Heat treatment of cold-sprayed C355 Al for repair: microstructure and mechanical properties

Cold gas dynamic spraying of commercially pure aluminum is widely used for dimensional repair in the aerospace sector as it is capable of producing oxide-free deposits of hundreds of micrometer thickness with strong bonding to the substrate, based on adhesive pull-off tests, and often with enhanced hardness compared to the powder prior to spraying. There is significant interest in extending this application to structural, load-bearing repairs. Particularly, in the case of high-strength aluminum alloys, cold spray deposits can exhibit high levels of porosity and microcracks, leading to mechanical properties that are inadequate for most load-bearing applications. Here, heat treatment was investigated as a potential means of improving the properties of cold-sprayed coatings from Al alloy C355. Coatings produced with process conditions of 500 °C and 60 bar were heat-treated at 175, 200, 225, 250 °C for 4 h in air, and the evolution of the microstructure and microhardness was analyzed. Heat treatment at 225 and 250 °C revealed a decreased porosity (~ 0.14% and 0.02%, respectively) with the former yielding slightly reduced hardness (105 versus 130 HV0.05 as-sprayed). Compressive residual stress levels were approximately halved at all depths into the coating after heat treatment, and tensile testing showed an improvement in ductility

Corrosion Study of Cold Sprayed Aluminum Coatings onto Al 7075 Alloy

Peer reviewed: YesNRC publication: Ye

A laser thermal cycling rig as a new method to characterize the evolution of coating adhesion under thermal cycle

Thermal sprayed coatings are often used for high temperature applications and, per se, are subjected to transient temperature gradients during operation. The recurrent temperature changes generate stresses that damage the coating with time, and can even lead to its delamination. The most common methods to evaluate coating behavior under thermal cycling are furnace testing or burner rigs. Both approaches cannot match the conditions reached in service for several applications, in term of the achievable heating rates for instance. As a consequence, a versatile and robust method to evaluate coating resistance to spalling under thermal cycles is still to be found. This paper presents the development of a thermal cycling rig where the heat input is provided by a laser. This rig allows easy testing of several samples jointly for heating rates as high as 55\ub0C/s and for thousands of thermal cycles. Preliminary trials have allowed the development of different spalling criteria. Finally, it was found that SS430-based materials arc-sprayed on Al substrates exhibit higher delamination resistance (life) under rapid heating/cooling cycles than SS304 coatings on the same substrate. For such high heating rates, the thermal stresses generated in the coating would be more critical than the thermal mismatch at the interface coating/substrate.Peer reviewed: YesNRC publication: Ye

Mechanical and microstructural characterization of cold-sprayed Ti-6Al-4V after heat treatment

The cold spray of Ti-6Al-4V coatings deposited on Ti-6Al-4V substrates has been investigated. Coatings were produced using nitrogen and helium as propellant gases and subsequently heat treated with various temperature-time conditions. The microstructure was characterized by SEM and optical microscopy while mechanical properties were measured by microhardness and tensile testing. It is shown that coatings sprayed with nitrogen gas were relatively porous in comparison to the nearly completely dense coatings obtained with helium gas. In the as-sprayed condition, coatings displayed high hardness but low tensile strength. Heat treatments at temperatures of 600 C and higher resulted in a decrease in hardness due to microstructural changes within the particles including recovery, recrystallization, and/or phase transformation. However, an increase in tensile strength was attributed to improved inter-particle bonding due to an observed change from brittle to ductile features on the fracture surface. The highest strength coating produced was a helium-sprayed coating annealed at 600 C, which featured a tensile strength ~85% of the minimum required bulk value and coating/substrate microstructures similar to the as-received powder/substrate microstructures. \ua9 2013 ASM International.Peer reviewed: YesNRC publication: Ye

Simulation of gas-substrate heat exchange during cold-gas dynamic spraying

Peer reviewed: YesNRC publication: Ye