Mechanism Of Melt Pool Evolution Process Of Laser Additive Deposition Under Gravitational Effect

This thesis takes NV E690 marine high-strength steel as the research object,based on the current research status,combined with the actual development of the process technology,the mechanism study of the evolution process of the laser additive deposition molten pool under the gravity effect was carried out.An orthogonal visual inspection platform was built to investigate the melt pool evolution process when the substrate was horizontal and inclined to the gravity direction under the action of continuous wave laser.A paraxial vision detection platform and a height detection platform were built to study the influence of pulsed laser on the gravity induced humping effect and the cladding height.The research contents and innovations of this thesis are as follows:(1)Based on the orthogonal visual inspection platform with a high-speed camera and a coaxial industrial camera to investigate the evolution of the molten pool when the substrate is perpendicular to the direction of gravity.The globular phenomenon of a rough cladding single-track boundary caused by the solidification of the droplets was discovered.The dynamics of droplet movement was studied,and the "spreading/solidification" competition model of droplets was established.Aiming at the globular phenomenon,combined with the contact angle theory,the influence of substrate preheating,oxidation and temperature on the wettability of the droplet was investigated,and a solution was proposed to reduce the globular phenomenon.(2)The laser energy distribution model on the surface of the molten pool when the substrate was inclined to the direction of gravity was established.Based on the orthogonal visual inspection platform,the variation of the coaxial geometric size of the molten pool under different substrate inclination angles was studied.The gravity induced humping effect contributed to a huge height fluctuation on the deposited single track.Based on high-speed camera images,the evolution process and evolution mechanism of the gravity-induced hump effect were explained.(3)A process method based on the time-sequence controlled laser to eliminate gravity induced hump effect was proposed.A paraxial visual inspection platform was established to investigate the influence of the time-sequence controlled laser on the melt pool evolution process.It was found that several molten pools occurred in a single laser spot,and the coalescence phenomenon between the melt pools was observed by forming a liquid bridge.The image processing technique was applied to extract the single-track cladding height data and study the influence of the pulse frequencies and the pulse duty cycles on the cladding height variations.The results showed that the light gap,based on time-sequence controlled cladding,could cool the molten pool and eliminate the hump effect caused by gravity.