A Research On Process Energy Efficiency Modeling And Applications For Hot-wire Laser Welding

Energy conservation and green manufacturing have become priorities in manufacturing industry,while the environmental compatibility of manufacturing has become the focus of research.Laser manufacturing as a kind of novel technology,offers the significant advantages of high energy density focusing,high flexibility,efficiency,and high quality.However,the connection and material removal in laser manufacturing process are based on the mechanism of vaporization and melting,the energy efficiency of laser manufacturing process is extremely low.Especially,the laser based processes energy efficiency might be difficult to be analytically modeled as the complex conditions and various parameters of the process.At present,many researchers investigated the energy efficiency at process level and many significant results have been provided,however,there is still lack of in-depth researches on the essence of energy transformation,especially the systematic study on the coupling mechanism and analysis of the correlations between the effective energy utilization in the process and the output of the properties from the perspective of sustainable manufacturing.Hot-wire laser welding,that is laser welding with a filler hot wire can considerably decrease the fitting precision requirements on parts,and the preheating wire can significantly reduce the input laser power requiring for metal melting,resulting an increase of the energy efficiency and a smaller heat-affected zone of the welding,and leading to a wide range of applications in the manufacturing industries.Therefore,in this dissertation,the hot-wire laser welding is taken as the research subject according to its significant improvement in laser energy absorption and good properties of welded joint.Based on the essence of energy transformation mechanism and characteristics of energy flows in the process of laser hot-wire welding process,the measurement methods and parametric equations representation model were obtained to reveal laws of the energy efficienc y ratio under different process conditions and different process schemes.Further,the synergy mechanism of energy efficiency and welding quality of laser welding process were analyzed to achieve a sustainable and systematic evaluation of the technologies.The main research work and innovative achievements in this dissertation are :(1)For a general problem of multi-energy sources,complex and dynamic energy conversion and dissipation in the hot-wire laser welding process,a multi-level subsystem partitioning method is suggested based on the physical essence of energy conversion,absorption and dissipation of electricity,light and heat,dividing the welding process into laser energy induced and transforming subsystem,filled wire preheating and conveying subsystem,and metal melting and welds forming subsystem.Therefore,the research system boundary is defined as the input of electricity,gas,and material,etc.and output of the specific process properties of welding parts,companied with energy consumed in the process.According to the energy flow characteristics of each level,a method is proposed to characterize the energy conversion quantitatively and energy efficiency of each sub-process system and the key energy efficiency parameters are also identified.Then,the laser energy induced and transforming subsystem is characterized by the photoelectric conversion efficiency of the laser and the absorption efficiency of the material to the laser,where the key parameters including the laser power,the welding speed,the focus position and the working condition.The filled wire preheating and conveying subsystem is characterized by the preheating heat efficiency of welding wire,where the key parameters including the wire heating current,the wire feed speed,the wire heating length,the wire diameter,etc.and the thermal physical properties of the wire material.And the metal melting and welds forming subsystem,being critical for welding quality is characterized by the minimum energy required to form the weld,where the key parameters including the heat conduction,heat convection on the surface of the workpiece and the energy loss of heat radiation.(2)As for the problem of being difficult to analytically model the energy efficiency o f laser based process,a quantitative description method with power input and output was also suggested.On the basis of energy transfer,such as electro-optic,electro-thermal and thermal-thermal,and according to various forms of energy losses,such as h eat conduction,heat convection and heat radiation,the energy input and output was measured and presented with key process parameters,including laser power,welding speed and wire feeding speed and heating current.At the same time,considering the influence of the butt joint gap and preheating temperature of wire on the power and energy utilization,based on energy conservation and changes in resistance heating enthalpy,a preheating temperature prediction model of hot-wire laser welding is established to reveal the influence of heating current,wire length,wire feed speed,wire diameter,wire thermal properties on preheating temperature.Further,by decomposing the laser power required for filled wire melting and the electrothermal power required for the wire preheating,a power model of the welding process is established to realize the parameterization of various energy forms and conversion efficiency in the process.The experiment results show that an average relative error of the theoretical calculations and the experimental measurements is 4.2% and 4.6% respectively,which verify both the preheating temperature prediction model and power model of hot-wire laser welding being quite reliable.(3)The differences in the energy sources,process conditions and process parameters,make it difficult to compare energy efficiency between manufacturing processes.In this dissertation,the laser power equivalent was initially introduced for energy conversation calculation and for characterizing the differences be tween the laser powers required for hot-wires laser welding and that required for cold-wire laser welding.The laser power equivalent depends on the key process parameters,offering a basic law for description and standardization of the energy saving rate among different manufacturing processes.The experiment results show that in the hot-wire laser welding process,a maximum energy savings of 17%~36% was realized over cold-wire laser welding.And the experiments under different butt gap showed that laser p ower equivalent has a positive linear correlation with preheating temperature,which could effectively improve the process energy efficiency.(4)Taking the significant differences between "quantity" and "quality" of the energy consumed in the manufacturing process,where in the hot-wire laser welding,namely energy transmitted to weld workpiece surface,the weld forming energy was still unknown due to thermal radiation,optical radiation and material reflection.Therefore,the welding gap melting volume specific energy consumption was proposed to estimating the energy effectively consumed at unit process level.The minimum energy required for the welding gap melting volume is used to define the useful energy output,and the laser energy required to melt th e base material,the laser energy required to melt the wire and the resistance heat required to preheat the wire were characteristic as a function of the thermal physical properties of the base material and the wire,the duration of the welding process,the preheating temperature of the wire ends,etc.Then a specific energy consumption model for hot-wire laser welding process was initially established.The experiment results show that an average relative error of the theoretical calculations and the experi mental measurements is 6.9%.To save energy,certain heating current,small wire diameter and appropriate the wire endpoint preheating temperature could be selected.(5)The weld penetration ratio is often used as a comprehensive index of welding quality and energy efficiency.In this dissertation,the welding gap melting volume was expressed as the preheating temperature of wire,penetration ratio and butt joint gap and so on,offering a basis for unveiling the coupling relationship between energy efficiency laws and weld quality.In addition,the trace element method was suggested for solving the measurement problem of melting volume.The experimental data show that the distribution of Ni in the weld was uniform then a simple,reliable measurement of the welding gap melting volume was achieved.(6)The problem of complex physical mechanism of laser welding and complicated energy consumption mechanism in welding process,which results in fuzzy relationship between energy efficiency and welding quality.A gr ay relational model between energy efficiency and welding quality was established to mapping correlations among the key feature sequences of hot-wire laser welding,where process parameters and melting volume were used as intermediate variables.Finally,a systematic evaluation of the sustainability of the process technologies was conducted according to the experiment results,which shows that increasing the laser power and heating current,and keeping the butt gap quite equivalent to the filled wire diamet er can significantly improve the process energy efficiency while ensuring the excellent quality of the weld.