Heat source and melting efficiencies

As the energy source in laser cladding is electromagnetic radiation, absorption and energy coupling are in decisive role to determine the heat source and melting efficiencies, i.e. how much of the available laser power actually participates in the cladding process (to heat the base material, to melt the coating material and weld it to the base material) and how much is lost to the surroundings due to back reflection, convection, radiation from the melt pool etc.

Together with the available laser power, it dictates the productivity and largely the cost efficiency of the process. There are few ways to measure and compare these efficiencies in laser cladding. One of the most used ways to define the heat source efficiency is to conduct measurements with a calorimeter during cladding. Miyamoto et al. [96] used such method during the 1-step off-axis CO2 laser cladding of Cu-based powder on Al-based base material.

Heat source efficiency was just 10% without powder feed (= remelting), but increased to 30%

when Cu powder was fed into the melt pool. This was claimed to be due to oxidation, which took place in melt pool during cladding. Marsden et al. [97] measured CO2 heat source efficiencies during the off-axis cladding of Stellite 6 powder (30-60 μm) onto mild steel (surface finish was not mentioned). According to calorimetric measurements, heat source efficiency increased from 15% to 25% when the powder feed rate was increased from 5 to 8 g/min while keeping other parameters constant. Frenk et al. [98] noticed later similar enhancement in CO2 heat source efficiency when the powder feed rate was increased (Stellite 6 on mild steel). With the traverse speed of 500 mm/min, heat source efficiency increased from 16 to 30 % when the powder feed rate was increased from 2 to 8 g/min. With the traverse speed of 1200 mm/min, heat source efficiency increased from 18 to 25 % when the

powder feed rate was increased from 3 to 12 g/min. When the powder feed was increased, coating thickness increased, simultaneously the incidence angle of laser beam changed due to more inclined melt pool. As a consequence of this and linearly polarized beam, the absorption was increased because angle of incidence approached the well-known Brewster angle. These results were verified by conducting laser remelting tests with varying angle of incidence.

Absorption increased from 10 to 37.5% when the angle of incidence (= angle with respect to surface normal) was increased from 0 to 80°. Ollier et al. [99] obtained similar results by changing the angle of incidence of linearly polarized CO2 beam towards “pushing” direction in relation to cladding direction. As a consequence of this, the heat source efficiency increased from 13 to 35 % when the incidence angle was increased from 90 to 150° (with respect to horizontal). Gutu et al. [100] took also the advantage of Brewster angle of linearly polarized CO2 beam. They developed the optics, which produced circular beam on the surface of base material at very high angles of incidence (70-80˚ with respect to surface normal). At least in surface heating of mild steel, this configuration enhanced the energy coupling by a factor of 2–4 as compared with normal incidence (0˚). They also demonstrated this configuration in cladding by equipping it with vertical off-axis nozzle but efficiency comparison was not made with respect to cladding in normal position. With randomly polarized beams characteristics for fiber-optic delivered beams and direct diode lasers, such positive Brewster effect cannot be utilized. On the other hand, change in incidence angle of beam does not have large influence on absorption assuming that average reflectivity Rave can be estimated as the average value of the reflectivities Rp and Rs according to Rave = (Rp + Rs)/2.

Bloehs et al. [101] reported calorimeter studies conducted during the cladding of Stellite 21 powder (45-90 μm) from off-axis nozzle onto milled 16MnCrS5 steel with CO2 and Nd:YAG lasers. In addition to heat sources (= wavelengths), they studied the influence of traverse speed, power density, surface finish of the base material and different shielding gases. The highest energy coupling rates in CO2 laser cladding (30–35%) were achieved without shielding gas. Coupling rates with Ar and N2 were approximately 28%. The lowest coupling rates were achieved with He (21–26%). Heat source efficiencies for CO2 and Nd:YAG laser cladding were 30 and 60%, respectively, in identical conditions. Increase in traverse speed increased the coupling rates slightly. This was claimed to be due to decrease in bead width, which allowed higher portion of irradiated beam to impinge to oxidized solid surface around the melt pool. Coupling rates also increased when the power density decreased. For high power densities the highly reflective melt pool, resulting in lower coupling rates, occupied most of the laser-irradiated area. Furthermore, coupling rates in CO2 laser cladding increased slightly in the following sequence as a function of base material surface finish; polished, milled, oxidized, grit-blasted. The positive effect of grit-blasting in CO2 laser cladding was also reported in Ref [40]. Bloehs et al. [101] calculated also melting efficiencies (ratio of the energy to heat and melt the powder to laser energy used) for CO2 and Nd:YAG laser cladding.

They were approximately 9% for CO2 and 18% for Nd:YAG. This calculation can be done with equation as follows:

η = melting efficiency (%)

Pc = power utilized to melt coating material and weld it to base material (W) P = delivered laser power (W)

Δm = mass of melted material (base material, coating) (g) ΔH = enthalpy term (cp · ΔT + Lm) (kJ kg^-1)

c(T) = specific heat capacity of material melted (base material, coating) (J kg^-1 K^-1) ΔT = difference between melt pool temperature and ambient (K)

Lm = latent heat of fusion of melted material (base material, coating) (kJ kg^-1) t = cladding time (s)

ρ = density of melted material (base material, coating) (g mm^-3) ΔV = volume of melted material (mm³)

Δs = clad length (mm)

A = melt cross-sectional area (mm²) Vb = traverse speed (mm min^-1)

Melt pool temperature used in calculations, which was measured by optical pyrometer was 1700˚C. Gedda [20] compared the heat source and melting efficiencies in laser cladding between CO2 and Nd:YAG laser. According to calorimetric measurements, he stated that 40%

of the used CO2 and 50% of the used Nd:YAG laser power was absorbed by the process while cladding Stellite 21 powder (30-60 μm) on grit-blasted mild steel with off-axis nozzle setup.

This small difference in absorption or energy coupling, however, resulted in substantial difference in melting efficiencies as calculated with the equation shown above. Pc for Nd:YAG laser cladding was twice as high as compared with CO2 laser cladding. This result meant that improved absorption, resulted from the shorter wavelength of the laser beam (1.06 vs. 10.6 μm), was given fully to Pc since the amount of power needed to heat the base material was equal in both processes. Melt pool temperature of 2027˚C was used for both processes.

His energy efficiency and redistribution results are summarized in Table 1.

Table 1. Energy redistribution in laser cladding [20].

Distribution CO2 Nd:YAG

Power reflected off the melt 50% 40%

Power reflected off the powder cloud 10% 10%

Power used to heat the base material 30% 30%

Power used to melt the clad layer and weld it to the base material 10% 20%

In addition to CO2 and Nd:YAG lasers, HPDL devices are increasingly used in laser cladding.

Nowotny et al. [16] studied the melting efficiencies of CO2 and HPDL (940 μm) lasers.

According to them, 2.5 times higher CO2 laser power was needed in comparison with HPDL to obtain single beads of Stellite 21 on mild steel with equal cross-sectional areas. This indicated the beneficial effect of shorter wavelength. Difference in heat source efficiencies between YAG (and other short-wavelength lasers; HPDL and fiber) and CO2 cladding is much less than predicted by the absorptivity values at RT. This is attributed to the different behaviours when the temperature rises. That is, CO2 absorptivity increases significantly with a rise in temperature [23, 102], whereas absorptivity for wavelength lower than 1.8 μm slightly falls [102].

Above mentioned values of 13-40% for CO2 and 50-60% for Nd:YAG laser cladding are rather low compared with heat source efficiencies reported in literature for conventional arc

and electron beam welding processes. Heat source efficiencies in welding processes measured by calorimetric techniques was given by Kou [103] and they were 50-70% for PTA, 60–80%

for TIG, 70–80% for MIG, 80–90% for SAW and EBW. Moreover, they are not largely dependent on materials and their surface properties.

In addition to roughening the surface, decreasing the wavelength of the laser beam and utilizing the Brewster angle of linearly polarized beam, it has been reported that using reflective dome, extraordinary high traverse speeds or fine powder particles help to improve efficiencies in laser cladding. As the largest unnecessary loss of energy in laser cladding is the back reflection from the melt pool [20], it is reasonable to attempt to recycle it back to the melt pool. This was described in Refs. [40, 41], where the optical feedback system, which consisted of clean reflecting dome, increased covering rates by a factor of 1.5–1.7 in CO2

laser cladding. Seefeld et al. [80] found out that with traverse speeds as high as 10 000 mm/min substantially higher covering rates were achieved compared with normal traverse speeds used in traditional laser cladding (300–1500 mm/min). With Nd:YAG laser power of just 1.2 kW, Stellite 21 coating layer, 0.5 mm in thickness on mild steel, was laser clad with a rate of 0.23 m²/h. Drawback of this high traverse speed cladding technique enabled by high power density (= small spot, Db = 0.8 mm) was the low powder catchment efficiency, which was just 15–35 %. Partes et al. [104] verified that this improved efficiency resulted from the diminished heat conduction losses to the base material as well as increased energy input into the powder cloud. This latter effect was due to thicker powder cloud and associated increase in absorption due to multiple reflections. Thicker powder cloud resulted from the higher powder feed rate since f/v (g/mm) was kept constant. They also found out that the use of finer powder particles (10–45 μm) instead of coarse ones (45–150 μm) enhanced melting efficiency further by a factor of two. Pelletier et al. [74] noted similar increased absorptivity as a function of powder feed rate due to multiple reflection phenomenon as discussed earlier. Sears [105] suggested that improved absorptivity results from the denser powder cloud, which returns more reflected energy back to the surface. Another factor, which has been reported to enhance the melting efficiency is the possible energy release due to exothermic reaction, which takes place in melt pool. For instance, some carbides (TiC), borides (TiB2) and intermetallics (Ni3Al) [106] have large negative formation enthalpies, which could improve the process efficiency to some extent.

1.3.2.1 Productivity

Productivity or other commonly used term, deposition rate, is strongly related to the amount of absorbed laser power, which mainly depends on the laser power available and factors discussed above. Deposition rates in laser cladding collected from the literature are plotted in Figure 8. They concern the cladding trials of monolithic , Ni- and Co-based alloys on Fe-based base materials. It can be seen that short-wavelength lasers and hybrid processes increase productivity significantly. The following net deposition rates can be selected: ~0.5 kg/h with 3 kW CO2 + reflective dome, off-axis blown powder process [23]; 1.4-1.9 kg/h with 4 kW HPDL, cold wire process [45]; 1.9 kg/h with 4 kW HPDL, cold strip process [54]; 3 kg/h with 6 kW CO2, hot wire process [15]; 4.5 kg/h (Stellite 6 assuming that solid wire) with 5 kW CO2, hot wire process [43]; 10 kg/h with 4.4 kW Nd:YAG, hot wire process, [58]. For the sake of comparison, deposition rates for TIG (dilution 5-10%) are up to 2 kg/h, for PTA (dilution 5–30 % ) up to 7 kg, for MIG (dilution 10-25%) up to 10 kg/h, for SAW (dilution 15-35%) up to 70 kg/h and for electroslag up to 350 kg/h [107]. These reported values for conventional welding processes are effective rates meaning that any necessary breaks in the deposition process such as changing electrodes are taken into account. They can be considered

also as net rates since filler forms consist mainly of wires, rods, strips and tubes excluding the PTA process where the filler is in powder form.

0

Figure 8. Deposition rates expressed in mm³/h for blown powder cladding process unless otherwise denoted [15, 18, 23, 43, 45, 54, 58, 61, 79-81].