditions both for the alloy 6005A in the T6 temper and for
the alloy 6056 in the T78 temper. The input of the thermal model are the total power Pin (W) for each
welding condition which is calculated based on the rotational speed x (rad/s), the torque Mz (Nm) and
the efficiency g, as P = gPin = Mzx [49]. The torque has been measured during welding [49,63] (most
dedicated FSW machines include a torque measurement). For the 6005A-T6 welds, the efficiency is
estimated by measuring the temperature during welding at two locations on the tool away from
the shoulder surface. The efficiency does almost not vary with the welding parameters and is close
to g � 0.95 [49].

Fig. 17 shows the variation of the torque, represented in terms of the variation of the corresponding
total mechanical power P as a function of the advancing speed for various rotational speeds and for
both alloys. Note that the welding parameters are different in the two alloys.7 A higher rotational
speed induces only a slightly larger power. The effect of the rotational speed on the total power is
more marked in the 6056 welds which could be explained by the more complex geometry of the tool
(i.e. a triflute tool) compared to the 6005A welds which were performed using a simply threaded pin.
The total power increases with increasing advancing speed in the 6005A-T6 welds while no pro-
nounced effect of the advancing speed can be observed in Fig. 17b for the 6056-T78 welds. Note how-
ever that for an advancing speed equal to 1000 mm/min the power of the 6005A-T6 welds and the
6056-T78 welds is similar, probably due to a minor difference in the room temperature yield strength
between the 6005A-T6 and 6056-T78 base materials, see Table 9. The variation of P with respect to the
advancing speed is almost linear P (kW) = 2.66 + 0.00322 � v (mm/min) [49] for low advancing speeds
and tends to saturate for high advancing speeds. Use of the vertical force to infer the mechanical power. When the measurement of the torque is
not available, the measurement of the vertical force (Fz) could be used to infer the total power
7 The welds made of the 6005A alloy were performed with a very large span of welding parameters in order to test the extreme
welding parameters still giving sound welds. The welds made of the 6056-T78 alloy were performed in order to reach the best
tensile performances and hence the lower advancing speeds were not tested.


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Fig. 19. Assessment of the thermal model for various welding parameters applied to the 6005A alloy welds, see also Ref. [63].
Maximum temperatures at 7.2, 14 and 20 mm from the weldline on the advancing side of the weld.


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