I.M. Richardson

The Theory of Laser Materials Processing

In this chapter some aspects of the interaction of femtosecond laser pulses with metals has been considered and compared with the effects of longer pulses. The most important point is, perhaps, that metals exposed to femtosecond laser pulses are no longer in local thermal equilibrium. On short time scales the electron and phonon subsystem are decoupled and may have very different temperatures. Moreover, the electrons are governed by a nonequilibrium distribution and not by the Fermi-Dirac distribution even for low intensities. In many cases, depending on the value of the electron temperature relaxation time, the parabolic heat conduc-tion (TTM) is no longer suitable for the determination of the electron temperature distribution and should be replaced by the ETTM.

Grain refinement due to grain detachment in electromagnetically stirred AA7020 welds

Abstract For the first time evidence is presented that supports the role of the grain detachment mechanism during grain refinement of aluminium welds as a result of stirring. These grains are localised in the centre of the weld pool and have not been fully melted. The results of energy dispersive spectroscopy analysis of these grains indicate the similarity of their chemical composition to that of the grains in the base metal. This suggests that these grains are the partially melted grains, present at the fusion line, that are brought into the weld pool by the action of stirring.

Kinetics of the martensitic transformation in low-alloy steel studied by means of acoustic emission

Abstract The kinetics of the martensitic transformation in three carbon steels (C60, C70 and C80) have been studied using the acoustic emission (AE) technique. It is demonstrated that the volume fraction of martensite f as a function of time t during cooling can be derived from the measured AE power ℧ 2 , since ℧ 2 ∝ d f /d t . An analysis of results shows that the fraction data as a function of temperature T can be described by the Koistinen and Marburger (KM) equation with high accuracy. This indicates that the nucleation of martensite takes place heterogeneously and that the average volume of martensite crystals is constant over the extent of the transformation. The change in kinetics with carbon content is attributed to the amount of dislocations created in the neighboring austenite, which influences the degree of autocatalysis. Also, the acoustic emission energy generated per unit volume of martensite has been found to scale with the change in dislocation density in the formed martensite as the carbon content is varied.

Keyhole behaviour during laser welding of zinc-coated steel

The production of consistent, high-quality laser welds on zinc-coated steels for the automotive industry remains a challenge. A simple overlap joint geometry is desirable in these applications but has been shown to be extremely detrimental to laser welding because the zinc vapour formed at the interface between the two sheets expands into the keyhole and disrupts fluid flow in the melt pool, which often leads to metal ejection. In this work, laser welding on sheets with various coating thicknesses has been performed and it is observed that the sheets with thick coatings (?20 ?m) show surprisingly good weldability. High speed video camera visualizations of the keyhole provide insight into the keyhole dynamics during the process. It appears that the dynamic pressure of zinc vapour can effectively elongate the keyhole and the process can reach a stable state when an elongated keyhole is continuously present. A simple analytical model has been developed to describe the influence of zinc vapour on keyhole elongation.

Pendant droplet oscillation during GMAW

Abstract This paper deals with pendant droplet oscillation in pulsed current gas metal arc welding (GMAW). Using a constant current power source, bead on plate welds were made on mild steel plates. The frequency of the pendant droplet oscillation was monitored visually (using a high speed video camera) and electronically (by analysis of the voltage signals). The results obtained are compared with the results predicted by a model based on a mass spring system. It was found that the oscillation frequency of small droplets revealed by both monitoring techniques matches that predicted by the model. For larger droplets, the oscillation frequency measured by voltage signal analysis tends to be higher than that revealed by video analysis and that predicted by the model. This implies that during droplet oscillation in (GMAW) the arc behaviour may change and as a result the arc voltage is not any more proportionally related to the arc length.

A stereo vision method for tracking particle flow on the weld pool surface

The oscillation of a weld pool surface makes the fluid flow motion quite complex. Two-dimensional results cannot reflect enough information to quantitatively describe the fluid flow in the weld pool; however, there are few direct three-dimensional results available. In this paper, we describe a three-dimensional reconstruction method to measure weld pool surface features based on a single high-speed camera. A stereo adapter was added in front of the high-speed camera lens to obtain two images in the same frame from different view points at the same time. According to machine vision theory, three-dimensional parameters can be reconstructed based on two such images. In this work, three-dimensional velocity fields have been obtained using this method. Based on the calibration technique employed, the associated error is estimated to be less than 11.4%. Quantitative experimental results are useful for understanding the flow pattern, and possibly for controlling the flow of liquid in the weld pool.

Conceptual model for stress and strain development during welding with trailing heat sink

Abstract It has been proven by several research groups that welding with a trailing heat sink can reduce the buckling distortion of welded thin sheet. However, the transient mechanical behaviour during welding with a trailing heat sink was never fully understood. This paper presents a conceptual model that is capable of explaining both the longitudinal plastic strain and the longitudinal stress development during welding with an active cooling source. The model is based on an existing three bar model for stress and strain development during conventional welding. Comparison with three-dimensional finite element calculations shows that the model gives qualitatively realistic results. The model provides insight into the relevant mechanisms involved in stress development during welding with a trailing heat sink, which is essential for determining the optimum cooling source parameters.

Influence of clamping on distortion of welded S355 T-joints

Abstract Welding almost always leads to some distortion of a structure due to its strongly time dependent and localised heat input. This distortion is usually undesirable and causes problems related to alignment and fit-up tolerances with adjacent structures. Considerable research has been carried out on different mitigation methods to reduce distortion to manageable levels; however, most of this has been for a single, simple set of clamping conditions. In practice, clamping is often required and the choice of restraint conditions and subsequent influence on distortion should be explored. This paper examines gas metal arc welds made on S355 T-joints under different clamping conditions. Measured welding temperatures and subsequent part distortions are shown for two different clamping conditions and compared to simulations. The interaction of the clamps and the workpiece due to the material reaction is examined by recording the transient clamping force. It is shown that the clamping condition strongly affects the welding distortion amplitude and might even alter the distortion mode.

A study of acoustic emission energy generated during bainite and martensite formation

Acoustic emission (AE) signals generated during bainite and martensite formation in steel C45 have been measured, and the AE energy has been correlated with the strain energy accompanying both displacive transformations. The gas tungsten arc welding process was used to vary the volume transformation rates of bainite and martensite formation. The root mean square (rms) voltage Urms of the continuous AE signals was measured during travelling arc welding and after spot welding. Depending on the cooling rate and the mean austenite grain size, martensite or bainite is formed in the weld. After spot welding with moderate arc currents, only martensite was formed during cooling, which was reflected by a peak in the Urms data: the martensite peak. An analysis of the results shows that the AE energy produced during the transformation ( ¯ U 2 m dt) is proportional to the volume Vm of martensite in the spot weld, with proportionality factor km. During travelling arc welding, bainite and martensite formation occur simultaneously and both displacive transformations contribute to the measured AE power at each moment. The AE power due to bainite formation ( ¯ U 2 b ) was calculated using the obtained proportionality factor km and was found to be proportional to the volume rate of bainite formation dVb/dt with proportionality factor kb.

HYBRID LASER/GMA WELDING ALUMINIUM ALLOY 7075

Aluminium alloy 7075 is a heat treatable alloy based on Al-Mg-Zn system (5–6 wt % Zn, 2–3 wt % Mg and 1–2 wt % Cu) with the high strength obtained by precipitation hardening treatment. This alloy has the characteristics of a high thermal sensitivity and a low eutectic liquidus temperature range, which make it difficult to weld. The severe HAZ softening, the cracking in the weld and the material lose by vaporisation are the major problems encountered during fusion welding. In this study, the weldability of AA 7075 (T6) sheet using the hybrid laser/GMA welding process was examined. The hybrid welding set-up comprised a 3 kW Nd:YAG laser and a pulsed GMA welding machine. Both bead-on-plate welding and butt-welding were performed under various conditions. The results indicate that hybrid laser/GMA welding process at the optimised conditions can successfully fusion weld AA 7075. The welds produced by the hybrid welding process have a similar HAZ width as the weld produced by autogenous laser welding. Whereas the hybrid process overcomes the problem of severe top bead undercut created in autogenous laser welding. Transverse solidification cracking, however, was found to appear when the arc power was higher than a certain level. Below the level the welds produced were cracks free. Careful thermocouple measurements of the temperature profile in the workpiece were carried out during the welding. The temperature isotherms show that the solidification cracking was related to the elongation of the mushy zone formed in the high speed welding process.