Control of meltpool shape in laser welding

In laser welding, the achievement of high productivity and precision is a relatively easy task; however, it is not always obvious how to achieve sound welds without defects. The localised laser energy promotes narrow meltpools with steep thermal gradients, additionally agitated by the vapour plume, which can potentially lead to many instabilities and defects. In the past years, there have been many techniques demonstrated on how to improve the quality and tolerance of laser welding, such as wobble welding or hybrid processes, but to utilise the full potential of lasers we need to understand how to tailor the laser energy to meet the process and material requirements. Understanding and controlling the melt �ow is one of the most important aspects in laser welding. In this work the outcome of an extensive research programme on understanding the meltpool dynamics and control of bead shape in laser welding is discussed. The results of instrumented experimentation, supported by computational �uid dynamic modelling give insight into the fundamental aspects of meltpool formation, �ow direction, feedstock melting and the likelihood of defect formation in the material upon laser interaction. The work contributes to a better understanding of the existing processes, as well as development of new range of process regimes with higher process stability, improved e�ciency and higher productivity than standard laser welding. Several examples including, ultra-stable keyhole welding and wobble welding and a highly e�cient laser wire melting are demonstrated. In addition, the authors present a new welding process, derived from a new concept of the meltpool �ow and shape control by dynamic beam shaping. The new process has proven to have many potential advantages in welding, cladding and repair applications.


Introduction
Prevention of defects and control of weld quality is one of the biggest challenges of high productivity power beam welding processes, such as laser welding.A typical melt pool in laser welding is subject to many competing forces, which can be di cult to balance.The highly focused laser energy can easily lead to a steep temperature gradient, which then induces rapid convective ow in order to equalise the temperature in the liquid metal.Several studies showed the underlying mechanisms of different components contributing to the net ow in laser welding, such as Marangoni convection, Buoyancy ow, ow due to friction of vapour gases and capillary ow due to the welding speed 1 .All these components have a major in uence on the nal weld and depending on the viscosity of the material and processing conditions (laser power, travel speed and spot size), a variety of different regimes can be achieved.This means that sometimes altering one of the processing parameters can lead to a remarkable difference in the weld pro le or likelihood of defect formation.Furthermore, unlike in most low productivity processes, in laser welding, the timescales for defect formation and the solidi cation time are very short making an insitu defect detection more challenging 2,3 .
In most scenarios the Marangoni convection is one of the dominant forces in the liquid metal ow and its direction usually follows the temperature gradient.This opens a window for control of this ow and improvement of various aspects of weld quality.Modi cation of the laser pro le or beam shaping is one of the most common approaches of exible control of the energy distribution of a laser beam and hence the temperature gradient.Several different techniques of beam shaping are possible but the most common and industrially viable use diffractive optics 4,5 refractive optics 6,7 or spatial manipulation of bre delivery optics 8,9 .In all cases a precisely engineered manipulation of the optical path redistributes the incoming laser beam and modi es its energy distribution in the workpiece.Positive effect on the microstructural development, bead quality improvement and melt ow modi cation have been demonstrated 8,10,11 .In addition, cyclic oscillation of a circular laser spot by galvo-scanning optics, referred to as wobble welding gained attention in the last years.Several bene ts on the weld pro le and process stability have been reported to be achieved with this technique, mostly in micro-welding applications [12][13][14] .Many publications focus on technical aspects of beam shaping or its in uence on the nal weld, however, there is no general understanding of their effect on the melt pool dynamics and overall process stability.In other means, when it comes to practical aspects of laser welding, it is unclear how to set-up the process in order to operate it in the most stable conditions with well-balanced forces within the meltpool.
In this paper we report the results of an ongoing research programme on the understanding of the phenomena responsible for weld bead formation and feedstock melting in laser welding and additive manufacturing.Various cases from numerical modelling and experimental validations are shown.Two new welding regimes are proposed, and the work draws some useful recommendations for laser users.

Methods
The main aim of this research was to understand the ow behaviour in different processing regimes by means of uid ow modelling and several validation experiments.Various laser systems were used, as summarised in Table 1.Each was optimised for a speci c process, such as conduction welding, laser wire melting, keyhole laser welding, wobble welding, and some new regimes.In all cases two different camera systems were used to observe the metpool, a compact welding camera (Xiris XVC1000) for generic observations and a high speed camera (HSVC NAC/Phantom 710S) with a laser illumination (Oxford Lasers FireBIRD 1000w) for detailed studies of the melt ow, as shown in Fig. 4. In addition, in some cases, tungsten carbide particles were added to the liquid metal to trace the ow pattern.In all cases, pure shield argon was used.

Wobble welding (System 2)
Wobble laser welding experiments were carried out on a 500 W (SPI Lasers) bre laser coupled with a Raylase galvo scanner equipped with a 400 mm lens, as shown in Fig. 2. Pure shield argon was supplied through a ceramic nozzle to provide shielding from ambient air.All wobble welds were carried out in 2 mm thick 1xxx series aluminium plates.

Concentric twin spot welding (System 3)
After understanding the main requirements for stable keyhole welding, a new improved keyhole regime was investigated, referred to as concentric twin spot welding.The experiments required a complex energy distribution pro le, which was achieved with two separate lasers, a 3 kW bre laser (SPI Lasers) with 100 µm spot size and an 8 kW bre laser (IPG Photonics) defocused to 10-15 mm spot size.Both laser heads were mounted onto a Fanuc robotic manipulator, as shown in Fig. 3.The rst beam generated a keyhole regime with deep penetration, whilst the second beam generated a conduction regime with large meltpool around the keyhole.The liquid metal was protected by a specially designed trailing shielding nozzle supplied with pure shield argon.The material welded was S355 steel with a thickness of 10 mm.

Multi energy source MES process
A new laser wire melting process was investigated, which required integration of a 6 kW bre laser delivered by means of a high power galvo scanner (IPG Photonics), a plasma transfer arc source (EWM) with a wire feeder (Dinse).The laser was interfaced with a 5-axis gantry motion system (Aerotech), as shown in Fig. 4. The galvo-scanner was used for beam shaping.The laser spot was being oscillated in a linear pattern with variable transient speed to vary the applied energy density over the programmed pattern, as shown in Fig. 5.The laser spot size and amplitude of oscillation were 4 mm and 20 mm respectively.The average oscillation speed was adjusted to achieve different energy pro les.In this case, due to complexity of the set-up, the whole system was enclosed in a exible argon enclosure lled with pure shield argon.10 mm Ti6Al4V plates and a 1.2 mm Ti6Al4V wire were used as the substrate and feedstock material.

Fluid ow modelling
For all the cases apart from the wobble welding Fluent commercial CFD solver Ansys-Fluent was used to solve the governing equations.The volume of uid (VOF) method was used to track dynamic free surfaces.The liquid Metal was assumed to be a laminar, Newtonian, incompressible uid.The wire movement was described by using the mixture theory and Euler method.All details and boundary conditions can be found in 15 .
All wobble welding cases were solved using a commercial Flow 3D weld package.

Results and discussion
One of the main advantages of lasers is the ability to operate in a variety of regimes and processing conditions.The melt pool development and its stability are one of the primary challenges in laser welding, and can in uence the bead pro le, defect formation and microstructural development.Some regimes are rather simple with only a few variables to control and only one or two major forces acting on the meltpool.An example of such a regime is conduction welding with the surface tension gradient, referred to as a Marangoni convection as the main driving force for the melt ow.In contrast, keyhole or wobble welding require control of several variables and many driving forces can in uence the ow of the liquid metal.Normally, the fewer the components contributing to the net ow, the easier it is to predict and control it.Hence, it is important to understand different regimes separately.

Surface tension regime
In the absence of vaporisation, when the energy density of the laser beam is below the vaporisation threshold for a given material, the melt pool tends to acquire a symmetric shape with a hemispherical fusion boundary, as shown in Fig. 6.The ow velocity vectors follow the temperature gradient, but their magnitude is low.The fusion boundary is slightly asymmetric, due to additional in uence of the welding speed.However, generally, this regime does not experience any complex ow pattern and its direction and magnitude is mostly entirely dependent on the temperature gradient.Hence in conduction welding the intensity distribution pro le of the laser spot has a major in uence on the ow pattern and hence beam shaping is one the most effective ways of controlling it.
The situation is slightly more complex with the addition of feedstock wire.Most liquid metals require signi cant effort to overcome the cohesive force between the molecules as the system tries to minimise its surface energy.Therefore, any interruption of the meltpool by an external object normally results in altering its shape and ow pattern.In Fig. 7 an example of melt ow behaviour in conduction regime with the additional a ller wire is shown.When the droplet is not in contact with the main meltpool, the forces are quite balanced without any dominant direction of the ow.However, as soon as the droplet makes the contact with the meltpool, we can observe a sudden change of its shape and alteration of ow direction.
The droplet and the main meltpool become slightly elongated, the meltpool becomes shallower due to the drag force of the cohesive forces.In addition, the velocity vectors develop directionality, indicating a weak ow towards the wire.As soon as the droplet detaches from the wire tip the meltpool tries to relax to its steady-state shape driven by the surface tension force.
The meltpool experiences periodic interruptions every time the droplet is detached, followed by its relaxation to its nominal shape, before being interrupted again.Note that in the case presented in Fig. 7 the droplet detachment interval was approximately 0.8s.Hence, the duration between two subsequent droplet detachment events was not su cient for the meltpool to achieve fully nominal shape.The fact that a perfectly stable meltpool can be interrupted by the contact with an external object, such as a feedstock wire or a droplet, indicates that the strength of the temperature gradient driven ow in liquid metals is relatively low.

Drag enhanced regime
In kehole laser welding, apart from the Maraggoni convestion, other forces, such as the recoil pressure, friction of escaping gases and capilary force, are amongst other dominant forces inducing motion of the liquid metal.At low welding speeds, the recoil force of the vapour pressure is dominant resulting in a downward ow of the liquid metal, as shown in Fig. 8.This can lead to the accumulation of heat at the bottom of keyhole, which then tends to induce a strong circular ow driven by Marangoni convection to balance the temperature distribution.The downward ow is know to be undesirable due to the high risk of gas entrapment and other defects 16 .Also, this kind of system with a deep and narrow keyhole does not allow for e cient ejection of vapour gases and is likely to generate spatter.
An increase of welding speed is known to cause an inclination of the keyhole and simultaneous change of ow direction.The escaping gases from the bottom of keyhole induce a shear force onto the rear wall of the keyhole, resulting in enlarging its exit diameter, as shown in Fig. 9. On the one hand, this upward ow and increased diameter have a positive effect on porosity mitigation and keyhole stability.On the other hand, the ow pattern can be quite dynamic, resulting in strong vortexes and spatter.This is an example of drag force enhanced melt ow, which counteract the Marangoni convective ow.
A similar principle of elongated keyhole is used in wobble welding.In this regime, a relatively small keyhole is rapidly moved in a periodic pattern to mechanically spread the meltpool and avoid localised overheating of the material.This mechanical oscillation not only affects the spread of the liquid metal but also allows the control of the ow direction.In certain cases, the melt ow due to Marangoni convection is completely suppressed and a new ow by the drag force due to motion of the beam becomes dominant.The ow direction, its magnitude and the meltpool stability are dependent on the beam motion parameters.As shown in Fig. 10a, at optimum conditions, the keyhole is stable despite signi cantly bigger melt pool to keyhole size ratio.The keyhole is well balanced, and both the convective ow and keyhole natural perturbations are mostly suppressed.The right combination of angular velocity and size of the metpool resulted in a slightly elongated keyhole with highly stable meltpool.Note that angular velocity in this case was controlled indirectly by the oscillation frequency and amplitude.Increasing the power (Fig. 10b) or frequency (Fig. 10c) resulted in either too large meltpool or too high angular velocity and led to destabilisation of the system.An increase of laser power resulted in excessive accumulation of energy, which could not be redistributed fast enough even at higher frequency (angular velocity) and led to instabilities.At higher frequency (Fig. 10c) the beam motion induced excessive drag on the meltpool and accelerated the liquid metal beyond the stable limit for surface tension forces, which led to melt expulsion, spatter and dynamic instabilities.This indicates that to successfully apply dynamic control of the keyhole and meltpool, the right velocity needs to be applied for given material viscosity, density and thermal properties.

Balanced drag enhanced regime
As demonstrated in the previous section, keyhole regime can be di cult to get it right.At slow speeds and high powers, the large meltpool surrounding the keyhole can lead to necking and temporary keyhole collapse, which increases the likelihood of keyhole defects.The drag force of escaping vapour gasses from inclined keyhole at high speeds stabilizes the keyhole, but also often leads to melt expulsion and spatter.A stable keyhole requires an optimum exit diameter, which should allow for e cient removal of gases but also it needs to maintain high pressure to enable deep penetration.A stable meltpool requires temperature gradient to be kept to the minimum.Usually, it is challenging to optimise both of these important aspects of keyhole and meltpool stability with a standard laser beam with a Gaussian or a top hat energy pro le.A small spot size, on the one hand, promotes stability of keyhole by minimising size of the meltpool but the small keyhole and high temperature gradient induce strong melt ow.A big spot size, on the other hand, leads to a larger keyhole size with better vapour gas removal ability but the larger meltpool can lead to keyhole collapse.
It is possible to achieve a well-balanced keyhole with a stable meltpool without applying dynamic beam oscillations, just by application of an advanced control of the energy pro le of a laser beam.In an example in Fig. 11, a complex beam pro le with a high intensity central section and a low intensity but high energy outer section was applied.The high intensity central section generated a deep and narrow keyhole, whilst the low intensity high power section provides a unagitated melpool with low temperature gradient.The keyhole is stable, and the liquid metal does not tend to close it.The resulting weld pro le ensures deep penetration and good quality.Another bene t of a bead pro le with small root size and wide top bead is its excellent resilience to overpenetration and root sagging.Note that, in this particular case, this complex beam pro le was achieved utilising two separate laser heads, but a similar effect can be potentially achieved with beam shaping.In Fig. 12, another example with even larger low intensity section is shown.The process exhibited an excellent stability and the resulting weld pro le shows an interesting alternative to normal keyhole welds.

Non-surface tension wire melting regime
Traditionally, in most laser wire melting applications, the laser is operated in conduction regime, where a large laser spot generates the meltpool, into which the feedstock wire is immersed.The small meltpool in keyhole regimes requires high precision in pointing the wire and is less common than conduction regime.However, due to the unidirectional character of the heat conduction in metals, it is impossible to decouple the bead width from dept of penetration, i.e. to achieve wide meltpools with low dilution or other way round.In other words, conduction welds are highly susceptible to 3-dimensional heat ow.However, it is possible to decouple heat conduction from convective ow with the application of a beam shaping and dynamic oscillation.In Fig. 13, an example of beam shaping with the use of galvo scanner, referred to as dynamic beam shaping 17 , is shown.The combination of a multi energy source (MES), consisting of a laser and an arc source, and beam oscillation by a galvo-scanner resulted in a complex energy pro le (Fig. 13a) in the workpiece and a fully controlled melt ow pattern.Evenly distributed energy resulted in a low temperature gradient and hence negligible Marangoni convection.The resulting top beads and cross sections are shown in Fig. 14.Since the ow in the meltpool is almost entirely controlled by the dynamic beam shaping it was possible to tailor it and achieve different bead pro les.When applying a simple oscillation without any sophisticated control of the temperature gradient, a weld-like bead shape with a convex reinforcement was achieved (Fig. 14a).Also a typical shape of the solidi cation line, elongated towards the opposite direction to the welding direction can be seen.This suggest that this process was able to control the weld width, but not to affect the melt ow signi cantly.However, when applying a more sophisticated oscillation pattern with a control of temperature gradient, not only a atter top bead could be achieved, but also the shape of the solidi cation line is completely different, suggesting a non-standard meltpool shape prior to the solidi cation (Fig. 14b).This bead exhibits much atter reinforcement and the solidi cation pattern is narrow in the longitudinal direction, which is not normal for such a large meltpool.This indicates that the ow magnitude in the longitudinal direction was low, which is consistent with the modelling data from Fig. 13c.The lack of strong longitudinal ow meant that the liquid metal could be more easily controlled and transferred to the transverse direction for instance.The comparison of macrographs in Fig. 14 show atter reinforcement and marginally shallower penetration for the sample achieved with the more advanced oscillation (Fig. 14d), but in any way in both cases good quality beads with low aspect of depth to width were achieved, which is not possible with a standard laser beam pro le.This demonstrates a great potential of advanced beam shaping in laser material processing applications.

Conclusions
Melt ow and bead shape development in several laser welding regimes was studied using CFD modelling and high-speed imaging.The following conclusions can be drawn from this research: In conduction laser welding or laser wire melting processes, the main ow is caused by the Marangoni convection, which can be controlled effectively by beam shaping.
In keyhole regime, the melt ow is more complex with several factors contributing to the net ow.Balancing all forces in keyhole regime is more challenging with standard laser beams, which may lead to a narrow operating window for stable processing conditions.In this work, a new approach with concentric twin spots was demonstrated to stabilize keyhole.
Laser wobble welding requires optimum oscillation parameters for a given laser power and type of material.The process should be operated with optimum oscillation velocity for a given amplitude.
A new concept of dynamic beam shaping has been developed and its ability to freely tailor the melt ow demonstrated.

Figure 7 Laser
Figure 7

Figure 10 Effect
Figure 10