GB2322102A

GB2322102A - Chemical etching end-point determination

Info

Publication number: GB2322102A
Application number: GB9702962A
Authority: GB
Inventors: Nigel Haworth; Allister Mccool
Original assignee: British Aerospace PLC
Current assignee: BAE Systems PLC
Priority date: 1997-02-13
Filing date: 1997-02-13
Publication date: 1998-08-19
Also published as: GB9702962D0

Abstract

The chemical etching or chemical contour machining (CCM) of a metallic component is terminated when a sacrificial control piece is etched through. The end-point is determined by a rapid rise in the electrical resistance of the control piece. The control piece may be strip 10 having unmasked etching areas 16,18 and electrical connections 12 to a resistance meter, the strip having a thickness equal to the etch depth. Alternatively the control piece may be a length of wire.

Description

CHEMICAL CONTOUR MACHINING PROCESS Field of Invention The present invention relates to chemical contour machining (CCM) which is a technique shaping metallic parts by selectively removing material from a workpiece using an etchant.

Background Art Volume 16 of the Metals Handbook (1989, published by ASM International) describes CCM and methods of assessing the amount of material removed.

In the aerospace industry, CCM is used to shape components, particularly made of aluminium and titanium. However, CCM can also be used on components made of mild steel, low alloys steels, high alloy steels, stainless steels, conducting superalloys, magnesium alloys and niobium alloys.

According to a standard procedure, components are immersed in a tank containing an etchant, which will generally be an acid; in the case of titanium, the etchant can be a mixture of hydrofluoric and nitric acids. Prior to immersion, one or more selected areas of the component are covered by a mask so that the etchant only comes into contact with.

and removes material from, the unmasked selected areas.

The depth of material removed (the etch depth) in those areas is proportional to the submersion time. In order to control the etch depth, the etching rate of the particular etchant is measured at the beginning of a work shift by exposing a control piece made of the metal that will be etched in the bath; the control piece is of known thickness and is placed in the etchant bath for a known time period. By measuring the thickness of the control piece before and after exposure to the etchant bath (e.g. using ultrasonic thickness measuring equipment), the etch rate of the particular etchant can be calculated.

Using the etch rate, the predicted immersion time necessary to remove the required material is calculated. However, it has been found that such calculations are highly unreliable. Because the components subjected to CCM are expensive and would have to be discarded if too much material were removed the component is generally immersed in the etchant for only about 90% of the predicted etching time. The component is then removed from the bath and the thickness of the component is measured and a revised etch rate is calculated. The component is then returned to the bath and submitted to further CCM. After a further period of time, the component is again removed from the bath, rinsed and its thickness measured again; this process is repeated until the component has the designed thickness, within a given tolerance, which is typically + 0. lmm.

However, the etch rate can change over the course of an etching step and so it is often necessary in practice to remove and measure the components on several occasions until it has reached a thickness within the design tolerance. Each time the component is removed from the etching bath to measure its thickness, the etchant must be washed off, and the component cleaned prior to moving it to a work station for measurement. This involves a substantial amount of manual intervention and makes CCM time-consuming and expensive manufacturing procedure.

There is thus a need for a simple and reliable method of assessing when a component has been exposed to the etchant for long enough to achieve the required etch depth within the design tolerance.

Various attempts have been made to predict the etch rate and to maintain it within known parameters, including the completion of regular titration tests. However, using such techniques, it has not been possible to achieve the required accuracy.

In an article entitled "Determination of Depth and Rate of Etch based on Sacrificial Dissolution of One Arm of a Ratiometric Transducer" in Review of Scientific Instrumentation, 5 May 1992 it has also been proposed to determine when a required etch depth has been achieved. In this method, one half of a control sample is masked while the other is unmasked. A voltage is applied across each half of the control sample and the measured differential in the voltage through each half is used to calculate the relative thicknesses of the material in the masked and unmasked halves. The control sample and the component being etched are simultaneously placed in a bath; the component will have the required etch depth when the difference in thickness between the masked half and the unmasked half equals to the etch depth. However, the control sample is complex and also the calculation is based on the relationship between the cross sectional areas of the two halves of the control piece and the voltage. The voltage levels required in order to operate the proposed methodology effectively are high, demanding a high current to be passed through the sample.

The present invention provides an improved CCM method whereby it is possible to predict simply and accurately when a component has been exposed for a sufficient time to achieve a required etch depth.

Disclosure of the Invention The present invention provides a simple method of predicting when a predetermined thickness of metal has been removed from the metallic component, allowing the identification of the end point of a metallic etching step to be accurately determined without constant removal of the component from the etching bath for measurement.

According to the present invention, there is provided a method of chemically etching a metallic component to remove a predetermined thickness of metal in at least one localised area, which method comprises a) exposing the said at least localised area to an etchant bath, b) simultaneously exposing a sacrificial control piece made of the same metal as the component to the same etchant bath, the sacrificial control piece having a thickness such that the sacrificial control piece is substantially etched through when the said predetermined thickness of metal has been removed from the metallic component, c) monitoring the integrity of the sacrificial control piece and d) terminating the etching of the metallic component when the sacrificial control piece has been substantially etched through.

The monitoring of the integrity of the sacrificial control piece is preferably performed by passing an electric current through it and monitoring the resistance of the sacrificial control piece. As the sacrificial control piece approaches the point at which it has been substantially etched through, its resistance will rise steeply and the component should be removed from the etching bath at this time. In order to provide a criterion for removal of the component and in order to prevent premature termination due to small fluctuations in the measured resistance, it is preferred to monitor the resistance of the sacrificial control piece throughout the etching method and calculate the standard deviation from the mean resistance measured. The CCM method can then be terminated when the measured resistance exceeds a predetermined number of standard deviations from the mean resistance measured hitherto in the method; preferably, the predetermined number of standard deviations is a value between 1 and 7, more preferably between 2 and 5 and most preferably about 3.

Using the above method, it has been possible to predict accurately the depth of etching of a component to within a tolerance of +O.lmm and in many tests, a tolerance of +0.02mum or less has been achieved, as is reported below.

The sacrificial control piece preferably has the same thickness as the predetermined thickness of metal to be removed from the component.

In a preferred embodiment, the a sacrificial control piece comprises a strip of the metal concerned, a pair of electrical contacts in electrical contact with the strip each of which is protected to prevent the etchant bath eroding the contacts, and an unmasked area located between the contacts and exposed to the etchant bath.

It is the unmasked area that is etched through to indicate the termination of the etching step.

Preferably, oniy a small area between the contacts is exposed to the etchant with the remainder of the sacrificial control piece being masked. The masked area may extend over the whole of one side of the strip, i.e. the opposed side to the area exposed to the etchant. However for reasons that will be apparent later, it is preferred that both sides of the strip remain unmasked in the edge region of the exposed area of the strip.

Detailed Description of the Drawings Figures la and ib are, respectively, a plan view and a sectional view of a sacrificial control piece for use in the method of the present invention (the section shown in Figure ib being taken along the line "b-b" of Figure la); Figures 2a and 2b are, respectively, a plan and a bottom view of an alternative sacrificial control piece for use according to the present invention; Figure 3 shows a masked component that is subjected to etching according to the present invention; Figure 4 shows a different component for etching according to the present invention; Figure 5 shows a plot of the resistance of the sacrificial control piece with time, also showing the standard deviation of the resistance reading from the mean resistance reading; Figure 6 shows a masked component having a masked off area for etching according to the method of the present invention; and Figure 7 shows the same component as shown in Figure 6 but with a different masked off area.

Detailed Description of the Invention Referring initially to Figures la and ib, there is shown a sacrificial control piece for monitoring the CCM of a titanium component; the control piece comprises a strip 10 of titanium having a pair of contact leads 12 connected at each end of the strip. A mask 14 covers most of the strip except for a central etching area 16 and two control pockets 18.

The material used for making the mask and the method of applying it to the sacrificial control piece are well known in the CCM art and accordingly will not be further described here except to say that the whole of the strip is initially covered in the mask and the central etching area 16 and two control pockets 18 are cut out by scribing through, and peeling off, the mask in those areas.

In order to prevent the etchant from penetrating through the mask in the region of the electrical connections 12, additional masks 20 are placed around the connections 12.

The resulting sacrificial control piece was placed in a frame and the leads 12 were connected in series with a multi-meter that can measure the resistance of the titanium strip 10 between the electrical connections 12. The sacrificial control piece was then immersed in a known etchant bath consisting essentially of a mixture of hydrofluoric and nitric acids.

An electric current was passed through the titanium strip via leads 12 and the multi-meter (not shown) was used to calculate the resistance of the strip. The resistance fluctuated between 0.2 and 0.4 ohms for six minutes and fell to a range of between 0 and 0.3 ohms for 26 minutes. The resistance then fluctuated between 0.2 and 0.7 ohms for a period of 7 minutes before climbing to approximately 20 ohms, at which point the resistance levelled out. The sacrificial control piece was then removed from the etchant, washed and examined. The central region of the etched area 16 had been etched through, leaving tangs of titanium along edges 22 and 24; these tangs of titanium were the reason why the sacrificial control piece remained conductive. However, the rapid rise of the resistance when the central area had been etched through showed that the sacrificial control piece of Figure 1 can provide a sharp end point for indicating when a component subject to chemical controlled machining should be removed from the etching bath.

Example 2 In order to eliminate the residual tangs 22 and 24, a revised mask pattern was devised and this is shown in Figures 2a and 2b. The sacrificial control piece is identical to that shown in Figure 1 except that a portion of the mask has been removed on the underside of the control piece (underneath where the tangs were formed) as shown by areas 25 in Figure 2b.

A titanium component that is to be subjected to CCM is shown in Figure 3. The component is to be etched only in area 30 and accordingly the rest of the component is masked off (as shown by marked off area 32).

The component of Figure 3 together with the sacrificial control piece of Figure 2 were fixed in a frame. The leads 12 of the sacrificial control piece were connected in series with a multi-meter and the frame, the component and the sacrificial control piece were immersed in a hydrofluoric acid/nitric acid etchant.

The component of Figure 3 had a thickness of 1.22 mm and the titanium strip of the sacrificial control piece of Figure 2 had a thickness of 0.75 mm.

The resistance values observed were generally the same as those stated in connection with Example 1 above and the components and the sacrificial control piece were removed from the etchant bath when a step change in the resistance of the sacrificial control piece was noted. Both the component and the sacrificial control piece were then washed and examined.

The sacrificial control piece had been etched through and no etched tangs (corresponding to tangs 22 and 24 in Figure 1) were observed.

The component was found to have a thickness of 0.61 in the area 30 after the CCM step, which means that a thickness of 0.61 mm had been removed in area 30. This represents 0. 14 mm less than the thickness of the sacrificial control piece.

The control pockets 18 were then examined and they were found to have been etched through at their perimeters, while the centres of the control pockets had a thickness of approximately 0.14 mm. It was therefore clear that the scribing of the mask to expose the etching area 16 and the control pockets 18 had caused accelerated etching in those areas under the scribe.

Example 3 The experiment of Example 2 was repeated except that the blade used to scribe the mask in order to expose etching area 16 and control pockets 18 was controlled to ensure that it did not come into contact with the titanium surface along lines 26 shown in Figure 2.

Again, the etching process was terminated when a step in the value of the resistance of the sacrificial control piece was detected. The results are shown in Table 1.

Table 1

Example Control Component - Component Component control piece Number Piece start - final - etch depth thickness - start thickness thickness minus etch thickness depth 2 0.75 1.22 0.61 7 0.61 0.14 3 0.73 1.23 0.49 0.74 -0.01 4 0.73 3.35 2.63 0.72 0.01 5 0.73 3.33 2.58 0.75 -0.02 6a 0.73 3.33 2.62 0.71 0.02 6b 0.73 2.58 1.85 0.73 0 All thicknesses are given in millimetres.

Example 4 Although the step change in the resistance of the sacrificial control piece was steep, it was not vertical and it was therefore proposed to apply an exact regime whereby the etching process is terminated. This regime is described below: The method described in connection with Example 3 was repeated using the sacrificial control piece shown in Figure 2 and the component shown in Figure 4.

The component of Figure 4 differs only from that shown in Figure 3 in that it is larger: like the Figure 3 component, it includes a central exposed area 30 and a masked off area 32.

The resistance of the sacrificial control piece 16 was monitored by the multi-meter every 15 seconds and the results are shown in Figure 5. The resistance fluctuated over a period of time and the standard deviation from the mean resistance was calculated by a computer and is also plotted on Figure 5. As can be seen, there is a dramatic rise in the resistance as the sacrificial control piece approaches being etched through. In order to provide a precise cut off point that is not triggered by a fluctuation in the resistance, the component is removed from the etching bath when the resistance reading had varied from the mean of the resistance readings of the sacrificial control piece by three standard deviations.

Thus, as the etching process proceeds, the mean resistance and the standard deviation are calculated dynamically and when the resistance differs from the mean resistance by more than three standard deviations, the component and the sacrificial control piece were removed from the etchant and examined. As can be seen from Table 1, the etch depth in the component was within 0.01 mm of the original thickness of the sacrificial control piece.

Example 5 The methodology set out in connection with Example 4 was repeated except that the component had the configuration shown in Figure 6. The results are given in Table 1.

Example 6 The mask of the component shown in Figure 6 was removed following etching and a new mask was applied to the component (see Figure 7); the upper part of the previously etched area 30 from Figure 6 was masked by a mask 40. The exposed area in Figure 7 was partly composed of area 42 that had previously been exposed to etchant in the process of Example 6 and partly of area 44 which had previously been masked by mask 32 in Example 6.

The control piece of Figure 7 was placed in a frame together with a fresh sacrificial control piece of the design of Figure 2 and immersed in an etchant bath. The component was removed when the resistance value of the sacrificial control piece exceeded its mean resistance over the course of the etch by three standard deviations. The component was then washed and examined.

The component had a stepped configuration, with area 42 being more deeply etched than area 44 because it had been exposed to two etching steps whereas area 44 had only been exposed to one etching step.

Table 1 shows the result of the further etching where the values for Example 6a represent the etching in area 44 and Example 6b shows the etching in area 42. As is clear from the results, the etching depth is accurately controlled within both the single etched area 44 and the double etched area 44 to within 0.02mm of the thickness of the sacrificial control piece.

From the results of the above Examples, it can be seen that, by making the sacrificial control pieces substantially the same thickness as the desired etching depth of the component, and by removing the component when the resistance of the sacrificial control piece rises in a step fashion, the completion of the etching of the component can be accurately determined without having to repeatedly measure the thickness of the component and return the component to the etching bath until the required etch depth has been achieved.

The method of the present invention provides a number of benefits: 1. The etching bath is extremely corrosive and toxic; the method of the present invention is considerably safer than that of the prior art because it is not necessary to repeatedly remove the components during the etching step for thickness measurement.

2. Because the etching step will be conducted in a single operation without the need to remove the component to measure the etch depth, the etching process can be conducted much more rapidly and therefore at a reduced cost.

3. Because it is possible to accurately determine when the required etch depth has been achieved, there is a reduced risk of overetching the component, which would cause the component to be rejected.

4. Because each etching step is conducted in a shorter time, the potential through-put of an etching plant can be significantly increased.

Although the above Examples use a sacrificial control piece having substantially the same thickness as the required etch depth, this is not an essential requirement of the present invention. In particular, if the control sample were exposed at both faces, the sacrificial control piece can be thicker than the required etch depth. According to a possible embodiment of the invention, the control sample can be in the form of a wire that is immersed in the etching bath and the etching process is terminated when the wire has been etched through (the etching through of the wire can be determined by passing a current through the wire and determining the resistance of the wire in the same way as the current is passed through the control piece of Figures 1 and 2). In these circumstances, it will be appreciated that, because the component is only exposed to the etching bath on one surface whereas the wire is completely surrounded by the etching bath, the wire will have to be thicker than the required etching depth. The required thickness of the wire can easily be determined by routine trial and error.

Although the detection of the etching-through of the control piece has been described in the Examples as being achieved by monitoring the resistance of the sacrificial control piece, other methods of assessing the integrity of the sacrificial control piece are also within the scope of the present invention.

Claims

1. A method of chemically etching a metallic component to remove a predetermined thickness of metal in at least one localised area, which method comprises: a) exposing the said at least localised area to an etchant bath, b) simultaneously exposing a sacrificial control piece made of the same metal as the component to the same etchant bath, the sacrificial control piece having a thickness such that the sacrificial control piece is substantially etched through when the said predetermined thickness of metal has been removed from the metallic component, c) monitoring the integrity of the sacrificial control piece and d) terminating the etching of the metallic component when the sacrificial control piece has been substantially etched through.

2. A method as claimed in claim 1, wherein the monitoring of the integrity of the sacrificial control piece is performed by passing an electric current through it and monitoring the resistance of the sacrificial control piece.

3. A method as claimed in claim 2, wherein the etching of the metallic component is terminated when the said resistance rises step-wise.

4. A method as claimed in claim 2 or claim 3, wherein the resistance of the control piece is monitored throughout the etching and the standard deviation from the mean measured resistance is calculated and the etching is terminated when the measured resistance exceeds the mean measured resistance by a predetermined number of standard deviations.

5. A method as claimed in claim 4, wherein the predetermined number of standard deviations is a value between 1 and 7, preferably between 2 and 5 and most preferably about 3.

6. A method as claimed in any one of claims 1 to 5, wherein the sacrificial control piece has the same thickness as the predetermined thickness of metal to be removed from the component.

7. A method as claimed in any one of claims 1 to 6, wherein the sacrificial control piece comprises: a strip of the same metal as the component, a pair of electrical contacts in electrical contact with the strip, each of which is protected to prevent the etchant bath eroding the contacts, and an unmasked area exposed to the etchant bath and located between the contacts.

8. A method as claimed in claim 7, wherein the unmasked area between the contacts is constituted by a first portion on one side of the strip extending across the whole width of the strip and second portions formed by the edge regions of the part of the opposed side of the strip underlying the first portion.

9. A method as claimed in any one of claims 1 to 5, wherein the sacrificial control piece is a wire.