Microstructure And Properties Of Laser Cladding TiAl Coating On TC4 Alloy Surface

Titanium alloy has low density,small specific gravity,non-magnetic weldability,high specific strength and corrosion resistance.Compared with magnesium alloy and aluminum alloy,it has more excellent high-temperature performance,and is widely used in aerospace,shipbuilding,petroleum and military industries.However,its low hardness and poor wear resistance limit its application in high temperature components due to its poor high temperature resistance compared with Ni-based superalloys.Compared with titanium alloy,TiAl alloy has good high temperature performance,low density,high elastic modulus and specific strength,and has good metallurgical bonding performance with titanium alloy,which can be used as surface protection coating for titanium alloy parts under high temperature service conditions.In this paper,commercial Ti4822 powder was used to prepare TiAl alloy protective coating on TC4 alloy by laser cladding,so as to improve the surface properties of TC4alloy.The effects of laser power and scanning speed on the coating quality were explored.The phase distribution,hardness distribution,high temperature cyclic oxidation resistance and friction and wear properties of the coating were analyzed by scanning electron microscope,EDS spectrometer,XRD diffraction,hardness test,cyclic oxidation test and friction and wear test.The coating detection and experimental results are as follows:(1)When the laser power is 2300 W,the scanning speed is 4%and the overlap rate is50%,the surface quality of the coating is better.The coating and the substrate are metallurgically bonded without cracks and holes.Excessive or too slow laser power will cause rough surface of the coating.The coating is mainly composed of duplex structure and near lamellar structure,and the microstructure distribution of lap coating is relatively uniform.The average hardness of the single coating is more than 470 HV,1.44 times that of the substrate.The hardness of the lap coating near the lap ridge is more than 490 HV,and the hardness between the ridges is 470 HV.(2)The oxidation of TC4 substrate above 600°C will generate obvious oxide layer,and the oxidation at 600°C will generate intermediate products Ti₆O and Ti₃O.The oxidation products above 800°C are mainly Ti O₂.Ti O₂is not dense and has no protective effect,and the substrate will be continuously oxidized.However,TiAl coating will undergo(3)a small amount of localized oxidation at 600°C,and an obvious oxide layer will be formed on the coating surface between 800°C and 900°C.However,the coating surface is not completely oxidized after 50 h of cyclic oxidation.At 1000°C,the oxide film will lose its protection from the edge oxidation damage.The oxidation products of the coating are mainly Ti O₂and Al₂O₃,and the oxide film with the outer layer of Ti O₂,the middle layer of Al₂O₃,and the inner layer of Ti O₂and Al₂O₃mixed layer will be formed,and the Al₂O₃layer plays a major role in antioxidant protection.The oxidation resistance of TiAl coating and the protective effect of the oxide film make the oxidation failure temperature of the coating between 900°C and 1000°C,while the uncompact oxide film of TC4 substrate does not have protective effect,which makes the oxidation continue,and the oxidation failure temperature does not exceed 600°C.(4)The friction and wear properties of TiAl coating and TC4 substrate at different temperatures were compared and analyzed.The wear between the sample surface and the friction pair was directly observed below 200°C,and the friction and wear resistance of the coating was better than that of the substrate.At 400°C,the TC4 substrate gradually transforms into the wear between the oxide layer and the matching pair,while the TiAl coating needs to be above 600°C to gradually transform into the wear between the oxide layer and the matching pair.The friction and wear resistance of TC4 substrate is better than that of TiAl coating at 400°C-600°C.At 800°C,TiAl coating and TC4 substrate are transformed into the wear between the oxide layer and the matching pair,and the wear performance of TiAl coating is better than that of TC4 substrate.The larger the load is at high temperature,the rougher the wear surface of the coating is,and the larger the friction coefficient is.The wear rate of load exceeding 10N increases with the increase of load.The wear surface is rough when the rotational speed is low at high temperature.The wear rate and friction coefficient decrease with the increase of rotational speed below 100 r/min.When the rotational speed is 200 r/min,the friction heat-generating material further softens and the cutting effect is strengthened,resulting in the increase of wear rate and friction coefficient.