| Laser metal deposition(LMD)can create parts with complex shells and shorten the manufacturing cycle drastically,which provides a new possibility for the rapid manufacturing of thin-walled parts.Although the technology has been widely used in iron-based,titanium-based,and nickel-based materials,it still faces a great challenge to deposit aluminum alloys.Aluminum alloys possess the characteristics of high laser reflectivity,high thermal conductivity,and Al alloy powders are easy to be oxidized.Porosity may occur and the molten pool is unstable,which brings great difficulty to control the geometry and mechanical properties of deposited parts and made the technology hard to be applied and promoted.In this study,Al Si10 Mg aluminum alloy powders are used to investigate the geometry and mechanical properties control of thin-walled structures with laser metal deposition process.Combining experiment,numerical simulation,and process monitoring,as-deposited Al Si10 Mg with high strength and high plasticity were made.And the geometry of thin-walled shells with different shapes were controlled accurately.The influence of oxygen concentration in atmosphere,laser scanning speed and other process factors on porosity in deposited parts is studied.Besides,a numerical model is built to describe heat and mass transfer of interaction between powder flow and molten pool during laser metal deposition of Al Si10 Mg,in order to investigate the effect of molten pool on porosity.Flow field of molten pool under the impact of powder flow is analyzed.The forming mechanism of porosity during Al Si10 Mg laser metal deposition process is clarified considering oxygen concentration,flow field of molten pool,solidification rate and other factors.Effective porosity control strategy was brought forward,so that the porosity can be controlled below 0.5%.The effect of laser power,laser scanning speed,laser scanning direction and powder feeding angle on process stability of laser metal deposition of Al alloys thinwalled parts is systematically investigated.Optimized technological parameters for as-deposited Al Si10 Mg with high strength and plasticity are obtained.The tensile strength of deposited samples has reached 355 MPa,and elongation has reached10.1%.The microstructure of the deposited sample is composed of α-Al dendrites and Al-Si eutectic.At appropriate laser power,large-scaled rod-like phase will appear.The misorientation angle,texture and grain size is quite different from typical α-Al dendrites.Especially,Σ3 grain boundary are found,which play an important role in the improvement of mechanical properties.In order to solve the geometry problems such as concave defects and wave-like defects,online monitoring system for forming quality is established.The correlation between reflected laser signals and accumulated layer height,infrared light signals and layer width is built,repectively.The monitoring machenism of reflected laser is clarified.Monitoring capacity is enhanced based on kurtosis.Signal process is completed and feature information is extracted.Layer height and layer width during laser metal deposition of aluminum alloys in monitored in real time.Furthermore,regarding circular shells,variable cross-section shells and shells with large tilt angle as research subjects,control strategy for process parameters is proposed based on heat accumulation at typical location.Combining with the monitoring technology,Al Si10 Mg shells with above features are additive manufactured by laser metal deposition.The reliability of monitoring technology and process strategy is verified.In summary,through the research on porosity control,geometry control and process monitoring for aluminum alloys thin-walled structure by laser metal deposition,these works can provide some theoretical basis and technical support for the application of laser metal deposition of Aluminum alloy thin-walled structure.And these works can also also illuminate the related mechanism of laser metal deposition of aluminum alloys. |
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