| GMAW arc additive manufacturing(AM)is widely used in metal manufacturing due to its low cost and efficiency.However,its large heat input influences its forming precision and performance which limits the further improvement of efficiency.Moreover,it is lack of comprehensive research on improving the efficiency and quality of 316L stainless steel GMAW arc additive manufacturing with various processes and arc modes.In this paper,aimed at improving the forming efficiency of 316L stainless steel deposition,a quantitative comprehensive evaluation system for the quality of arc additive manufacturing was established,then the process optimization methods of efficiency and performance improvement were systematically studied from the process parameters of single wire,different modes of double-wire to auxiliary gas process by MIG and CMT processes.The main contents of the thesis are as follows:(1)Based on the electrical waveform,forming and microstructure image analysis,and mechanical properties,the multi-information fusion quantitative evaluation system of AM was established to provide a tool for evaluating the forming quality of AM process experiments.The evaluation standard and criterion system for the quality of arc additive manufacturing was constructed by four first-grade evaluation indexes of deposition process stability,molding quality,molding efficiency and performance,and several second-grade evaluation indexes.Concentration ratio K value of voltage and current probability density was extracted by statistical methods.While based on cross-section image recognition,forming efficiency,forming roughness S_a,S_b and S_c in three directions and microstructure characteristics were extracted by cross-section,difference value and particle ranging method.Moreover,the eigenvalues of forming defects and bending properties were obtained by fuzzy quantization method.Then these characteristic values were normalized to realize the quantitative comprehensive evaluation with multi-index fusion of AM.(2)A comparison test of multi-process parameters of single-wire 316L stainless steel arc-AM was carried out by MIG welding.Combined with the evaluation results,the influence mechanism of the process parameters on forming,grain growth and properties was revealed,and the process parameters were optimized by emphasizing"efficiency"and under the condition that the performance was up to standard.The results showed that the bottom gradually decreasing current could stabilize the deposition process and optimize the bottom molding,while increasing the scanning speed or cooling time could reduce the heat input and thus improve the performance.The effective deposition rate of Speed Arc-XT welding with diameter of 1.2 mm was higher than that of other arc modes,which was about 82.2%higher than that of0.8 mm diameter deposition.Then,the contrast test between single-and double-wire with the same power supply was carried out,the results showed that the total and effective deposition rate of double-wire part could reach to 1.15 kg/h and 0.82 kg/h,respectively,which was about23.2%higher than that of single-wire part.Due to the increased filling amount of double-wire and the stirring effect of arc force on the molten pool,the microstructure of the upper and middle parts was refined,and the average tensile strength was increased by 20.4 MPa.Moreover,the microstructure of all samples showed columnar crystal morphology.Due to the preheating effect of the previous layers on the subsequent layers,and the thermal cycling treatment effect of the fllowing layers on the previous layers,the bottom hardness was larger than the upper and middle part,and it was anisotropic in tensile properties,which could be up to 14.38%,moreover,the tensile fracture was typical ductile fracture.(3)The process of the same phase current and non-synchronous arcing and extinguishing of double-wire were put forward by exploring the influence mechanism of double-arc interferences,which could reduce the mutual interference of two arcs and improve the forming ends.Moreover,the stirring effect of CMT+P mode on molten pool and the tensile anisotropic fracture mechanism of arc-AM were revealed by deposited experiments of different CMT arc modes and the metallographic analysis of tensile fracture specimens,which proved that CMT+P mode was not only beneficial to molding,but also could improve efficiency by continuous deposition.The results showed that the horizontal tensile force made the columnar crystal wide along the direction perpendicular to the spindle and the grain deform,which made the fracture occur at the grain boundaries.However,the columnar crystal by vertical tensile force did not deform obviously,and the fracture occured at the interlayer remelting cell crystals,indicating that the horizontal plasticity was better than the vertical direction.As the columnar grain boundaries are weak links in tensile fatigue,the tensile strength of horizontal specimens with more grain boundaries was less than that of vertical ones.Moreover,the ratio of wire feeding speed to scanning speed of well-formed 316Lstainless steel by double-wire arc AM was about4.2.When the scanning speed was 120 cm/min,the maximum and effective deposition rate were 5.41 kg/h and 3.91 kg/h,respectively.(4)A new process platform for CMT+P double-wire stainless steel arc AM with two-direction auxiliary gas was constructed.The cooling and stirring mechanism on molten pool of the auxiliary gas nozzle angle,the auxiliary gas flow rate and the height between the auxiliary gas nozzle and the substrate were explored by deposition from the single directional auxiliary gas and one-layer deposition to two directional auxiliary gas and multi-layers deposition.Then,the regression equation between the comprehensive evaluation results and three auxiliary gas parameters was established by the evaluation system and the BBD response surface method.The model predicted that the optimum auxiliary gas process parameters of the auxiliary nozzle angle,the auxiliary gas flow rate and the height were 17.4°,25 L/min and 10.44 mm,respectively.Then,the comparing test of double-wire 50-layer depositions by the optimal auxiliary gas process showed that the microstructure of the optimum auxiliary gas process part was refined due to the stirring and cooling effect of the auxiliary gas flow,moreover,the effective deposition rate,hardness and maximum bending force were increased by about 24%,6.85%and 12.96%,respectively,while the tensile anisotropy was only 3.7%,which was obviously weakened.When the wire feeding and scanning speed reached to 5.5 m/min and 1.5m/min,respectively,the 50-layer deposited part with the optimal auxiliary gas process was well formed,and the average hardness and tensile strength could get to 167±5.01 HV and521.45±10.25 MPa,respectively.Moreover,the total and effective deposition rate were as high as 5.95 kg/h and 4.61 kg/h,respectively,which realized the high efficiency arc AM of stainless steel. |
PDF Full Text Request