Properties of alloy 625 claddings made with laser and CMT methods
Research output: Book/Report › Doctoral thesis
|Publisher||Tampere University of Technology|
|Number of pages||182|
|State||Published - 23 Nov 2018|
|Publication type||G4 Doctoral dissertation (monograph)|
|Name||Tampere University of Technology. Publication|
Cladding, overlay welding, or weld overlay, is a method where some part of a metal component is coated with a layer of another metal, to obtain the better, more desirable properties of corrosion or wear resistance to this layer. By these means it is possible to get better properties to the section of the component that needs to withstand higher corrosion and/or wear load.
Two methods, were examined in this study, laser cladding with powder as a filler material and arc welding with CMT process using wire as a filler material. Laser cladding is a process whereby metal powder is injected into a laser beam through either coaxial or lateral nozzles. The laser beam melts the powder and a thin layer of the substrate surface. The melt solidifies very rapidly and forms a fine-grained microstructure with good mechanical and corrosion properties. CMT is an acronym for Cold Metal Transfer, a process developed and owned by the Austrian welding power source manufacturer, Fronius. The CMT process is an advanced, digitally controlled version of the traditional MIG/MAG arc welding process. The characteristic feature of the CMT process is cyclic retraction, a backward motion of the wire with a frequency of 70–80 Hz.
Laser cladding tests were performed with several alloy 625 powders as feedstock material. The tendency of these powders to produce hot cracks was studied with single bead tests. It turned out that there were variations in the content of the elemental impurities carbon, phosphor, sulphur and boron, as well as in the silicon, manganese, aluminium and titanium content of the powders. Powder that was free of aluminium and titanium, but contained a relatively large amount of boron turned out to be very crack sensitive. It was proved that the aluminium and titanium content of the alloy 625 feedstock powder affects the direction of the melt flow in the melt pool. Aluminium and titanium free powder produces an inward flow in the melt pool that leads to deeper penetration, steeper thermal gradients and a columnar dendritic structure, so that the last of the melt to solidify occurs at the weld centreline and hot cracks may form before that part of the melt which has a lower solidification temperature has totally solidified. In contrast, alloy 625 feedstock powder containing some aluminium and titanium leads to an outward melt flow in the melt pool, which in turn leads to shallower penetration, a lower temperature gradient and more equiaxed dendritic structure, so that the last of the melt to solidify melt is not at the weld centreline.
The applicability of using the CMT process in cladding was studied with single and adjacent beads with a stringer motion by using Ø1.2 mm alloy 625 wire as a feedstock material. Some tests were also conducted with the conventional MIG and pulsed MIG processes especially aimed at comparing the actual process power in all these three types of MIG processes: conventional MIG, pulsed MIG and CMT. The results of these tests showed that the actual arc power is lower with the CMT process than it is with conventional MIG and pulsed MIG processes, although with the CMT process the actual arc power determined with AIP method was clearly higher than that determined with the common method where average the current is multiplied with the average voltage.
The CMT process allows the use of a relatively high travel speed of 1000 or 1200 mm/min. This is a relatively high travel speed for the arc welding process, but not for the laser cladding process. Higher travel speeds, in the range of 1400-1500 mm/min, led to instability in the melt pool, uneven beading and occasional underfill-type defects. However, it is also not possible to decrease the thickness of the cladding to below 2 mm by increasing the travel speed, or with any other method. With the CMT process, it was possible to use a relatively large transition, track displacement, of around 4 mm between the cladding passes, which increases the overall cover rate.
The results show that it is possible to produce flawless, low-dilution cladding with the CMT process, and the deposition rate can be up to 5 kg/h with a cover rate of over 0.2 m2/h. These values are about twice those achieved by the laser cladding equipment used in this study. In addition, when energy consumption values were multiplied with process efficiency values, the utilization of electrical energy in the CMT process was shown to be much more efficient than that of the laser cladding process. The calculations showed that the electrical energy needed to produce one square meter of alloy 625 cladding with the laser cladding process is 373 MJ, while it was only 66 MJ with the CMT process. In other words, the CMT process only consumes about one sixth (17%) of the energy needed for the laser cladding process.