DESCRIPTION

MANUFACTURING OF STANDARD TEST BLOCKS CONTAINING ARTIFICIAL DEFECTS FOR ULTRASONIC INSPECTION

This invention is relevant, for calibration and comparison purposes in especially the ultrasonic testing (UT) of aircraft engine parts and other engineering structures and materials that require ultrasonic inspection, to the manufacturing methods of standard test blocks made of Steel, Superalloy or Titanium alloys that contains artificial defects of known type, shape, size and with known spatial position in the subject part.

In today's technology, aircraft engines parts are tested by using ultrasonic inspection method for structural defects that may have formed during manufacturing. In the current ultrasonic inspection method, depending on the defect type if the defect is above a certain size it presence in the part being tested can be realized but as the defect type cannot be determined a realistic estimation of service life for such a defect-containing part can not be made. By producing metallic blocks to be used as standard test blocks in ultrasonic inspection that contain defects of known type, shape, size and spatial position in the part it will be possible to analyze the ultrasonic inspection signals depending on the defect types. Thus it will be possible to determine the service life of actual parts realistically and to use them more economically.

The blocks are composed of two parts, one being thinner than the other one (1 and 2), and made of the materials used in production of aircraft engine parts, namely steel, superalloy and titanium alloys. The thicknesses of the parts are related to the dept of the location of the defects with reference to the surface after the two parts are bonded by diffusion, and thus chosen. Since it has no relevance to the process itself different thicknesses may be chosen as desired and even a matching stepped configuration may be manufactured. Artificial defects of various kinds, shapes and dimensions (6 and 7) are placed in the cavities (5) prepared on the surfaces (3 and 4) to be bonded by diffusion. These cavities are prepared, using a high precision electric discharge machine (EDM), marginally

smaller than the defects to be placed in and thus the defects sink into the matrix (host material) during the following high temperature process resulting in a perfect contact interface. If the desired defect is a void then the cavity is prepared with final dimensions. After placing the defects in then- positions the two block pieces are pressed together and bonded by diffusion using Vacuum Hot Pressing (VHP) or Hot Isostatic Pressing (HIP) employing different temperature, time and load parameters depending on the material. The defect types with various shape and sizes placed in steel matrix are spherical carbides (e.g. tungsten carbide) or oxides (e.g. Al ₂ C^); in superalloys matrix carbides, oxides or compositional defects (phases of different composition compared to the matrix); and in titanium alloy matrix carbides, or hard-alpha phase (an alpha titanium phase that has much higher hardness levels compared to normal alpha titanium due to dissolved oxygen plus nitrogen content up to 25%).

Furthermore, regardless of the matrix material, by opening matching cavities of any size and shape on the surfaces of the upper and lower block halves to be bonded, void type artificial defects are obtained in the final block following diffusion bonding that represents void type actual defects that can form during the production of the material.

Yet another defect type that may occur in metallic materials is locally large-grained regions that have abnormally large grains compared to the rest of the matrix. The gram size difference between such regions and the rest of the matrix may be between 2 to 10 in

ASTM grain size scale. Such large-grained regions can be formed on the surface of one or, in a matching configuration, on both of the metallic blocks (steel, superalloy or titanium alloy) to be bonded, and thus by diffusion-bonding these pieces via VHP or HIP blocks containing artificial defect regions made of large-grains with known location, dimensions and ASTM grain-size are obtained. To form such large-grained regions in a controlled manner, with known grain-sizes and dimensions three methods were used. In the first method, spot welding pulses were applied on a bonding surface until desired grain-size and large-grained region size is obtained. Then, the recast top layer is removed by fine machining or grinding. Thus, only the heat-affected bottom area of the location where multiple spot-weld pulses remains. It is this area that is encapsulated in the final block as the large-grained defect region following the diffusion bonding of the second

block to this surface. In this process, the grain size and the dimensions of the artificial defect region is adjusted by the power of spot-welding and the number of weld pulses. The necessary power and number of pulses are determined on preliminary trials depending on the starting material.

The second way of obtaining a large-grained region as artificial defect is, through the same logic and process sequence as in spot welding method, to apply repeated laser pulses on the selected spot of the surface. The large-grained region beneath the recast area where the laser is applied is utilized to constitute the artificial defect region. For this purpose the recast top layer is removed via fine machining or grinding operation. Thus, where the laser is applied only the heat-affected region remains. It is this area that is encapsulated hi the final block as the large-grained defect region following the diffusion bonding of the second block to this surface. In this process, the grain size and the dimensions of the artificial defect region is adjusted by the power of laser and the number of laser pulses. The necessary power and number of pulses are determined on preliminary trials depending on the starting material.

As a third way of obtaining a large-grained artificial defect region, first the material identical to the block matrix and of desired shape and dimensions is heat treated at necessary temperature and for time length depending on the material until the desired ASTM grain size is achieved. Then, the pieces of desired dimensions prepared from this large-grained material are placed into the cavities opened by using EDM or fine machining on the surfaces of the blocks to be bonded. Thus, when the two blocks are diffusion-bonded via VHP or HIP large-grained artificial defect regions of known location, dimensions and gram size are obtained hi the final block. When very small regions to constitute the artificial defect region are desired to be prepared this way if EDM or fine machining becomes difficult on large-grained material, pieces cut to the desired shape and dimensions while identical to the matrix should be heat treated under vacuum to achieve large gram sizes. Thus, the pieces with desired ASTM gram sizes obtained without surface oxidation to constitute the artificial defect regions are placed into the cavities prepared precisely via EDM or fine machining on the surfaces to be

bonded. Thus, when the two blocks are diffusion-bonded via VHP or HIP, large-grained artificial defect regions of known location, shape and grain size are encapsulated within.

Different temperature, time and load parameters were used in VHP and HIP processes depending on the matrix material and the defects to be encapsulated. The reason for this was both the different diffusion requirements for each matrix material as well the necessity to preserve the shape and the dimensions of the artificial defects and/or matrix materials, and the microstructure of the defect and/or matrix material (e.g. grain size or chemical composition), and the spatial location of the artificial defect within the block.

The VHP and HIP parameters determined are: For blocks made of steel the temperature range: 1000 ⁰ C to 1250 ⁰ C, applied stress range: 25Kg/cm ² to 200 Kg/cm ² , time length: 1 to 5 hours; for Inconel 718 blocks temperature range: 1000°C to 1250°C, applied stress: 35Kg/cm ² to 200 Kg/cm ² ; time length: 1.5 to 5 hrs; and for Ti6A14V alloys temperature range: 750°C to 1050°C; applied stress: 20Kg/cm ² to 150 Kg/cm ² , time length: 1 to 4 hours. Under appropriate pressing parameters the blocks halves are bonded to each other completely due to atomic diffusion. The blocks thus formed that carry artificial defects within were subjected to ultrasonic inspection tests. During these ultrasonic inspections different signals were obtained and recorded from the artificial defects of known spatial location, shape, and dimensions within the blocks. These recordings were later used as references for the ultrasonic inspection of actual parts.

Manufacturing of the artificial defects made of hard alpha titanium (hard alpha) that are placed in titanium alloy blocks is another route in this process (the process of manufacturing standard test blocks containing artificial defects to be used in ultrasonic testing). Hard alpha titanium phase is the alpha titanium phase that has increased hardness due to the dissolved oxygen and/or nitrogen atoms. Three methods were employed to produce this phase. The first method was to form hard alpha phase of a known thickness on the surface of one or both titanium alloy blocks that will constitute the final block following diffusion-bonding, and then remove the undesired portions of this phase via EDM or fine machining (machining or grinding) in a fashion that this phase

is maintained in desired location and geometry followed by encapsulation within the titanium block following VHP or HIP. In the second method, instead of forming this phase directly on the surfaces of the parts that will constitute the final block, a titanium alloy piece of sufficient thickness was transformed into hard alpha completely, and then pieces that will constitute the artificial defects of certain shape and dimensions were cut from this hard alpha piece and placed into the cavities prepared for them in appropriate dimensions on the untransforaied titanium alloy blocks, and were encapsulated within the final block following diffusion bonding via VHP or HEP. In the third method, first the titanium alloy pieces of suitable size and shapes that will constitute the artificial defects were cut from titanium alloy via EDM, machining or grinding, and then these pieces were transformed into hard alpha phase and placed into the cavities of fitting dimensions on the block surfaces and followed by encapsulation into the final titanium alloy block by diffusion bonding during VHP or HD? process. For all three methods, the formation of hard alpha phase was achieved via heat treatment of titanium alloy pieces wrapped in zirconium foil at a temperature range of 750°C to 95O ⁰ C for 3 to30 hours depending on the temperature chosen. The purpose of zirconium foil here is to ensure dissolution of oxygen in the structure by suppressing the oxidation due to reduced oxygen partial pressure as the zirconium scavenges some of the available oxygen. Otherwise, if the part is being heat treated under atmospheric conditions the surfaces will be oxidized completely.

Likewise, if carbide type artificial defects are to be formed by conversion of the surface carbon is placed on the selected area with desired diameter and the whole sample is wrapped again with zirconium foil. Thus, during the carbide forming heat treatment excessive oxidation due to contact between the metallic surface and carbon with the oxygen in the air is prevented. Depending on the metal used local carbides can be formed via heat treatment at a temperature range between 700 and 1000 ⁰ C for 1 to 2 hours. Carbon enters the metallic structure over a limited region via diffusion and forms carbide with carbide forming elements. The surface is then cleaned by a final machining method and then j oined to a second block by diffusion thus encapsulating the carbide within.

Here the subject of manufacturing ultrasonic inspection blocks containing artificial defects does not present an innovation in terms of the final process that creates diffusion bonding (VHP or HDP) of two metallic blocks so as to encapsulate the artificial defects in between. This part of the process was previously emphasized in some Japanese patents. However, in those patents the process of producing the defect and the types of defects employed are different than those mentioned here. This is to say that the production of the defects via conversion of the surfaces (be it the large-grained region, or the formation of hard alpha or carbide phases that are produced by conversion of the surface) brings about an advantage. This advantage may be expressed as the following: if the defect is formed on only one surface and then covered by an un-treated block, at least in one direction the interface between the defect and the matrix is excellent (perfect transition). If the defect is formed by conversion of the surfaces on exactly matching locations of both blocks to be bonded then when inspecting ultrasonically from both surfaces of the block a perfect transition will be achieved. Otherwise, when defects are placed into the pre-prepared cavities on the surfaces it is very difficult and often impossible to achieve perfect bonding with the host matrix in every direction.

EXPLANATIONS OF THE FIGURES

The metallic blocks produced for this invention to reach its goal are shown in the annexed figures showing:

Figure 1-2: View of the upper part (thinner) of the metallic test block. Figure 3-4: View of the lower part (thick) of the metallic test block. Figure 5-6: Front view of the metallic upper and lower blocks following bonding via pressing.

Figure 7-8: Side view of the metallic upper and lower blocks following bonding via pressing.

The parts hi the figures are numbered as described below:

1) Upper (thin) block

2) Lower (thick) block

3) The lower surface of the upper block with machined cavities for the defects. 4) The upper surface of the lower block with machined cavities for the defects.

5) The cavities prepared on the surfaces for the defects to be placed in.

6) The defects of different dimensions and shapes that were formed by conversion of the surface at certain locations.