Characterisation of the cold metal transfer (CMT) process and its application for low dilution cladding

The process characteristics of the synergic cold metal transfer (CMT) process have been examined for welding aluminium alloy. Utilising a simple backlighting system and through the arc monitoring the droplet transfer modes were identified. Whilst the modified short circuit mode was evident for the lower parameter range, a two part transfer mode based upon a combination of spray and short circuit transfer was observed for the mid to upper parameter range. The technology was also explored as a cladding process for applying to ternary alloyed (Al–Cu–Mg) aluminium plate. This alloy system is known to be susceptible to solidification cracking when MIG welded using the binary Al-2319 (Al–Cu) filler wire, this being due to the wide element freezing range of the weld resulting from mixing with the base material. Utilising this filler, weld dilution ratios for both CMT and pulsed welding were identified across the examined parameter range. The CMT process exhibited greater control of dilution that enabled deposition of a quasi-binary (Al–Cu) layer exhibiting a less crack susceptible composition. Onto this layer conventional MIG welding could be applied which could potentially eradicate cracking using a binary fi

Microstructure of interpass rolled wire + arc additive manufacturing Ti-6Al-4V components

Mechanical property anisotropy is one of the issues that are limiting the industrial adoption of additive manufacturing (AM) Ti-6Al-4V components. To improve the deposits’ microstructure, the effect of high-pressure interpass rolling was evaluated, and a flat and a profiled roller were compared. The microstructure was changed from large columnar prior beta grains that traversed the component to equiaxed grains that were between 56 and 139 μm in size. The repetitive variation in Widmanstätten alpha lamellae size was retained; however, with rolling, the overall size was reduced. A “fundamental study” was used to gain insight into the microstructural changes that occurred due to the combination of deformation and deposition. High-pressure interpass rolling can overcome many of the shortcomings of AM, potentially aiding industrial implementation of the process.EPSRC, AirBu

Investigation of low current gas tungsten arc welding using split anode calorimetry

Most previous split anode calorimetry research has applied high weld currents which exhibit pseudo Gaussian distributions of arc current and power density. In this paper we investigate low current arcs and show that both the current and power distributions have minima in the centre – varying significantly from the expected Gaussian profile. This was postulated due to the formation of the arc with the copper anode and the tungsten cathode. Furthermore, a number of parameters were varied including the step size between measurements, anode thickness and anode surface condition as well as cathode type and tip geometry. The step size between measurements significantly influenced the distribution profile and the anode thickness needed to be above 7 mm to obtain consistent results

Morphology investigation on direct current pulsed gas tungsten arc welded additive layer manufactured Ti6Al4V alloy

The effects of pulsed gas tungsten arc weldingparameters on the morphology of additive layer manufacturedTi6Al4V has been investigated in this study. Thepeak/ base current ratio and pulse frequency are found tohave no significant effect on the refinement of prior betagrain size. However, it is found that the wire feed ratehas a considerable effect on the prior beta grainrefinement at a given heat input. This is due to the extrawire input being able to supply many heterogeneousnucleation sites and also results in a negative temperaturegradient in the front of the liquidus which blocks thecolumnar growth and changes the columnar growth toequiaixal growth

Dissimilar metal joining of stainless steel and titanium using copper as transition metal

Joining of stainless steel and titanium dissimilar metal combination has a specific interest in the nuclear industry. Due to the metallurgical incompatibility, it has been very difficult to produce reliable joints between these metals due to the formation of FeTi and Fe2Ti types of intermetallic compounds. The metallurgical incompatibility between both materials is enhanced by the time–temperature profile of the welding process used. Brittle intermetallics (IMCs) are formed during Fe–Ti welding (FeTi and Fe2Ti). The present study uses the low thermal heat input process cold metal transfer (CMT), when compared with conventional GMAW, to deposit a copper (Cu) bead between Ti and stainless steel. Cu is compatible with Fe, and it has a lower melting point than the two base materials. The welds were produced between AMS 4911L (Ti-6Al-4V) and AISI 316L stainless steel using a CuSi-3 welding wire. The joints produced revealed two IM layers located near the parent metals/weld interfaces. The hardness of these layers is higher than the remainder of the weld bead. Tensile tests were carried out with a maximum strength of 200 MPa, but the interfacial failure could not be avoided. Ti atomic migration was observed during experimental trials; however, the IMC formed are less brittle than FeTi, inducing higher mechanical properties.EPSR

Performance testing of a low power consumption wireless sensor communication system integrated with an energy harvesting power source

This paper presents the performance testing results of a wireless sensor communication system with low power consumption integrated with a vibration energy harvesting power source. The experiments focus on the system’s capability to perform continuous monitoring and to wirelessly transmit the data acquired from the sensors to a user base station, completely battery-free. Energy harvesting technologies together with system design optimisation for power consumption minimisation ensure the system’s energy autonomous capability demonstrated in this paper by presenting the promising testing results achieved following its integration with Structural Health Monitoring (SHM) and Body Area Network (BAN) applications

Parameters controlling weld bead profile in conduction laser welding

In laser welding and other processes, such as cladding and additive manufacturing, the weld bead geometry (depth of penetration and weld width) can be controlled with different parameters. A common practice is to develop process parameters for a particular application based on an engineering approach using the system parameters i.e. laser power and travel speed. However, in such a case the process is optimised for a particular system only. This study is focused on understanding of the phenomena controlling the weld profile in conduction welding for a wide range of beam diameters from 0.07 mm to 5.50 mm. It has been shown that the weld bead geometry can be controlled by the spatial and temporal distribution of laser energy on the surface of workpiece, such as power density, interaction time and energy density. This means that similar depths of penetration can be achieved with various optical set-ups. It has been also found that it is more difficult to achieve pure conduction welds with small beam diameters, which are typically used in powder bed additive manufacturing, due to high conduction losses and low vaporisation threshold

Application of laser in seam welding of dissimilar steel to aluminium joints for thick structural components

Laser welding-brazing technique, using a continuous wave (CW) fibre laser with 8000 W of maximum power, was applied in conduction mode to join 2 mm thick steel (XF350) to 6 mm thick aluminium (AA5083-H22), in a lap joint configuration with steel on the top. The steel surface was irradiated by the laser and the heat was conducted through the steel plate to the steel-aluminium interface, where the aluminium melts and wets the steel surface. The welded samples were defect free and the weld micrographs revealed presence of a brittle intermetallic compounds (IMC) layer resulting from reaction of Fe and Al atoms. Energy Dispersive Spectroscopy (EDS) analysis indicated the stoichiometry of the IMC as Fe2Al5 and FeAl3, the former with maximum microhardness measured of 1145 HV 0.025/10. The IMC layer thickness varied between 4 to 21 μm depending upon the laser processing parameters. The IMC layer showed an exponential growth pattern with the applied specific point energy (Esp) at a constant power density (PD). Higher PD values accelerate the IMC layer growth. The mechanical shear strength showed a narrow band of variation in all the samples (with the maximum value registered at 31.3 kN), with a marginal increase in the applied Esp. This could be explained by the fact that increasing the Esp results into an increase in the wetting and thereby the bonded area in the steel-aluminium interface

Laser stabilization of GMAW additive manufacturing of Ti-6Al-4V components

GMAW (Gas Metal Arc Welding) of titanium is not currently used in industry due to the high levels of spatter generation, the wandering of the welding arc and the consequent waviness of the weld bead. This paper reports on the use of laser welding in conduction mode to stabilize the CMT (Cold Metal Transfer), a low heat input GMAW process. The stabilization and reshaping of Ti-6Al-4 V weld beads was verified for laser hybrid GMAW bead on plate deposition. The laser beam was defocused, used in conduction mode, and was positioned concentric with the welding wire and the welding arc (CMT). Finally, the results obtained for bead-on-plate welding were applied to an additively manufactured structure, in which a laser-hybrid stabilized sample was built and then evaluated against CMT-only sample. This work reveals that laser can be used to stabilize the welding process, improve the weld-bead shape of single and multiple layer depositions and increase the deposition rate of additive manufacture of Ti-6Al-4 V from1.7 kg/h to 2.0 kg/h

Assessing the effect of TIG alternating current time cycle on aluminium wire + arc additive manufacture

The effect of electrode positive time cycle (% EP) of the alternating current TIG process has been investigated for aluminium wire + arc additive manufacture of linear walls. The study considered the effect on oxide removal, linear wall dimensions, microstructure, mechanical properties as well as the effect on electrode wear. The results showed that the effective wall width was minimum at 20%EP with a corresponding maximum in layer height. It was also observed that increasing the% EP increased the electrode wear rate, which in turn affected the arc stability. Microstructure analysis showed a noticeable increase in the grain size for higher% EP. The study also showed that% EP had no significant effect on mechanical properties. From a heat input analysis, a direct correlation was observed between the arc voltage and the% EP. The study also indicated that there could be other contributing factors to wall dimensions. For aluminium wire + arc additive manufacture of linear walls, minimum cleaning ranged between 10%EP and 20%EP