Fiber Laser Cutting-welding Integrative Technique Of High Strength Aluminum Alloy

Rocket tank has been usually made of high strength aluminum?Al?alloys because of light weight and high specific strength.It has been fabricated by manual cutting,and arc welding.From the cutting to welding,the parts are disassembled and assembled repeatedly,causing a complex process,uncontrollable quality and low efficiency.Laser cutting and welding integrative technique is a new process by integrating laser cutting and laser welding to one process.It could reduce the clamping time,shorten the process,and then improve the precision and quality of manufactured parts.However,it is difficult to obtain good quality for both laser cutting and laser welding of Al alloys due to its high reflectivity to laser beam and special physical-chemical properties.In order to achieve the high quality fabrication of rocket tank,some key techniques and mechanisms in laser cutting and welding integrative technique of Al alloys are studied in detail.The results are listed as follows.The effects of cutting parameters on the surface morphological features of fiber laser cutting of 4mm-thick AA2219 Al alloy were studied.It was found that the cutting surface was characterized by three zones with different striation patterns,which were smooth horizontal striation zone at the upper,vertical striation zone at the middle and coarser oblique striation zone at the lower.The method to assess the kerf quality was proposed by the fraction of the height of oblique striation zone with maximum roughness to the full thickness of workpiece,?.The kerf is at the I-class quality?ISO 9013:2002?when the?is less than 0.2and the dross height is less than 0.3 mm.The optimal parameters for the I-class quality are laser power greater than 4.5 kW,cutting speed at 3⁴ m/min,and gas pressure no less than1.3 MPa.The kerf quality is improved by increasing the laser heat input and gas pressure,which decrease the viscosity of liquid metal at cut front edge,increase the driving force caused by gas flow,and then reduce the?and the root dross.The effects of the temperature at cut front edge(T_ce)on the kerf quality were studied.The optimal T_cerange was determined as 1800°C¹950°C,whose existence mechanism was testified by calculating the forces acting on the liquid metal of molten pool.When the T_ceis at this optimal range,the angle of the resultant to the gravity,?is minimum,about 38°.This angle is most favorable to push the liquid metal away molten pool and obtain the best kerf quality.Increasing or decreasing the T_ceoutside of the optimal range increases the fraction of oblique striation zone and the height of the dross,and decreases the kerf quality.Moreover,a method to improve the kerf quality at initial stage was put forward according to the optimal T_cerange,which was demonstrated by a designed experiment.The results are significant for the establishment of on-line monitoring in laser cutting.The effects of four kinds of kerf on the bead shape and the porosity of laser-arc hybrid welds were studied.It was found that weld porosity was proportional to the thickness of kerf remelting layer?d?,but seemed independent of the types of the kerf.The porosity reaches B-class standard?ISO 10042-2005?,and is less than 2%when the d is less than 136?m.The porosity is affected by two factors.When the d is less than 150?m,the laser keyhole purification is the dominant factor,which removes the remelting layer by the gasification and ionization process during keyhole formation.It reduces the number of the bubbles formed by oxide decomposition,and then decreases the porosity.When the d is more than 150?m,the effect of oxygen content on the melt flow driven by surface tension becomes to be a dominant factor.The surface tension temperature coefficient is changed from negative to positive by the oxide in remelting layer,which is not removed clearly by laser keyhole purification and enters into the molten pool.This change causes the melt flow reversed,which prevents the escaping of the bubbles,and increases the porosity.The effects of four kinds of kerf on the microstructure and mechanical properties of laser-arc hybrid welds were studied,demonstrating the feasibility of laser cutting and welding integrative technique of high strength Al alloy.The ultimate tensile strength and the elongation of Argon-weld are equal to that of Machined-weld when the d is less than 70?m,which are 278 MPa and 3.1%,respectively.The tensile properties of Nitrogen-weld are slightly lower than that of Argon-weld because of the formation of nitrides in the weld.Both Air-weld and Oxygen-weld are with low tensile properties because of coarsened grain and the formation of a large amount of oxides.Considering weld porosity and tensile properties comprehensively,it can be found that the weld meets the fabrication requirement of rocket tank when the d is less than 136?m,at which the porosity is at B-class standard and the ultimate tensile strength is above 250 MPa.According to the structure characterization of the bottom part of rocket tank,the effects of the weldment angle,?on the bead shape,microstructure and mechanical properties of the3-dimensional weld were studied.The?is at the range of 0°⁹0°on the basis of bead shape and weld porosity,it was demonstrated that the 3-dimentional hybrid welding of Al alloy only employed the upward sloping and vertical welding,rather than the downward sloping and vertical welding.For a given?,there has an optimal range of arc current to obtain B-class weld.It is 270³00 A when the?is at the range of 0°¹5°,equal to that of usual downward welding.It is 240²70 A when the?is at the range of 15°⁶0°,and reduces to only 270 A when the?is at the range of 60°⁹0°.The forces analysis of droplet transition and molten pool indicates that the improvement of keyhole stability,the acceleration of melt flow and the elongation of molten pool are the main factors to cause above changes.In addition,the columnar grain zone is disappeared when the?is larger than 60°,causing the weld is totally composed of equiaxed dendrites.The ultimate tensile strength and the elongation of 3-dimensionla welds reaches the maximum when the?is 60°,which are 320 MPa and 5%,respectively.The fracture surfaces show that tensile properties are in good agreement with weld porosity and microstructure.Both the cutting and welding techniques developed in this thesis were verified on the reconfigurable and multi-axis CNC machines developed by the same project.In comparison of the present process of water cutting and variable polarity plasma arc welding,the cutting speed increases by 6 times,the surface roughness of the kerf reduces by 25%³3%,the welding speed increases by 11 times,and the ultimate tensile strength of hybrid weld increases by 11%²4%.The results showed that the laser cutting and welding integrative technique is transplantable,and the results would provide a technical storage and some theoretical support to the fabrication of large scale structures of Al alloy.