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Effect of Initial Microstructure on the Performance of 6XXX Al-alloy Laser Welds: A Computational Study

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Abstract

Laser welding (LW) offers an attractive joining technique for Al-alloys. The performance of laser welds usually suffers from mechanical strength degradation in the heat-affected zone (HAZ). In the present study, the effect of the initial-aged microstructure on the post-welded state of 6XXX Al-alloys laser welds was examined via computational modeling techniques. A well-established and detailed precipitation model was used, coupled with a strength model. The influence of the main process variables for aging heat treatment (time and temperature) and LW (power and speed) on the mechanical integrity of weld joints and specifically in the yield strength profile in the HAZ was analyzed. Also, a simple method for the prediction of the width of HAZ is provided. It is concluded that more coarsened microstructures show better performance (compared with the aged state) due to lower degradation of mechanical strength and narrower width of HAZ on the post-welded state. This study provides a method for the selection of the appropriate process parameters for aging and LW of 6XXX Al-alloys.

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Abbreviations

A :

Lattice constant (m)

b :

Magnitude of Burgers vector (m)

\(\bar{C}\) :

Mean solute concentration in matrix (wt.%)

C 0 :

Initial solute concentration (wt.%)

C eq :

Equilibrium solute concentration (wt.%)

\(C_{i}\) :

Solute concentration in particle/matrix interface (wt.%)

\(C_{\text{p}}\) :

Particle concentration (wt.%)

C p :

Heat capacity (J/kg K)

\(D_{\text{c}}\) :

Diffusion coefficient (m2/s)

\(\bar{F}\) :

Mean obstacle strength (N)

f :

Particles volume fraction (%)

G :

Shear modulus (N/m2)

\(G\left( {D,t} \right)\) :

Growth/dissolution rate of particles (m/s)

\(J\left( t \right)\) :

Nucleation rate (#/m3s)

k :

Thermal conductivity (W/m K)

K :

Boltzmann constant (J/K)

k j :

Constant in solution hardening model (Pa wt.%(−2/3))

\(k_{\text{ppt}}\) :

Constant in precipitation hardening model (Mpa m)

M :

Taylor factor (#)

\(N_{0}\) :

Number of nucleation sites (#/m3)

\(N_{i}\) :

Number of particles of size i per unit volume (#/m3)

\(n\left( {D,t} \right)\) :

Number density distribution function (#/m3m)

Q:

Welding power (W)

r:

Particle radius (m)

\(r^{*}\) :

Critical particle radius for nucleation (m)

\(r_{i}\) :

Particle radius of size i (m)

\(r_{\text{c}}\) :

Critical particle radius for shearing/bypassing transition (m)

\(\bar{r}\) :

Mean particle radius (m)

R g :

Universal gas constant (J/kmol)

R(\(R = \sqrt {x^{2} + y^{2} + z^{2} }\)):

Distance to the center of laser beam (m)

s :

Particle/matrix interface position (m)

t :

Time (s)

t peak :

Time when the peak strength is achieved (s)

T :

Temperature (K)

T 0 :

Ambient temperature (K)

u :

Welding speed (m/s)

\(V_{\text{m}}\) :

Particle molar volume (m3/mol)

w(\({w = x - ut}\)):

Distance in x-direction in a moving coordinate of speed u (m)

xyz:

Cartesian coordinates

\(Z\) :

Zeldovich factor (m)

\(\text{HAZ}_{y}\) :

Width of HAZ

α :

Thermal diffusivity (m2/s)

β :

Constant for dislocation line tension (#)

\(\beta^{*}\) :

Addition rate of solute atoms to critical nuclei (#/s)

\(\gamma\) :

Interfacial energy (J/m)

\(\delta \left( {D - D^{*} } \right)\) :

Delta Kronecker function (#)

\(\Delta G^{*}\) :

Activation energy (J/K)

ΔG 0 :

Energy term related to nucleus/matrix interfacial energy (J/K)

ΔH 0 :

Enthalpy for the Mg2Si formation reaction (J/mol)

ΔS 0 :

Entropy for the Mg2Si formation reaction (J/Kmol)

μ :

Gaussian distribution parameter (m)

ρ :

Aluminum density (kg/m3)

σ :

Overall macroscopic yield strength (Pa)

σ i :

Lattice resistance (Pa)

σ WH :

Work hardening (Pa)

σ GB :

Grain boundary hardening (Pa)

σ ss :

Solid solution strengthening (Pa)

σ p :

Precipitation hardening (Pa)

\(\sigma_{0}\) :

Sum of lattice resistance, work hardening, and grain boundary hardening (Pa)

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Acknowledgments

The author acknowledges Dr. A. Keramopoulos for his useful comments and proofreading.

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    Spiros N. Samaras

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Samaras, S.N. Effect of Initial Microstructure on the Performance of 6XXX Al-alloy Laser Welds: A Computational Study. J. of Materi Eng and Perform 25, 1729–1740 (2016). https://doi.org/10.1007/s11665-016-2037-0

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  • DOI: https://doi.org/10.1007/s11665-016-2037-0

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