| Due to the obvious advantages such as unrestricted forming size,high flexibility,wide material selectivity,and easy automation,coaxial powder feeding laser cladding has become the preferred additive manufacturing technology for net forming of large metal parts,preparation of gradient materials,parts repair and surface modification.Since the coaxial powder feeding laser cladding process takes place in an open environment,defects such as porosity cracks are easily generated to affect the cladding performance,and in addition,the powder addition and dynamic evolution of the melt pool have a significant impact on the cladding quality.It is difficult to obtain the temperature and flow field distribution of the melt pool,the influence of the powder on the melt pool,and the mechanism of the formation and evolution of pores by experimental means only.Since Inconel718 alloy has excellent elevated-temperature mechanical properties and is widely used in chemical machinery,aerospace,and other industries,the coaxial powder feeding laser cladding of Inconel718 alloy is used as the research object,combined with multi-physics field coupling simulation and experiment,to study the evolution of the temperature field,flow field and geometry of single-pass laser cladding layer,the formation process of porosity defects.The results of the study can provide theoretical guidance for the optimization and quality control of coaxial powder feeding laser cladding.The main research contents of this paper are as follows.(1)Taking into account the interaction between laser and metal material,changes in thermophysical properties,phase change,convection,the impact of powder on the melt pool,and other phenomena,simplified the powder temperature rise model,proposed an improved double ellipsoidal heat source model,and the high-speed camera is used to obtain the initial velocity of powder,and a model of the whole process of laser cladding with gas-liquid-solid multiphase coupled was established.The evolution of the melting temperature field under different process parameters was obtained.The maximum temperature of the melt pool increases with the rise of laser power,the height,width,and depth of the melt pool are growing as well;with the increase of scanning speed,the maximum temperature of the melt pool starts to decrease,the melt pool decreases in height and width,the depth of melt pool changes less.The temperature gradient at the front end of the melt pool is the largest,and the temperature gradient at the back end is smaller.(2)The evolution law of the melt pool of coaxial powder feeding laser cladding is obtained.Before the powder reaches the substrate,the melt pool has been formed,and the flow form of the melt pool at this time is mainly Marangoni convection caused by the surface tension gradient,and the vapor recoil pressure makes the surface of the melt pool fluctuate slightly.Based on the Pelect number analysis,it is concluded that once the melt pool is formed convection is the main mode of heat transfer.When the powder particles hit the melt pool,the radial outward flow is generated from the impact point,while the vertical direction of the impact point instantaneously generates downward flow and fluctuates up and down afterwards,at this moment the impact of the powder on the melt pool becomes the main driving force of the melt pool flow and intensifies the convection of the melt pool.When a large amount of powder is injected into the melt pool,the flow is more complex and variable,and the trend is downward flow at the impact point and radial outward flow on the surface,and the free surface of the melt pool fluctuates drastically.It is found that the cladding layer usually produces sticky powder on the surface,which is mainly caused by the unmelted particles being trapped by the area to be solidified at the back end of the melt pool,and the application of an auxiliary heat source behind the laser spot can effectively extend the melt pool length and reduce the surface sticky powder.(3)A "sandwich" observation system was established to analyze the generation and evolution of bubbles in situ during the coaxial powder feeding laser cladding process.The results show that most of the bubbles in the melt pool are generated by the precipitation of gas dissolved in the melt due to the change of solubility,and a few are generated by the melting of hollow powder and the impact of powder particles on the melt pool surface,and most of the bubbles are generated at the front end of the melt pool and solidified at the back end of the melt pool.There are three main ways for bubbles to escape: floating out of the surface of the melt pool to escape through their buoyancy;moving with convection and contacting the surface of the melt pool to escape;violent breaking and escaping with fluctuations in the surface of the melt pool.When the bubble diameter is larger than 60μm,it is easier to escape through its buoyancy.96.94% of the porosity defects in the cladding layer obtained from the single-factor experiment are less than60μm in diameter.Porosity defects are closely related to energy density,higher or lower energy density will produce excessive porosity defects,the laser power has the greatest impact on porosity defects,and fewer porosity defects are generated near the 1300 W power.(4)The effect of process parameters on the morphology of the cladding layer was investigated by single-factor experiments combined with gray correlation algorithm and singlefactor analysis,and the results showed that the geometric morphology of the cladding layer was mainly influenced by the laser power and scanning speed.The dilution rate is more influenced by the laser power,and the contact angle is more influenced by the scanning speed.The relationship equation and evaluation function between the geometry of the cladding layer and the process parameters were established to provide a basis for the estimation of the geometry of the cladding layer.The influence of process parameters on microhardness was investigated.It was found that the highest value of microhardness is usually on the surface of the cladding layer,and the laser power and scanning speed are the main factors affecting microhardness. |
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