Fatigue behaviour of fastener holes in high‐strength aluminium plates repaired by cold spray deposition

Abstract

This work examines the fatigue behavior of additive cold spray (CS) repairs of AA7075 and AA2024 fastener holes. Structural ring repairs around fastener holes were made by machining blend-outs ranging from 1/8 to 1/2 the thickness of the plate, then refilling the section of removed material with CS deposition. The repairs were then tested in a lap shear geometry with the repair on both the free (outside), and the mating (inside) surfaces, as well as in remote uniaxial tension. CS repairs for the inside lap shear AA7075 repair configuration and the outside lap shear AA2024 repair configuration were found to have significantly increased fatigue lives even exceeding the number of cycles to failure of the undamaged, unrepaired control plates. Further, none of the CS repairs caused any detrimental impact on fatigue life, and microhardness results indicate that no thermal damage to the substrate occurred. Some interface cracking was seen in the CS repairs, however no separation of the repair from the substrate was observed. I. INTRODUCTION Cold spray (CS) is a solid-state material deposition process that uses the kinetic, rather than thermal, energy of the impacting particles to bond with the substrate. In the CS process, high pressure gas is expanded through a converging diverging (de Laval) nozzle causing it to accelerate, speeding up the suspended powder with it. The exit velocity depends on the gas type, input pressure, temperature, and nozzle geometry, and varies from 200-1500 m/s [1]. Various gases can be used in CS, including compressed air, N 2 , and He, the latter of which, because of its lower molecular mass allows for higher gas and thus particle velocities. The impact velocity is one of the primary parameters that determines the quality of the deposit. Increasing the particle velocity generally leads to enhanced particle deformation, bonding, and compaction, resulting in increased density, and electrical, thermal, and mechanical properties of the deposited material [1, 2].

Experimental Studies into the Analysis Required for the Durability Assessment of 7075 and 6061 Cold Spray Repairs to Military Aircraft

“…A range of examples are presented in which the interface between the SPD and the substrate only fails subsequent to crack growth in the substrate. This failure mechanism is similar to those seen in [13][14][15][19][20][21], in that they involve the nucleation and subsequent growth from small material discontinuities in the substructure, at the intersection between the cold spray and the substructure, followed by delamination at the SPD to substrate interface. We also show that failure of cold spray repaired/coated panels can be due to the nucleation and growth of cracks in the substructure immediately outside of the coated/repaired region.…”

“…As outlined in the detailed reviews presented in [1][2][3][4][5] and in [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22], cold spray, also known as Supersonic Particle Deposition (SPD), is increasingly being used to repair military aircraft. Whilst the initial focus was on geometry restoration [1][2][3][4][5][6], attention has subsequently expanded to include its use to maintain the structural integrity of aircraft structural components [1,[7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. However, with the exception of [1,[7][8][9]…”

“…This meant that the (nucleating) crack was exposed to the environment-in this case, laboratory air. Since, as shown in [13][14][15][19][20][21], cracks that determine the fatigue life in cold spray repairs can nucleate, and subsequently grow, from small sub mm internal material discontinuities in the substrate at the intersection of the cold spray coating with the substrate, such cracks may not be exposed to the environment. This is an important distinction since, as explained in [14], such cracks grow in a "vacuum-like" environment and, therefore, as per [27][28][29][30], grow much slower than if they could "see" the environment.…”

Review of Requirements for the Durability and Damage Tolerance Certification of Additively Manufactured Aircraft Structural Parts and AM Repairs

“…Similarly, it has been shown [46] that for cold spray repair, which is sometimes referred to as supersonic particle deposition (SPD) [47] which is also being used to repair damaged aircraft structures [46,[60][61][62][63][64], the Hartman-Shijve equation can be used to compute the fastest growing crack in the substrate.…”

Cold-Sprayed Al6061 Coatings: Online Spray Monitoring and Influence of Process Parameters on Coating Properties

“…For example, corrosion resistance, electrical conductivity, wear resistance, functional properties, and repairing possibilities are application fields for cold-sprayed coatings. Especially, for corrosion protection and repairing possibilities, Al and Al-based alloy coatings have received a lot of interest due to their good material properties and high suitability for cold spraying as the feedstock materials [1][2][3][4][5]. In cold spraying, low melting point materials such as Al alloys can be sprayed without oxidation.…”