Geert Verhaeghe is a Principal Project Leader in the Laser and Sheet Processes Group, with particular experience in the welding of aluminium, robotics and automation for welding, weld distortion, arc and laser process monitoring, hybrid laser-arc processing and welding with high brightness laser sources.
Steve Shi is the section manager of the Laser and Sheet Processes Group at TWI. He has managed projects concerned with laser materials processing and joining of high strength steels, with particular involvement in laser and laser-arc hybrid welding, in-process monitoring and adaptive control of laser welding, resistance and laser welding.
However, as Steve Shi and Geert Verhaeghe report, the accuracy in part fit up required for autogenous laser welding is not always possible to achieve in today's production processes, which require flexibility and automation, while maintaining a high standard of weld quality. More tolerant welding procedures and/or process control must be integrated into automated welding systems to maintain weld quality.
The combination of an electric arc with the laser beam can significantly improve the gap bridging capability of the laser process, but whatever process is used, the first stage in any adaptive process control is to be able to monitor the welding process effectively in order to provide the necessary feedback parameters.
The availability of all the monitoring equipment has certainly contributed to the industrial acceptance of laser welding, particularly in thin sheet metal fabrication as found in the automotive industry, where laser welding is accepted as a process capable of achieving high weld quality with high productivity. But this is not yet the case for medium to thick section fabrications, where laser welding is still not widely exploited. Quality control procedures in these thicknesses are still predominantly based on post-weld destructive testing, which only allow spot checks of the manufactured parts.
This paper describes the result of work carried out at TWI as part of an EU funded project. The main objective of work was to assess the effectiveness of a range of commercially available photodiode based sensors to detect, in real-time, engineered factors causing weld imperfections, when welding T-joints in medium to thick section (6-12mm) steels.
Experimental work
Materials
The steel plate used in this work was 6mm, 8mm and 12mm thick S355J2G3 steel, in accordance with EN10025. A 1.0mm diameter A18 C-Mn steel filler wire (EN440) was used for the hybrid laser-MAG welding trials.
Simulation of weld imperfections
Based on knowledge of laser welding and hybrid laser-MAG welding potential parameter variations, which might be expected to arise in a production, were established. These factors, associated with process parameters, joint preparation and fit-up, are likely causes of different weld imperfections. In different ways, these imperfections were engineered into the T-joint welds to examine the detection sensitivities of the chosen sensors.
For autogenous laser welding, the main weld imperfections investigated were lack of side-wall fusion, lack of root fusion, incomplete penetration, surface pores, porosity, undercut and spatter. These imperfections were simulated by joint face contamination, variations in joint fitup and variations in laser process parameters.
For hybrid laser-MAG welding, the main weld imperfections investigated were lack of side-wall fusion, lack of root fusion, incomplete penetration, surface pores, porosity, undercut, incorrect weld toe, excess weld metal and spatter.These imperfections were simulated by changes to the workpiece geometry, joint face contamination, joint misalignment, hybrid welding configuration and hybrid process parameters.
Experimental set-up
The autogenous laser welding trials were carried out using a 4kW CW Nd:YAG laser, manufactured by Trumpf. The laser beam was delivered via a step index optical fibre, 600µm in diameter, to an output housing using a 200mmfocusing lens, to produce a nominal minimum spot size of 0.6mm in diameter.
A plume suppression gas jet (argon) was angled at 40° to the surface of the workpiece, trailing the laser beam, with an impingement point 1mm above the laser beam focus. A high pressure air knife was used to protect the laser optics during welding. The actual housing and seam tracker were mounted onto a JS30 Kawasaki robot, traversing the arrangement over the specimen, held in a stationary jig, during welding.
Hybrid laser-MAG welding trials were carried out using the same laser, in combination with a Lincoln PowerWave 450 synergic MIG/MAG welding power source. The MAG torch was attached to the laser output housing to provide an arc travel angle of 30° and a 16mm contact tip to workpiece distance.
For hybrid laser-MAG welding, the shielding gas used through the MAG torch was an Ar-20% CO2 mixture. The gas flow rate through the MAG torch, was 15l/min for all the hybrid laser-MAG welding trials.
In-process monitoring of welding
Three different sensors, manufactured by Precitec Optoelektronik GmbH, were used in this work. The first sensor, described by Precitec as the 'plasma' sensor, detects radiation in the wavelength range less than 600nm. The second sensor, referred to as the 'temperature' sensor, is sensitive in the infrared part (1100-1800nm) of the spectrum and was arranged to pick up signals from the molten weld pool. The third sensor, referred to as the 'back reflection' or' reflectivity' sensor, detects a narrow band of radiation centred around the wavelength of 1.06µm (i.e. the laser wavelength) reflected from the workpiece during processing.
The signals from the sensors were amplified as necessary and fed into a Precitec LMW900 weld monitor. The monitor allowed the signals to be displayed as amplitude versus time graphs, for each sensor; the time axis corresponding to distance along the weld. The amplitude of the sensor signals is displayed in arbitrary units. The frequency of sampling was 5kHz.
Experimental results and discussion
Autogenous laser welding with in-process monitoring
Welding trials were initially carried out to develop optimised conditions for achieving fully penetrating welds in 6-8mm T-joints. Imperfection free reference welds were produced and the sensor signals received were recorded during welding. These signals were then compared to welds made containing imperfections to assess the performance of the sensors.
Figure 1 shows the recorded sensor signals from a typical autogenous laser weld produced using the reference laser welding conditions. The particular weld corresponding to the trace in Fig.1, had a smooth top andunder bead and was free from visible imperfections.
Fig.1. Sensor signals from the temperature, and plasma sensors, arising from the reference autogenous laser welding conditions for a 6mm stiffener plate welded to an 8mm base plate:
a) Weld top bead
b) Temperature sensor response
c) Plasma sensor response
Figure 2 shows the sensor signals received from a T-butt weld produced using the reference set of welding conditions but with silver solder wire added to two areas in the joint. Both the recorded plasma signal and temperature signal showed changes corresponding to the placement of the wire, which resulted in the formation of surface pores, which can be seen in Fig.2a.
b) Temperature sensor response
c) Plasma sensor response
Tests were also carried out to simulate different imperfections by changing process parameters and joint fit-up conditions during laser welding, such as changes in laser beam focus (causing incomplete penetration), beam to joint misalignment (causing incomplete penetration and lack of fusion), and variation in plasma suppression gas flow rate (causing incomplete penetration).
It was noted that, for this particular joint/material combination, the temperature sensor detected most of the changes in joint fit up, process parameters and contamination of the joint face. It provided responses to added solder wire and grease (causing surface pores and excessive spatter), with step changes in the amplitude of the signal. The responses to the changes in characteristics of the optical signals caused by addition of wire and grease to the joint were different. The temperature sensor showed a step decrease in the area with added solder and a step increase in areas with applied grease.
The temperature sensor was also sensitive to variations in laser focus position and changes in laser beam position on the stiffener plate (caused incomplete penetration and lack of root fusion in welds). This sensor also responded to changes in laser focus (causing incomplete penetration), with gradual increase in the amplitude of the signal as the laser was moved away from the optimised focus position.
For autogenous welding the plasma sensor showed a similar capability to detect variations in emitted radiation caused by changes in joint fit up and surface condition as the temperature sensor, apart from not being able to detect the results of adding grease (causing excessive weld spatter). The combination of the plasma sensor and the temperature sensor was able to detect changes in the emitted radiation corresponding to all the simulated imperfections studied.
Sensor performance during hybrid laser-MAG welding
Similiar to autogenous welding, hybrid laser-MAG welding trials were carried out to develop optimised welding conditions for achieving fully penetrating welds on 8-12mm T-joints. Sensor signals recorded from the reference weld and welds with engineered imperfections were compared to assess the performance of these sensors for monitoring weld imperfections during hybrid laser-MAG welding.
Figure 3 shows the recorded sensor signals from a typical weld produced using the reference hybrid laser-MAG welding conditions. The weld had a smooth top and under bead and was free from visible imperfections. A smooth response in signal was recorded for the full length of the weld.
b) Temperature sensor response
c) Reflectivity and plasma sensor response
Figure 4 shows sensor signals received from a hybrid weld produced by changing the axial position of the beam focus along the beam incident axis from 2mm at the weld start to 12mm at its end. It is clear that all three sensors were sensitive to this change in laser focus position. Visual examination of the resulting weld indicated that the weld bead profile was similar to that of the reference weld for a laser beam focus position from 2 to 4mm above the stiffener plate, i.e. for the first third of the weld. There was little change in the sensor signals in this part of the weld (Fig.4a). In the second third of the pass, the weld still showed full penetration, but on the top of the weld, incorrect weld toe could be seen. In this region, the amplitude of all three detectors can be seen to rise. In the final part of the weld, the top bead became wider and more concave and weld penetration was eventually lost. In this region, the amplitude of the signals continued to rise for all three detectors, but no obvious step-change in signal could be seen on the change from full to partial penetration.
Fig.4. Sensor response from a hybrid laser-MAG weld made at the reference conditions but changing the axial position of the beam focus from 2mm away from the joint position at the start of the weld, to 12mmaway from the joit position at the end of the weld.
a) Weld top bead
c) Temperature sensor response
d) Reflectivity and plasma sensor response
Tests were also conducted to simulate various imperfections; by introducing contamination into the joint, variations in joint fit-up and hybrid laser-MAG welding parameters. This induced undercut and irregular weld bead profile and beam to joint misalignment. It also caused incomplete penetration and lack of fusion. Variations in wire feed rate introduced excess weld metal. Variations in arc voltage introduced weld spatter and irregular weld profile and changes in shielding gas flow rate induced porosity.
For hybrid welding, the plasma sensor exhibited clear responses to changes in joint fit-up, surface condition and process parameters causing undercut, incomplete penetration, excessive weld metal, excessive weld spatter and possibly internal porosity. The largest response from the plasma sensor was from the added silver solder, causing regions of undercut, and changes in arc current (wire feeding rate) resulting in the formation of excess weld metal.
Conclusions
The following conclusions could be drawn from this work:
The laser process monitoring system has shown itself to be an indicator of various factors causing weld imperfections during autogenous laser welding. The plasma sensors were able to pick up factors causing surface pores, incomplete penetration, and undercut during laser welding. The temperature sensor showed a similar behaviour to the plasma sensor, in terms of response to weld imperfections caused by changes in beam to joint alignment, but gave a larger response to joint contamination (causing excessive weld spatter) than the plasma sensor.
The largest response seen from both the plasma and temperature sensors was from factors causing surface breaking pores and incomplete penetration. For this particular joint/material combination, it was also possible to detect various factors causing weld imperfections during hybrid laser-MAG welding. The temperature and plasma sensors were able to detect the results of changes in arc parameters (current, voltage and shielding gas flow rate) causing excess weld metal, excessive weld spatter and incorrect weld toe, changes in beam to joint alignment and variation in axial focus position, resulting in undercut and incomplete penetration. The plasma sensor also showed responses to insufficient gas shielding, causing porosity in the weld. Both sensors exhibited the largest response to variations in arc parameters causing excess weld metal and spatter. The reflectivity sensor showed significant response to changes in laser focus position, resulting in incomplete penetration.
