A process model for the heat-affected zone microstructure evolution in Al-Zn-Mg weldments

Abstract

In the present investigation, process modeling techniques have been applied to unravel the sequence of reactions occurring during welding and subsequent natural aging of Al-Zn-Mg extrusions. The model uses a combination of chemical thermodynamics and diffusion theory to capture the heataffected zone (HAZ) dissolution and aging kinetics, with the particular feature of writing the constitutive evolution equation in a differential form. Separate response equations are then developed to convert the calculated values for the particle volume fraction and the matrix solute content into engineering quantities such as hardness or strength, for a direct comparison to experiments. The model indicates that particle dissolution is the main factor contributing to strength loss during welding. At the same time, growth of the remaining particles occurs during cooling, leading to solute depletion within the aluminium matrix. This, in turn, reduces the precipitation potential and contributes to the development of a permanent soft zone within the partly reverted region after prolonged roomtemperature aging. It is concluded that the combination of a microstructure model with an appropriate heat-flow model creates a powerful tool for alloy design and optimization of welding conditions for Al-Zn-Mg extrusions, and an illustration of this is given toward the end of the article.

A process model for friction stir welding of age hardening aluminum alloys

“…An interesting feature is that these welds do not reveal the characteristic drop in hardness within the partly reverted region of the HAZ, as frequently observed during fusion welding of similar materials. [25] It follows from Figure 10 that the strength recovery due to natural aging occurs more or less evenly across the entire soft zone of the welds. This suggests that the thermal conditions existing in FSW are different from those normally experienced during conventional fusion welding, where the total heat dissipation is significantly greater.…”

“…The models used to capture the HAZ microstructure evolution during welding of 6XXX and 7XXX series aluminum alloys have been adopted from Grong, [21] Myhr et al, [24,[26][27][28] q 0 ϭ ͐ R 0 4 2 PNr 2 dr ϭ 4 3 2 PNR 3 [4] and Bjørneklett et al, [25] respectively. These models use a combination of chemical thermodynamics and diffusion theory to describe the dissolution, reprecipitation, and natuFrom Eq.…”

“…[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] The symbols and units used throughout the article are In recent years, significant progress has been made in the defined in Appendix I. understanding of physical processes that take place during welding of aluminum alloys. [21][22][23][24][25][26][27][28] A synthesis of that knowl-II. MATHEMATICAL MODELING edge has, in turn, been consolidated into process models, which provide a mathematical description of the relation In the following, the different components of the process between the main welding variables (e.g., heat input, plate model are briefly outlined.…”

“…A more detailed description of thickness, and joint configuration) and the resulting weld the solution methods, algorithms, and computer programs properties, based on sound physical principles. [21,24,25,28] The referred to in the article can be found in the Ph.D. thesis of components of such a model will be as follows:…”

Recent Developments in Small-Angle X-Ray Scattering for the Study of Metals and Polymers

“…A major challenge for the optimisation of the properties of the welded part is to understand the response of the precipitates initially present in the microstructure to the highly non-isothermal temperature history experienced in the Heat-Affected Zone (HAZ) during the welding process. [8] A good level of understanding can be reached by combining two types of Small-Angle X-ray Scattering experiments: first, the Heat-Affected Zone can be probed by the X-ray beam, and the precipitate size and volume fraction can be inferred in each point of the microstructure. In parallel, the temperature history experienced by the material is known by a series of thermocouples placed in the sample during the welding process.…”