Welding Metallurgy of

IN view of the interest attaching to the vaporisation and diffusion of solids, the following observations may be worthy of record.

The Diffusion of Solids

Stochastic spectral methods for efficient Bayesian solution of inverse problems

This study is concerned with a correlation between the microstructure and the local brittle zone (LBZ) phenomena in high-strength low-alloy (HSLA) steel welds. The influence of the LBZ on toughness was investigated by means of simulated heat-affected zone (HAZ) tests as well as welded joint tests. Micromechanical processes involved in microvoid and cleavage microcrack formation were also identified using notched round tensile tests and subsequent scanning electron microscopy (SEM) analyses. The LBZ in the HAZ of a mUltipass welded joint is the intercritically reheated coarse-grained HAZ whose properties are strongly influenced by metallurgical factors such as an effective grain size and high-carbon martensitic islands: The experimental results indicated that Charpy energy was found to decrease monotonically with increasing the amount of martensitic islands, confirming that the martensitic island is the major microstructural factor controlling the HAZ toughness. In addition, microvoids and microcracks were found to initiate at the interface between the martensitic island and the ferrite matrix, thereby causing the reduction in toughness. These findings suggest that the LBZ phenomena in the coarse-grained HAZ can be explained by the morphology and the amount of martensitic islands.

Microstructure and Local Brittle Zone Phenomena in High-Strength Low-Alloy Steel Welds

Three-dimensional modeling of the microstructure evolution during metal additive manufacturing

The technique of solving the inverse heat conduction problem to determine the steady state welding conditions corresponding to observable or desired weld features is outlined. The objective function includes calculated and experimental values of the temperature related functions, constraints, and regularisation of unknown parameters. The algorithm used to determine the minimum of the objective function is presented. As an example, the temperature field and its corresponding grain structure have been calculated taking into consideration the latent heat and fluid flow in the laser welding of two aluminium alloys. The weld width and texture orientation have been used as the temperature related functions. The results of the simulations are directly compared with the experimental evidence.

Solution of inverse heat conduction problem for determining heat input, weld shape, and grain structure during laser welding

The technique of solving the inverse heat conduction problem (IHCP) to find the steady state welding conditions by observable or desired weld features is outlined. The objective function is an augmented sum of weighted squares of differences in value between calculated and measured temperature-related functions, constraints and regularisation of unknown parameters. The algorithm for finding the minimum of the objective function is based on the Gauss-Newton method. As examples, the heat input distributions during laser edge welding of aluminium alloys AC 120 (authors’ experiments), electron beam welding of low-carbon and stainless steel and as well as laser welding of aluminium alloy Al-Mg-Li-Cu-Mn-Zn (experimental data found in the published literature) (thin sheets and thick plates) are found. The weld interface, molten pool boundary and weld texture orientation have been used as the temperature-related functions. The results of simulation are directly compared with the experimental evidence. The suggested technique allows evaluating the heat efficiency of the welding process. It can be applied to an optimisation problem.

Inverse Modelling of Fusion Welding Processes

Thermal Processes in Welding

Experiments were carried out to determine the properties of the welded joints in 8 mm thick high-strength steels produced by quenching and tempering and thermomechanical rolling with accelerated cooling (tensile strength 821–835 MPa). The dependence of the strength, elongation, hardness, impact energy and crack opening displacement on the heat input in the range 1.0–0.7 kJ mm− 1 was determined. The results show that the dependence of the strength of the welded joints decreases and that of the elongation increases. The heat input has only a slight effect on the impact energy and crack opening displacement in the heat-affected zone.

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Effect of heat input on the mechanical properties of welded joints in high-strength steels

Based on the Green's function method, a mathematical model allowing for the latent heat of fusion and solidification is developed to describe the steady state, two-dimensional heat flow during welding of thin plates. It is demonstrated that the latent heat has a pronounced effect on shape and size of the weld pool and mushy zone. The thermal efficiency of base metal fusion by a line heat source η t can exceed 0·4839 considerably if the latent heat is taken into account. It is shown that the known simplified approaches for considering the latent heat can introduce large errors into the estimation of η t. The calculated and experimental weld pool shapes are compared.

Victor A. Karkhin

Papers

Solution of inverse heat conduction problem for determining heat input, weld shape, and grain structure during laser welding

Inverse Modelling of Fusion Welding Processes

Thermal Processes in Welding

Effect of heat input on the mechanical properties of welded joints in high-strength steels

Effects of latent heat of fusion on thermal processes in laser welding of aluminium alloys