Suppression of porosity in beam weaving laser welding

Abstract

The present paper describes a beam weaving laser welding technique to suppress argon or nitrogen porosity, which may appear during laser welding of low carbon steel. Bead on plate welding was performed using a 3 kW CO₂ laser. The weaving frequency was varied within 0–30 Hz and the weaving amplitude within 0–2 mm during welding. The experimental results show that under 2.4 kW laser power and 1.0 m min^-1 welding speed, the nitrogen porosity decreases remarkably with increasing frequency, and it can be eliminated for a weaving frequency of 22 Hz with 0.5 mm weaving amplitude. Under 2.4 kW laser power and 1.5 m min^-1 welding speed, beam weaving laser welding can also effectively reduce argon porosity at a weaving frequency of 22 Hz and amplitude of 1.0–1.5 mm.     

Liquid Al2O3-CaO-MgO inclusion entrapment at delta ferrite intercellular boundaries during solidification

Abstract

The behaviour of liquid Al₂O₃-CaO-MgO inclusions at the δ ferrite/melt interface in aluminium killed and calcium treated steel has been observed in situ using a confocal scanning laser microscope equipped with a gold image furnace. Movement of inclusions parallel to the solid/melt front was observed (a) during solidification rates below 1 μm s^-1 or with nearly static fronts, (b) during remelting of the front or (c) when inclusions at the solid/melt interface were pushed by a newly arriving liquid inclusion. Upon sliding, the inclusions tended to become entrapped in the intercellular regions of the front. After entrapment, a critical solidification velocity for pushing/engulfment was measured and compared with corresponding experimental results for pushing/engulfment at planar fronts. Engulfment at the intercellular boundaries occurred at solidification velocities larger than a critical velocity V_cr=9·91×10^-9 R^-1/2 in the present study. The result follows the same trend with respect to inclusion size as that found for planar interfaces, but indicates that the critical velocity is ~20%. lower. This is attributed to the observation that engulfment at intercellular regions involves replacing a section of the intercellular boundary with the inclusion, which decreases the surface energy change owing to engulfment compared with engulfment at planar fronts.     

Effect of beam interaction time on laser alloying with pulsed Nd–YAG laser

Abstract

Surface alloying of aluminium with nickel was carried out using a pulsed Nd–YAG laser. The effect of beam interaction time on laser alloying of aluminium with pulsed Nd–YAG laser has been studied. It was found that the beam interaction time of a pulsed laser has a significant effect on microstructure and properties of alloyed layers. The results indicated that with changes in the beam diameter, higher thickness of alloyed layer and higher microhardness are both obtained at a lower effective interaction time. When travel speed changes, the same conditions are obtained at a higher effective interaction time.     

Influence of laser surface alloying with chromium and nickel on corrosion resistance of type 304L stainless steel

Abstract

The influence of laser surface alloying (LSA) with Cr and Cr + Ni on the corrosion behaviour of type 304L stainless steel (SS) was investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) in chloride (0·5M NaCl) and acidic (1 N H₂SO₄) media. Surface alloying was carried out by laser cladding type 304L SS substrate with premixed powders of AISI type 316L SS and the desired alloying elements. The results indicated that Cr surface alloyed specimen exhibited a duplex (γ + α) microstructure with Cr content of ～24 wt-%, whereas Cr + Ni surface alloyed specimen was associated with austenitic microstructure with Cr and Ni contents of ～22 wt-% each. The potentiodynamic polarisation results in chloride solution indicated that LSA with Cr + Ni considerably enhanced the pitting corrosion resistance compared with LSA with Cr alone. In acidic media, such beneficial effects were not observed. Electrochemical impedance spectroscopy results showed an increase in semicircle arc for both chloride and acidic media for both Cr and Cr + Ni clad samples indicating improvement in the oxide film stability compared with untreated specimen. The polarisation resistance was higher and capacitance values of the laser clad specimen were lower than those in the untreated specimen. The microstructural changes and compositional variations produced by LSA are correlated to the corrosion behaviour.     

Laser cladding in repair of IN738 turbine blades

Abstract

Laser cladding with additive powder can be used for repairing high cost components such as industrial turbine blades. Repair of damaged blades is important for electrical power companies to reduce operating costs. The repair of IN738 gas turbine blades using laser cladding is reviewed. A laser cladding system for blade repair and the associated heat treatment procedures before and after repair are discussed. In general, laser cladding with IN625 powder does not require preheating and can be used for repairing IN738 blades in low stress areas. For laser cladding with a higher strength material, e.g. IN738 powder, preheating to high temperatures is necessary to avoid cracking. In a process simulation, test coupons were employed to compare the microstructures and mechanical properties of the IN738 clad region and base metal. Blade repairs with IN738 additive powder were found to be stronger but much more difficult to process than those with IN625 powder. Therefore, extreme caution is required when repairing IN738 blades in high stress areas using a matching filler metal.