Progress in the application of cold gas dynamic spraying to repairing continuous casting molds

A new continuous casting mold repairing method--cold gas dynamic spraying （CGDS） is introduced. The study investigates the advantages of the CGDS process regarding repairing operation, such as convenient, in-situ repairation,little heat delivery, microstructural and dimensional stability and other special applications. Microstructure and mechanical properties of the copper alloy coating, nickel coating, ceramic composite coating, and their interface to the substrates ,which are usually used in repairing operation have been researched by means of optical microscopy （ OM）, scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and micro-hardness tests. Experimental results have demonstrated the relative density of the copper alloy coating is as high as 98.7%, and that no obvious difference can be observed between the CrZrCu substrate and the Cu alloy coating in terms of microstructures; thus the interface is quite difficult to be identified. The bonding strength and micro-hardness of the Cu alloy coating reach up to 37 MPa and 310 HV0.2 ,respectively. The interface between the copper alloy coating and the nickel coating is either zigzag or wave shaped, and the cohesion is relatively good. As-sprayed nickel coating is dominated by severely deformed particles,and the relative density is up to 98.5%. Complete recrystallization occurred after annealing at 900℃ for one hour,while its micro-hardness remains as high as 124.1 HV02. All these results have indicated that CGDS is a promising technology for repairing the continuous casting mold and that its future development is prosperous as well.

Numerical simulation of the effects of the impact velocity on the particle deposition characteristics in cold gas dynamic spraying

In this study, the effects of the impact velocity on the particle deposition characteristics in cold gas dynamic spraying （CGDS） of 304 stainless steel （SS） on an interstitial free （IF） steel substrate are numerical simulated by means of a finite element analysis （FEA）. The results have illustrated that when the particle impact velocity exceeds a critical value at which adiabatic shear instability of the particle starts to occur. Meanwhile, the fatten ratio and impact crater depth （or the effective contacting area ） increase rapidly. The particle-substrate bonding and deposition mechanism can be attributed to such an adiabatic shear deformation induced by both the compressive force and the slide friction force of particle. The critical velocity can be predicted by numerical simulation, which is useful to optimize the CGDS processing parameters for various materials.

Microstructure and micro-hardness of the Inconel 718 superalloy coating prepared by cold gas dynamic spraying

Inconel 718 superalloy coating was prepared on a martensitic steel substrate by cold gas dynamic spraying （ CGDS ）. Microstructure and micro-hardness of the coating were characterized by means of scanning electronic microscope （SEM） ,energy dispersive spectrometer （EDS）, and micro-hardness and nano-indentation tests. The results have indicated that the coating＇ s thickness can reach more than 300 um, and there exists a good interfacial cohesion between the coating and the substrate. The quantity and size of the defects at the bottom, middle and top areas increase gradually. There is no significant change to the original microstructure of the Inconel 718 superalloy particle after the process of CGDS ;and an obvious torsional deformation on the particle＇ s profile is produced while little deformation at the center part occurs. The micro-hardness of the coating at the bottom, middle and top areas decreases in turn. Compared with the bottom area, the micro-hardness of the middle and top areas decreases by about 10% and 21%, respectively. The nano-hardness of the coating is much lower than the original particle,which decreases by about 13.5% at the bottom area and 28% at the top area,respectively. The distribution of micro-defects is an important factor to the micro-hardness of the coating.

Influence of the preheated substrate temperature on the microstructure deposition behavior of the 304 stainless steel coatings deposited by cold gas dynamic spraying

The effects of the substrate temperature on the deposition and microstructure of the 304 stainless steel （SS） cold gas dynamic spraying （CGDS） coatings were investigated. It was found that the higher substrate temperature could increase the deposition rate of the 304 SS particles,even the oxide films existed at the interface. There was a critical oxide film with a thickness of 3 -4 um which could be destroyed by the impacted particles. The micro-hardness and microstructure of the 304 SS coatings under different substrate preheating temperatures were almost the same.

Study on the 316L stainless steel coatings and their properties by cold gas dynamic spraying

316L stainless steel coatings were prepared by cold gas dynamic spraying （CGDS） on a carbon steel. A new heater of sprayed powders was added to the traditional facility. The results show that the coating quality is obviously improved, due to better interface bonding and reduced porosity.

Numerical simulation of particle tracks in the cold gas dynamic spraying process

In this study,the distribution behavior of the particle flow field in cold gas dynamic spraying （CGDS） was simulated through the Computational fluid dynamics （CFD） method. Traces of the particles with different diameters in the gas flow field were analyzed, and effects of fiat and sphere substrates on the particle tracks were also compared .Simulation results indicate that different escaping directions of particles flow with the two substrates. These investigations. gave instructions on how to design the powder recovery and dusting machines in a CGDS system.

Effects of standoff distance on the supersonic flow fields in cold gas dynamic spraying

In this study,the effects of standoff distance （SOD） on the supersonic flow fields （including gas and particle flow fields） during cold gas dynamic spraying （ CGDS ） are investigated by means of computational fluid dynamics （CFD）. The variation of velocity,temperature,pressure and density with different SoD is elucidated through the analysis of the distribution properties of the flow fields. It is found that the shock waves in front of the substrate remarkably influence the gas and particle flow fields. The wave system of expansion waves and pressure waves come into being continuously. The velocity of gas reaches the super.,;onic speed at the position of the Mach disc, while it decreases sharply when the gas flow crosses the Mach disc. The optirnal SoDs are 40 mm for 1 um particles and 50 mm for both 5 um and 22 um particles.

Elastic Modulus of 304 Stainless Steel Coating by Cold Gas Dynamic Spraying

304 stainless steel coating was deposited on the IF steel substrate by cold gas dynamic spraying （CGDS）, and the elastic modulus of the 304 stainless steel coating was studied. The elastic modulus of cold sprayed 304 stain- less steel coating was measured using the three-point bend testing and the compound beam theory, and the other me- chanic parameters （such as the equivalent flexural rigidity and the moment of inertia of area） of the coatings were also calculated using this compound beam theory. It is found that the calculated results using the above methods are accu- rate and reliable. The elastic modulus value of the cold sprayed 304 stainless steel coating is 1. 179 X 105 MPa, and it is slightly lower than the 304 stainless steel plate （about 2 X 105 MPa）. It indicates that the elastic modulus of the cold sprayed coatings was quite different from the comparable bulk materials. The main reason is that the pores and other defects are existed in the coatings, and the elastic modulus of the coatings also depends on varies parameters such as the feed stock particle size, porosity, and processing parameters.

Fracture behavior of 304 stainless steel coatings by cold gas dynamic spray

304 stainless steel coating was deposited on the Interstitial-Free steel substrate by cold gas dynamic spraying （CGDS）. Three-point bending test of the cold sprayed 304 stainless steel coating was tested by SHIMADZU electro-hydraulic servo-controlled fatigue testing machine and the fracture morphology was examined by scanning electron microscopy. The results showed that the fracture behavior of the cold sprayed 304 stainless steel coating was brittleness fracture. The crack in the coating occurred in the interfaces between particles and the crack extended to the internal of the coating with the increase of the load. When the crack has extended to the combination interface between coating and substrate, the crack extended to the two sides. The microstructure and mechanical property of the cold sprayed 304 stainless steel coating have been optimized after heat treatment.