Zhongjie Liu

Fiber laser welding of 780MPa high strength steel

A 2kW Ytterbium fiber laser, YLR-2000, was used for welding of 780MPa grade high strength steel. In the present work, this high strength steel is a kind of heat rolled, fine microstructure low carbon high strength steel used for welded constructions. The effects of defocusing distance, welding speed, shielding gas and its flow rate on cross section bead shape and penetration were studied in continue wave laser beam welding. The effects of frequency and duty cycle on the cross section bead shape and penetration were also studied in pulse fiber laser beam welding. In the fiber laser welding, square wave and triangle wave power output were adopted. The results showed that the pulse frequency and duty cycle of laser power had affected the bead shape and penetration in square wave Fiber laser welding. The pulse frequency of laser power had not effect on the bead width and penetration in triangle wave fiber laser welding. The microstructures of weld metals in different heat input were observed by optical microscopy. The hardness of welds was also tested and compared among welds welded at different welding speeds, in other words, in different heat inputs.A 2kW Ytterbium fiber laser, YLR-2000, was used for welding of 780MPa grade high strength steel. In the present work, this high strength steel is a kind of heat rolled, fine microstructure low carbon high strength steel used for welded constructions. The effects of defocusing distance, welding speed, shielding gas and its flow rate on cross section bead shape and penetration were studied in continue wave laser beam welding. The effects of frequency and duty cycle on the cross section bead shape and penetration were also studied in pulse fiber laser beam welding. In the fiber laser welding, square wave and triangle wave power output were adopted. The results showed that the pulse frequency and duty cycle of laser power had affected the bead shape and penetration in square wave Fiber laser welding. The pulse frequency of laser power had not effect on the bead width and penetration in triangle wave fiber laser welding. The microstructures of weld metals in different heat input were observed by optical micros...

Metallurgical study on laser-mag hybrid welding of HSLA-590 steel

The Laser-Arc hybrid welding of HSLA-590 steel has been studied using 2.4kW CO2 laser and 350A MAG welding machine. In this paper, the leading process, MAG current, welding speeds were investigated. The results show that arc leading hybrid welding is better than laser leading hybrid welding from the result of experiments. MAG welding current of 120∼180 ampere is the effective arc parameter to get better welds in 2kW CO2 laser hybrid welding. The acceptable distance DLA between Laser and Arc is also important parameter that is decided by observing the welding process and the cross section of welds. Gap tolerance and groove angle are also investigated in the experiments.Microstructure, cooling curve of HAZ and Hardness of welds under different hybrid welding conditions were studied. The cooling curve of HAZ was measured. The cooling time (cooling from 800°C to 500°C) affects microstructure of welds and grain size in HAZ. The average grain size in HAZ was measured. The results showed the hybrid welding was useful to increase the tough microstructure, such as ferrite and bainite, comparing with Laser welding and MAG welding individually under the same welding speed. The SEM (Scanning Electron Microscope) and DAS (Dendrite Arm Spacing) of base metal and weld metal were studied. The hardness of welds was measured, which showed the weld metal hardness of hybrid welds was lower than that of Laser welds and MAG welds.The Laser-Arc hybrid welding of HSLA-590 steel has been studied using 2.4kW CO2 laser and 350A MAG welding machine. In this paper, the leading process, MAG current, welding speeds were investigated. The results show that arc leading hybrid welding is better than laser leading hybrid welding from the result of experiments. MAG welding current of 120∼180 ampere is the effective arc parameter to get better welds in 2kW CO2 laser hybrid welding. The acceptable distance DLA between Laser and Arc is also important parameter that is decided by observing the welding process and the cross section of welds. Gap tolerance and groove angle are also investigated in the experiments.Microstructure, cooling curve of HAZ and Hardness of welds under different hybrid welding conditions were studied. The cooling curve of HAZ was measured. The cooling time (cooling from 800°C to 500°C) affects microstructure of welds and grain size in HAZ. The average grain size in HAZ was measured. The results showed the hybrid welding was u...

Crack behavior and characteristics of laser clad layer with powder mixture of CO-based alloy and WC

A powder mixture of Stellite-6 and tungsten carbide (WC) in the range of 0wt%∼47wt%WC with a normal multi-layer cladding and novel multi-layer cladding were deposited on mild steel (JIS-SM400B) plates by a 2.4kW cw CO2 laser. In the normal multi-layer cladding, the WC weight fractions at each layer are constant In the novel multi-layer cladding, the WC weight fractions at each layer are varied by controlling the disk rotation speed of the feeder. The microstructure, hardness and wear resistance of the clad layer were investigated by an X-ray diffractometry, energy dispersion spectroscopy (EDS), scanning electron microscope (SEM), laser microscope, Vickers hardness tester and wear tester. According to the analyzed results, the microstructure of the clad layer belonged to a hypoeutectic structure. In addition, the undissolved tungsten carbides dispersed in the matrix of the Co-based alloy. The Vickers hardness increases with the increase of WC weight fraction. Using a WC constant weight fraction in the third layer, the clad layers of normal multi-layer cladding and novel multi-layer cladding have almost same wear resistance. On the other hand, to decrease the crack sensitivity of clad layer, the influence of a preheating temperature and WC weight fraction in the normal multi-layer cladding and novel multi-layer cladding on the crack sensitivity was investigated comparatively. As the result, the cracks of clad layer made with the powder mixture of Stellite-6 and WC belonged to a quasi-cleavage fracture type crack. In the novel multi-layer cladding with gradual increase of WC particles clad at three subsequent layers shifts the critical amount of WC at which cracking appears to the higher level, or that it reduces the required preheating temperature.A powder mixture of Stellite-6 and tungsten carbide (WC) in the range of 0wt%∼47wt%WC with a normal multi-layer cladding and novel multi-layer cladding were deposited on mild steel (JIS-SM400B) plates by a 2.4kW cw CO2 laser. In the normal multi-layer cladding, the WC weight fractions at each layer are constant In the novel multi-layer cladding, the WC weight fractions at each layer are varied by controlling the disk rotation speed of the feeder. The microstructure, hardness and wear resistance of the clad layer were investigated by an X-ray diffractometry, energy dispersion spectroscopy (EDS), scanning electron microscope (SEM), laser microscope, Vickers hardness tester and wear tester. According to the analyzed results, the microstructure of the clad layer belonged to a hypoeutectic structure. In addition, the undissolved tungsten carbides dispersed in the matrix of the Co-based alloy. The Vickers hardness increases with the increase of WC weight fraction. Using a WC constant weight fraction in the thir...