A method for forming a metallic structure (701; 801) for use in an aircraft and having a non-linear aperture (716; 810; 902; 922; 942; 962) extending therethrough, wherein the metallic structure is for use in a gas turbine engine and the non-linear aperture provides a channel (908; 964, 966) for cooling fluid or acoustic attenuation, comprising: providing a main tool having a formation surface corresponding to a desired structure shape of the metallic structure; positioning a temporary tool into a slot defined by the main tool prior to forming the base layer of material on the formation surface; forming a base layer of material on the formation surface using a cold-spray technique prior to attaching a removable tool (712; 802; 900; 920; 940; 960) to the main tool, wherein the removable tool (712; 802; 900; 920; 940; 960) has a shape corresponding to a desired aperture shape of the non-linear aperture to the main tool, wherein attaching the removable tool (712; 802; 900; 920; 940; 960) to the main tool includes positioning a connection end of the removable tool into the slot defined by the main tool; depositing a layer of material on the base layer of material using the cold-spray technique in such a way that it surrounds the removable tool; removing the removable tool from the layer of material such that the layer of material defines the non-linear aperture; and removing the layer of material from the main tool to separate the metallic structure having the non-linear aperture from the main tool, wherein: the removable tool has at least two bends (728, 730) such that the non-linear aperture includes a first portion extending into the metallic structure from a first surface of the metallic structure, a second portion extending into the metallic structure from a second surface of the metallic structure, and a third portion extending from the first portion to the second portion; and / or wherein the removable tool resembles a grid, a rounded spiral, or a rectilinear spiral (900; 920; 940); and / or wherein a portion of the removable tool has a conical shape such that the non-linear aperture has a diffusion feature corresponding to the conical shape, the method further comprising removing the temporary tool from the slot after forming the base layer of material on the formation surface and prior to attaching the removable tool (712, 802, 920, 940, 960) to the main tool. providing a main tool having a formation surface corresponding to a desired structure shape of the metallic structure; positioning a temporary tool into a slot defined by the main tool prior to forming the base layer of material on the formation surface; forming a base layer of material on the formation surface using a cold-spray technique prior to attaching a removable tool (712; 802; 900; 920; 940; 960) to the main tool, wherein the removable tool (712; 802; 900; 920; 940; 960) has a shape corresponding to a desired aperture shape of the non-linear aperture to the main tool, wherein attaching the removable tool (712; 802; 900; 920; 940; 960) to the main tool includes positioning a connection end of the removable tool into the slot defined by the main tool; depositing a layer of material on the base layer of material using the cold-spray technique in such a way that it surrounds the removable tool; removing the removable tool from the layer of material such that the layer of material defines the non-linear aperture; and removing the layer of material from the main tool to separate the metallic structure having the non-linear aperture from the main tool, wherein: the removable tool has at least two bends (728, 730) such that the non-linear aperture includes a first portion extending into the metallic structure from a first surface of the metallic structure, a second portion extending into the metallic structure from a second surface of the metallic structure, and a third portion extending from the first portion to the second portion; and / or wherein the removable tool resembles a grid, a rounded spiral, or a rectilinear spiral (900; 920; 940); and / or wherein a portion of the removable tool has a conical shape such that the non-linear aperture has a diffusion feature corresponding to the conical shape, the method further comprising removing the temporary tool from the slot after forming the base layer of material on the formation surface and prior to attaching the removable tool (712, 802, 920, 940, 960) to the main tool. the removable tool has at least two bends (728, 730) such that the non-linear aperture includes a first portion extending into the metallic structure from a first surface of the metallic structure, a second portion extending into the metallic structure from a second surface of the metallic structure, and a third portion extending from the first portion to the second portion; and / or wherein the removable tool resembles a grid, a rounded spiral, or a rectilinear spiral (900; 920; 940); and / or wherein a portion of the removable tool has a conical shape such that the non-linear aperture has a diffusion feature corresponding to the conical shape, wherein the removable tool resembles a grid, a rounded spiral, or a rectilinear spiral (900; 920; 940); and / or wherein a portion of the removable tool has a conical shape such that the non-linear aperture has a diffusion feature corresponding to the conical shape, the method further comprising removing the temporary tool from the slot after forming the base layer of material on the formation surface and prior to attaching the removable tool (712, 802, 920, 940, 960) to the main tool.