WO2001049892A1

WO2001049892A1 - A method of forming on a substrate a coating of complex alloy containing elements whose evaporation temperatures differ by more than 350 °c

Info

Publication number: WO2001049892A1
Application number: PCT/UA1999/000033
Authority: WO
Inventors: Valery I. Topal; Alexandr F. Belyavin; Vasily Ja. Bratus; Frank Lemkey
Original assignee: United Technology Corporation Pratt & Whitney; International Center For Electron Beam Technologies Of E.O.Paton Electric Welding Institute
Priority date: 1999-12-29
Filing date: 1999-12-29
Publication date: 2001-07-12
Also published as: AU2957400A

Abstract

The present invention relates to the evaporation and deposition of complex alloys. A method for deposition onto substrate a complex alloy coating, which contains elements whose evaporation temperatures vary by more than 350 °C, is disclosed. The constituents of the alloy to be vaporized are forced to migrate throughout enriched with refractory material layer being preferably in solid-liquid state. The method provides high vaporization rate of complex and not complex alloys with good replicating the compositions of the alloys in the finished product.

Description

A METHOD OF FORMING ON A SUBSTRATE A COATING OF COMPLEX

ALLOY CONTAINING ELEMENTS WHOSE EVAPORATION

TEMPERATURES DIFFER BY MORE THAN 350°C

Field of the Invention

The present invention relates to the evaporation and deposition of complex alloys, more particularly to the production of protect coating and gas turbine superalloy articles by electron beam evaporation and deposition of starting complex alloy vapors with replicating the compositions of those complex alloys in the finished product.

Cross . reference is made to International Application No. PCT/US97/07541 of 05 May 1997 entitled "Method of High-Rate Vacuum Evaporation of Metals and Alloys Using an Intermediary Molten Metal".

Background of the invention

The deposition of materials by evaporation is a widely practiced process. In particular, this process is used in the gas turbine engine business to pro\ec\ turbine parts with helping of metallic and ceramic protective coatings. The process is performed under vacuum conditions and typically electron beam guns are used to melt the material to be vaporized and to heat the molten material to a temperature at which it vaporizes at a useful rate. The article to be coated is suspended above the melt pool and the vapors emitted from the melt pool condense on the article to provide the desired coating. There has been much development and work aimed at optimizing the process and improving the evaporation rate. Problems remain however in that the evaporation rate is slower than that which would be desired for economical coating application and difficulties are encountered in depositing complex alloys by vapor deposition. A typical gas turbine engine protective coating which would be applied by evaporation is that which is commonly referred to an MCrAIY type coating. In such a coating M represents a material selected from the group consisting of -Fe, Ni, Co and mixtures of Ni and Co, Cr is chromium, Al is aluminum and Y is yttrium; a typical composition range would be 5-40% Cr, 8-35% Al, 0.1-2% Y balance selected from the group consisting of Ni, Co and mixtures thereof. These coatings are described, for example, in U.S. patents 3,676,085, 3,754,903, and 4,585,481.

Table I lists the melting and boiling, temperatures and the temperatures at which the vapor pressure of various elements is from 1 to 10 Torr. It can be seen that the materials used in the MCrAIY type coating have 10 Torr vapor temperatures ranging from about 1380 C for Al to about 1715 C for Co. Hereinafter the term "evaporation temperature" of an element will be understood to mean the temperatures which produces a 10 Torr vapor pressure. This vapor pressure .correspond to the evaporation rate equal 10

-9 o -1

- 10 g-crrr²-sec . This relatively narrow range of temperatures in MCrAIY composition allows to vapor all elements and to provide composition of condensed coating- near composition of initial ingot. It is in fact the case that the MCrAIY compositions are successful in large part because of this narrow temperature range.

Similar evaporation temperatures mean that the materials all evapo- rate at approximately the same rate. Alloys which contain mixtures of constituents which have widely differing evaporation temperatures will exhibit preferential evaporation of the material whose evaporation temperature is lowest. Such process is analogous to fractional distillation. Thus, referring .again to Table I, it can readily be appreciated, that it would be difficult to evaporate alloys which contained both aluminum and tungsten because of the wide separation in the temperatures at which significant evaporation occurs. If such an alloy to be vaporized the aluminum would evaporate first at about 1380°C leaving the tungsten in a melt pool and the tungsten would only evaporate after the aluminum was completely evaporated and the melt pool temperature rose to the temperature at which significant tungsten evaporation occurs (approximately 3798°C). This preferential evapora- tion means tha coatings deposited from starting materials will have compositions which differ from that of the starting material. Therefore new generation of coatings on the MCrAIY base was obtained successfully only by method of plasma spraying. - This limitation on evaporating complex alloys has prevented the use of vapor deposition for the evaporation of structural superalloys. Structural superalloys generally contain refractory metals such as Hf, Nb, Mo, Ta, Re and W and also generally contain Zr, B and C. These materials are incompatible, in an evaporation sense, with Al which is an essential constituent of all nickel base superalloys.

Thus it has not been possible to date to vaporize complex alloys, alloys which contain constituents whose temperature of significant evaporation vary by more .than about 350°C from a single source to obtain coatings with the composition of starting alloys.^' This complex alloy evaporation problem has also hindered the evaporation of- most titanium alloys, steels, copper and aluminum-base alloys.

The evaporation rate, whether of non complex alloys such as MCrAIY. or complex alloys such as structural superalloys and titanium alloys, is also so slow with conventional evaporation processes as to make the processes only marginally economic. The evaporation rate can be increased by increasing the electron beam power but when the material being evaporated begins to boil vigorously spattering occurs. Spattering is the ejection of molten droplets from the melt pool. When such ejected droplets strike the work piece they cause unacceptable, surface defects and defects in depos- ited coating.

Figure 1 shows schematically commonly used apparatus and process for electron beam physical vapor deposition. Crucible 1 (of copper or copper alloy usually) has usually cylindrical form with open bottom. Crucible 1 is cooled with water circulated in canals (commonly used scheme is not shown). Ingot 2 of material to be vaporized is fed upwards through crucible 1. Electron beam (not shown) is melting upper portion of the ingot forming melt pool 4. Melt pool 4 is heated up to the temperature under which evaporation takes place. In common case it is not desired to heat the melt pool 4 to - boil point, because possible ejection of liquid metal (droplets) from the melt pool 4 and its deposition on coated surfaces with forming surface defects. As mention above, at such scheme the evaporation rate is- restricted and as result the deposit rate does not usually exceed 8 μ/min in order to maintain conditions preventing boiling and spattering.

Also, as mention above, evaporation of alloy constituents is in general proportional to their vapor pressure resulting difference between coating and starting material compositions, particularly when elements having high vapor pressure (such as Al and Cr) are vaporized in combination with elements having low vapor pressure (such as W, Re, Ta, Mo, Hf, Nb and so on).

^' The closest prior art for present invention is a method for depositing onto substrate a complex alloy coating which contains elements whose evaporation temperatures vary by more than 350°C, the method comprising of positioning the ingot of complex alloy to be vaporized into cooled crucible and using the electron beam as a heat source for vaporizing constituents of the complex alloy ("Resistance to heat of casting nickel alloys and their protection against oxidation", - Kiev, Naukova Dumka, - 1987. - pp. 196-200). Said method proposes making ingots for vaporizing starting constituents of a coating, said ingot dimensions corresponding to water cooled copper crucibles, feeding the ingots upwards from the bottom part of the crucibles to their upper part and simultaneous evaporation of tthe starting constituents, which are grouped according to their evaporation temperatures, from a set of crucibles to obtain desired complex alloy. Electron guns^' controlled individually ore used as heat sources. This allows to deposit simultaneously materials which contain elements with various evaporation temperatures. However this method is complicated and difficulties appear in connection with control of respect evaporation rates from a set of crucibles in order to obtain desired composition of coating material and its chemical homogeneity. Thus providing of efficient method for deposition of complex alloy coating comprising elements which evaporation temperatures vary more than 350°C is still very actual problem.

The invention essence

The invention is directed to create a high-productivity method for depositing onto substrate a complex alloy coating which contains elements whose evaporation temperatures vary by more than 350°C, which provides replicating the composition of vaporized complex alloy in the finished product and its chemical homogeneity due to modification of process of evaporation. This problem was solved by providing method for deposition onto substrate a complex alloy coating, which contains elements whose evaporation temperatures vary by more than 350°C, the method comprises of positioning the ingot of complex alloy to be vaporized into cooled crucible and using an electron beam as a heat source for vaporizing constituents of the complex alloy, wherein a c c o r d i n g - t o i n v e n t i o n refractory layer is formed above the ingot before evaporation, the layer consisting of at least 50% refractory elements and having solid lawer portion and upper portion in a form of a melt pool, which is maintained in liquid state by said heat source and from which the alloy constituents are vaporized, the ingot is fed upwards in said crucible to enrich the melt pool with consumable elements of the complex alloy by means of their migration from the ingot material to evaporation surface through solid portion of said layer.

It is such decision that allows to rise the temperature of material in the melt pool and thereby provides possibility of simultaneous evaporation of the material constituents whose evaporation temperatures vary in a great degree. This provides replicating the composition of vaporized complex alloy in the coating obtained by evaporation of complex alloy from single crucible and producing the coating with less energy consumption in com- parison with prior art. It is expedient to form the refractory layer above the ingot by means of addition of one or more refractory elements into the melt pool of complex alloy to be vaporize, which is formed on the ingot surface with electron beam. Due to high melting temperature of the refractory elements and heat removal through the walls of water cooled copper crucible such decision provides shallow molten portion (melt pool) of the layer, this portion comprising mainly of refractory elements, and lower solid portion under said melt pool, this lower solid portion consisting of solid refractory material and the complex alloy elements which migrate through this solid portion. Shallow molten portion of the layer has high temperature of boiling due to presence of refractory elements. This provides possibility to vaporize intensively and simultaneously all constituents of vaporized alloy.

Also it is expedient to form said refractory layer by means of placing a metal article above the ingot, which article has thickness from 10 to 35 mm and consists of at least 90 % (by mass) refractory elements.

Such decision simplifies the method according to the invention in a great degree and precipitates the process, excluding necessity to form layer adding refractory elements into the molten alloy to be vaporized in a step wise fashion.

It is expedient to use refractory elements, which . evaporation temperatures is equal or higher than the evaporation temperature of the vaporized alloy constituent having highest evaporation temperature, at that the upper portion of the layer has to consist of more than 90% added re- fractory elements.

At such decision the boiling temperature of upper molten portion of the refractory layer is much higher than evaporation temperature of all constituents of the alloy to be vaporized. This allows to rise significantly a temperature of the melt pool and to get similar evaporation rates for all con- stituents of the initial alloy with replicating the composition of evaporated complex alloy in the finished product. Due to above cause it is expedient to select refractory elements from the group consisting of W, Hf, Nb, Mo, Ta, Re, Os, Ru, Ir or mixture thereof.

In comparison with the closest prior art such method for deposition of complex alloy coating of certain composition by vaporizing the alloy of the same composition from single source in vacuum is distinguished by simplicity. It provides obtaining chemically homogeneous complex alloy coating through all coating surface. It simplifies the rate control to receive desired composition of the coating material and its chemical homogeneity. The method may be easily automated. Besides this method is more economical.

Brief Description of the Drawings

Fig. 1 shows a conventional prior art arrangement for the electron beam physical vapor deposition of alloys.

Fig. 2 shows a section of the apparatus of the present invention. Fig. 3 shows plan view of the apparatus of the present invention.

Figure 4 shows distribution of temperature in plane shown in Fig. 3.

Detail description of the preferred embodiments

The method and apparatus for performing the method according to the present invention will be described with references to the accompanying figures.

Fig. 2, 3 and 4 show schematically method and apparatus according to the present invention. Fig. 2 shows section of the apparatus according to the invention. Cylindrical crucible 1 is made of copper or copper alloy and cooled with water flowing through inner canals (the canals are not shown). Cooling is enough to maintain temperature of the copper crucible below its melting temperature (copper melting point is } 054^QC).

Solid-liquid metal 3 is fed upwards through crucible 1.

Electron-beam heat sources (not shown) are heating upper surface of refractory metal 3. Central zone A" of refractory metal 3 is heated up to higher temperature (about 3000°C). Under such high temperature strength of refractory metal is law, so pressure of the material 2 to be vaporized which is fed upward is enough to deform the refractory metal upward with forming a convex zone A" . It was found that successful vaporizing correlates in a great degree with forming the central convex zone A" above refractory metal 3. Probably, during evaporation process refractory metal 3 in the crucible center and in the zone of its contact with alloy 2 to be vaporized is under temperature which is high enough to cause (at least partly) melting material 2 to be vaporized. As a result shallow (about 1 μ) liquid zone will be formed between metal 2 to be vaporized and central zone A" of refractory metal. From estimations, molten zone consists of constituents of al- y 2 in combination ith some quantity of dissolved refractory metal 3.

At the evaporation process the constituents of alloy 2 to be vaporized come through liquid zone (by diffusion and/or convection process) into refractory metal 3 and then by the way of diffusion in solid or liquid phase they migrate to evaporation surface, which includes zone A', closed to the crucible, intermediate concave zone 4 and central convex zone A".

Convex zone A" is hottest zone (its temperature is about 3000°C) and a power of electron beam is maintained such to provide temperature about- melting point but less then it. In dependent on particular conditions, including a composition of refractory metal, the average temperature and dimensions of convex zone 4" may be varied. Zone 4' closed to crucible is more colder because water cooling maintains lower temperature between refractory metal 3 and crucible wall in order to prevent the crucible melting. Zone .4 has temperature between temperature of convex zone 4" and zone A'.

From our observation during evaporation the constituents of alloy 2 which have lowest pressure of vapor (Hf, Si, W, Nb, Ta and so on) are evaporating mainly from convex zone A". The constituents of alloy 2 which have highest pressure of vapor (such as Al and Cr) are evaporating mainly from zone A', and the constituents with intermediate vapor pressure (Ni, Co, Fe, Y, Si, Ti) are evaporated from intermediate zone 4. As it is shown in fig. 2, intermediate zone 4 is some deeper than zones 4' and A". It is this zone where the stable melt pool is observed to form. Telling the truth, above idea about the process reflects our to-day understanding of its essence. This understanding is not exhaustive because the process takes place ,at very nonequilibrium conditions (vacuum, great gradients of concentrations and temperatures and so on). Main effect of described method is producing the coating, which composition corresponds exactly to composition of initial material 2, with a deposition rate of 6-8 times larger then the rate under conventional process of evaporating from melt pool.

Fig. 3 and 4 together with fig. 2 shows practical side of the invention. Fig. 3 shows plane view of the apparatus shown in fig. 2, factually it is section of crucible 1. Surface, from which evaporation takes place, divides into three zones: central convex zone I marked as A" in fig. 2, zone II, which corresponds to convey zone 4 in fig. 2, and zone III, which corresponds to outer zone 4' in fig. 2. As it was said previously, temperature is highest one in the center of evaporation surface marked as zone- 1 and is lowest one in zone III. Fig. 4 shows approximate temperature distribution across evaporation surface (the. crucible diameter is 70 mm). It shows high temperature about 3000°C in zone I and lower temperature in other zones. Such temperature distribution is defined by distribution of becoming power of electron beam and loss of heat due to radiation, heat removal through the crucible and heat transferring into the metal to be vaporized.

In preferable variant of the invention a power supplied to vaporization surface by electron gun distributes between said three zone in such proportion. Zone I gets about 30-35% of total supplied energy. Zone II gets about 50-60% of supplied energy of electron beam, and zone III gets the rest, i. e. about 20% of supplied energy of electron beam. Such energy distribution was found to give the most stable results.

According to our evaluations, when diameter of evaporation surface is about 60-70 mm and power of electron beam is about 65 kW, zone I has diameter about 30-40 mm, width- of circular zone III is about 5 mm, the rest ^■ (ring with width of 20-25 mm and average diameter of 50-55 mm) is zone II. Referring again to fig. 2, 3 we note such experimental facts. Chemical composition in zone A" (fig. 2) and respect zone I (fig. 3) is distinguished by ^' high - above 90-95% - content of refractory (W, Nb, Ta and so on) elements.. Total amount of refractory elements in zone 4 (fig. 2) and in respect zone II ( and partly in zone III, fig. 3) is from 70 to 90%. Zone 4' (fig. 2) and respect part of zone III (fig. 3) is characterized by lowest summary content of refractory elements (to 40%), the rest is constituents of the alloy 2 (fig. 2). These data allow to say nothing about real distribution of chemical elements during the evaporation process and show only significant inertia of described process of evaporation, namely continuation of evaporation of alloy 2 constituents from the melt pool surface (mainly from zone 4 in fig. 2) for a some time after stoppage of power feed with electron beam. Other ^"aspects of the invention will be better understood from consideration of the following examples which are only illustrations but not limitation.

Example 1

A 63 mm diameter ingot of a alloy known as Inconel 718 manufactured by Inter Nickel Corporation- was prepared. The composition of the starting material was 19% Cr, 2.2% Mo, 5.0% Nb, 0.6% Al, 1.2% Ti, 20% Fe, 0.2% Mn, 0.2% Si, 0.04% C, no intentional additions of B and Zr, balance essentially Ni. Nb and Mo presence (vapor pressure is 10^"1 Torr at temperature 3250°C and 3046°C, respectively) in fixed combination with other elements, such as Ni and Al (vapor pressure is 10^"1 Torr at temperature 1680°C and 1380°C, respectively) refers this alloy to group described above.

A water cooled cylindrical crucible of 65 mm in diameter and 100 mm in height was prepared. The Inconel 718 ingot was placed into said crucible. To start the deposition process the Inconel 718 ingot was fed upwards from the bottom of the crucible. The top portion of the ingot was melted by an electron beam (all processing was performed in a vacuum of better than 10^" Torr). Additions were made consisting essentially of 500 grams of W and 100 grams of Nb. These additions were made to the melt pool in a step wise fashion, approximately 200 grams in each of three steps. The second and third additions were made only after the previous additions had been completely melted. The total mass of 600 grams corresponded to a cylinder 63 mm in diameter and approximately 20-22 mm in height.

At this step due to the high melting points of refractory metals and the heat removal through the water cooled copper crucible walls there was a shallow melt pool (consisting essentially of the refractory elements) having a depth of approximately 2-4 mm and under the melt pool there was a layer of solid refractory material (of about 16-20 mm in depth) and below the solid refractory material there was the Inconel 71 8 composition. We be-

- lieve that there was a thin (1 -5 mm) layer of molten Inconel 718 directly below the solid refractory material layer but we have not confirmed this. The electron ^' beam had a power of approximately 65 kW {ai accelerating voltage 18.5 kV current was 3.5 A). A 20 x 20 cm metal plate (a substrate) was supported about 300 mm above the melt pool. The constituents of the Inconel 718 material were vaporized from the surface of the melt pool and condensed on the substrate plate at a rate of between 40- 50 microns per minute. The rate of feeding the ingot to evaporation zone was about 1 mm per minute. The deposited material was analyzed and found to contain 8- 19% chromium, 2-2.6% molybdenum, 2-6% niobium, 0.6-0.7% aluminum, 1 -1 .6% titanium, 18-22% iron, manganese and silicon in trace amounts and 0.01 % zirconium, carbon and boron were not ana- lyzed. The found zirconium is believed to be the result of impurities. In the preceding example the refractory material was largely tungsten because of its high boiling point. But a significant addition of niobium was made because niobium is the primary refractory metal in the Inconel 71 8 composition. This addition of niobium to the refractory metal portion enabled the rapid achievement of compositional identity between the starting material and the deposited layer. If the refractory metal had consisted only of tung- sten it would have taken more time for the composition of the deposited material to have a niobium composition comparable to that in the starting Inconel 718 material. Thus it is preferred aspect of the invention to include in the refractory metal the primary refractory elements in the alloy to be deposited. It Is preferable also to use mixture of refractory metals to provide refractory composition which is melting within some temperature range, but not under definite temperature. It was mentioned earlier, that convex was observed in the center of layer enriched with refractory elements after achieving equilibrium process (the process with stable form of the surface). We believe it is better to place some quantity of refractory metal (preferably metal with highest melting temperature) into the refractory layer center and melt it with electron beam to create convex form of the melt pool before starting evaporation. Such action will accelerate achieving equilibrium process.

Example 2

Example 2 was performed in the fashion described in Example I ex- . cept that the -alloy deposited was Waspaloy having a nominal composition of 19.5% Cr, 13.5% Co, 4.3% Mo, 1.3% Al, 3% Ti, 0.08% C, 0.006% B, balance Ni. The starting refractory material consisted of 650 grams of tungsten and 100 grams of molybdenum and was melted into the molten Waspaloy composition in a step wise fashion using four additions to produce a tungsten rich layer whose approximate thickness was between 20-22 mm. During the deposition process the depth of the melt pool was about 3 mm and the depth of the solid tungsten rich portion was about 18 mm. Again as in above example deposition rates between AO-50 microns/minutes were observed at the same parameters of electron beam and substrate dimensions. The composition of the deposited material was 20% Cr, 13% Co, 3.9% Mo, 1.3% Al, 2.7% Ti, 0.01% Zr, C and B were not analyzed and the balance Ni. One can see that composition of deposited finished layer is nearly identical with starting alloy composition. Example 3

• The alloy, containing 20% Co, 20% Cr, 1 1.5% Al, 1.3% Si, 0.4% Y,

0.4% Hf, Ni - balance was prepared. This alloy (improved MCrAIY or Ni-

CoCrAIY) is used as high-temperature protective coating and is usually de- posited by process of plasma spraying. It was impossible to evaporate uni- ormly this alloy by EB PVD due to the essential vapor pressure difference.

Using the above described technique (namely the scope of invention) the process of electron-beam evaporation with standard water-cooled cooper crucible of 70 mm in diameter was done. The go-between pool (i. e. re- fractory pool) consisted of 300 grams of W, 100 grams of Ta and 100 grams of Hf. The deposition rate at rotating (20 min ^_1 ) substrate in deposition plane at a distance of 300 mm from the crucible to substrate was about 20 μm/min. The chemical composition of the deposited condensate was: 19- 20% Co, 8-12% Cr, 7-9% Al, 1.1 -1.5% Si, 0.3-0.45% Y, 0.1 -0.15% Hf.

Table 1

Claims

Claims:

1. A method for deposition onto substrate a complex alloy coating, which contains elements whose evaporation temperatures vary by more than 350°C, the method comprises positioning the ingot (2) of complex alloy to be vaporized into cooled crucible (1) and using an^" electron beam, as a heat source for vaporizing constituents of the complex alloy, characterized in that refractory layer (3) is formed above the ingot (2) before evaporation, the layer (3) containing at least 50% refractory elements and having solid lower portion and upper portion in a form of a melt pool (4), which is maintained in liquid state by said heat source and from which surface the alloy constituents are vaporized, and at evaporation the ingot (2) is fed upwards in said crucible (1) to enrich the melt pool (4) with consumable elements of the complex alloy by means of their migration from the ingot material to evaporation surface through solid portion of said layer.

2. A method according to claim 1, characterized in that said refractory layer (3) is formed above the ingot (2) by means of addition of one or more refractory elements into the molten vaporized complex alloy on the ingot surface, which is melted with electron beam.

3. A method according to claim 1, characterized in that said re- fractory layer is formed by means of placing a metal article above the ingot, which article has thickness from 10 to.35 mm and contains at least 90 % (by mass) refractory elements.

4. A method according to claims 1 - 3, characterized in that refractory elements are used which evaporation temperatures is equal or ex- ceed the evaporation temperature of the vaporized alloy constituent having highest evaporation temperature, at that the upper portion of the layer contains more than 90% added refractory elements.

5. A method according to claims 1-4, characterized in that refractory elements are selected from the group consisting of W, Hf, Nb, Mo, Ta, Re, Os, Ru, lr,or mixture thereof.