SPRAY DEPOSITION PROCESS

Description

[0001] The present invention relates to a spray deposition process and in particular to a metallurgical spray deposition process.

[0002] The process is also useful for producing thick and thin coatings and other sprayed metal deposits sprayed onto substrates of all kinds, particularly where some of the topographical features are difficult to cover or fill due to complex geometries.

[0003] It is known to use metallurgical spray deposition techniques for producing tools, moulds, dies and other bodies of significant thickness. Problems have been encountered in using such techniques from the point of view of inherent porosity in the spray deposited material and internal stresses which arise during the spray deposition processes. Attempts have been made to deal with these problems and known techniques are described in, for example, WO-A-96/09421 PCT patent application GB97/00590, and US 5 340 090.

[0004] In a wide variety of commercially important thermal spray processes, the manner in which sprayed droplets impinge, spread and solidify on deposition is critical in influencing the subsequent properties of the manufactured coating or deposit. The first droplets to be deposited will determine the properties at the coating/substrate interface. In the case of spray forming of free standing shapes for mould tooling, the first deposited droplets determine the accuracy of replication and tooling wear properties. As deposition continues, droplet deposition behaviour controls the bulk microstructure (such as volume fraction, morphology and size of porosity) consequently determining the coating or deposit bulk properties. At all stages of deposition, droplet rebounding or splashing leads to a reduction in process yield. Recent experimental evidence suggests that droplet splashing occurs to a significant and greater extent than previously believed.

[0005] A further problem with sprayforming onto patterns or substrates having an object surface of varied (three dimensional) topography arises due to the fact that metal spray processes are "line of sight" processes in which known effects of shadowing and bridging occur for certain configurations of object surface topography.

[0006] A further problem occurs at internal and external edges of object surface topography, where poor quality deposit integrity can occur resulting in poor quality integrity to the deposit. This can result in flaking or crumbling of the deposit at corners and edges.

[0007] According to the invention given in claim 1 we provide a process for producing a metallic tool, mould, die or other body of significant thickness or a coating, the process comprising directing a spray comprising molten metallic droplets carried by a propelling gas toward an object surface of a substrate or pattern so as to build up a metallic deposit or coating comprising the mould, tool, die, body or coating on the object surface of the substrate or pattern, wherein at one or more predetermined stages during spraying droplets of a size distributed about a Gaussian mean of 200 microns and above are sprayed and at one or more other stages droplets having a size below 150 microns are sprayed.

[0008] The larger droplets are initially sprayed onto the object surface of the substrate or pattern preceding the spraying of relatively smaller droplets. Desirably, where the substrate or pattern includes topographical relief features, the spraying of the relatively larger droplets is dependent upon the nature of and/or the location of the topographical relief features.

[0009] It is a feature of the invention that when spraying over portions of the object surface comprising topographical features with a depth/width aspect ratio for example, or into or onto sharp corners or the like, that metallic droplets of relatively large sizes are sprayed, at least initially, in order to reduce shadowing, bridging, and poorly defined edge or corner detail which has been found to be a problem with prior art techniques.

[0010] It has been found that for producing detail and reducing shadowing or bridging, metallic spray droplets of mean diameters of substantially 200 microns and above (preferably substantially 350 microns and above) produce highly beneficial results. This result is surprising because trends in spray deposition research and practices have herebefore tended to suggest that finely sprayed - droplets and relatively higher droplet spray velocities should produce improved results during the manufacture of coatings and most other products when using spray deposition techniques. Smaller droplet sizes have also been preferred because deposit porosity is minimised by using smaller droplet sizes. This is one of the premises behind the development of techniques such as plasma spraying and high velocity oxy-fuel metallic spraying techniques.

[0011] Additionally, spraying of larger droplet sizes for initial deposition (including for coatings) has been found to result in reduced porosity in the deposited material immediately adjacent the substrate or pattern.

[0012] It is preferred that the propelling gas of the spray is within a pressure range lower than that normally recommended for use with a particular metal spraying apparatus. The operating pressure will therefore be different for different equipment, but is preferably at or about 3 bar or less. This results in the relatively large droplets desired, and relatively low droplet velocities compared with known techniques.

[0013] It is preferred that the droplets are produced by arc spraying, because arc spraying typically produces coarser droplet sizes than other known spray deposition processes. Conventional arc spraying apparatus has however not been designed for spraying at the larger droplet sizes of the present invention and modification and frequent cleaning of arc spray guns during proving of the invention has been found to be necessary. It is believed that this is strongly indicative of the process of spraying metallic droplets at the dimensions preferred being novel and inventive per se.

[0014] Preferably relatively high currents are used in the arc spraying process compared with the currents used in conventional arc spraying techniques.

[0015] It is important to control internal stresses in relatively thick deposited bodies formed in metallurgical spray deposition processes. WO-A-96/09421 discloses metallurgical spray deposition techniques which may be used to control internal stresses in deposited bodies. The relatively large droplet sizes required to improve reproduction of detail and edge definition from the object surface of the substrate or pattern (and also to inhibit bridging or shadowing) have however been found not to be suitable for control of stresses. It is therefore preferred that in the process according to the invention, process parameters are varied such that the relatively larger droplets are sprayed during the period and over the areas where detail of the substrate is required to be reproduced. Relatively smaller droplets are sprayed after the period when the detail has been replicated as required, and in other portions of the deposit where it is not required to replicate detail, preferably under conditions to control internal stresses in the deposit.

[0016] Typically therefore, the process according to the invention may comprise initial spraying of relatively large droplets onto the object surface of the substrate or pattern where detail replication is required (such as, for example, edge definition is to be reproduced, and/or where shadowing is to be avoided), and then subsequent modification of the spray parameters, (preferably as soon as possible after the said pattern detail and edge definition have been achieved), so as to spray relatively smaller droplets onto the object surface of the pattern or substrate. (Preferably in order to bring stress control into operation, as described, for example, in WO-A-96/09421).

[0017] Alternatively, the process may comprise spraying of relatively large droplets from one spray source onto the object surface of the substrate or pattern where detail is required (such as edge definition is to be reproduced, and/or shadowing effects are to be ameliorated), and introducing a further spray of relatively smaller metallic droplets from a second spray source (preferably concurrently with the first spray), the further spray preferably being tailored to minimise internal stresses in the deposit.

[0018] It is preferred that the control step c) is operated by control means (preferably computer control means) and pre-programmed.

[0019] To achieve the required control, one or more spray guns are preferably mounted on manipulator means, such as an industrial robot which is preferably programmed, advantageously together with the spray guns, by the control means. Alternatively, or additionally, the one or more sprays of metallic droplets generated by the spray guns may be scannable, in which case the means for scanning the sprays is preferably co-ordinated and controlled, preferably by the same control means.

[0020] The invention will now be further described, by way of example only, and with reference to the accompanying drawings in which:

Figures 1(a) and 1(b) are explanatory sectional views of known spray deposition processes, highlighting the problem of shadowing and bridging known in the prior art.

Figure 2 is a schematic view of apparatus for use in the process according to the invention.

[0021] Referring to Figure 1(a), there is shown a substrate 101 rotated beneath a pair of arc-sprayed metallic droplet sprays 103 and 104. The arc sprayed metallic droplets are sprayed from two arc spray guns mounted on a 6-axis industrial robot (not shown) to produce a sprayed footprint 102, which is moved over the substrate by the robot manipulating the guns together, to produce a deposit 105 re-producing the object surface 106 of the substrate 101. The surface topography of object surface 106 is such that it is provided with a channel 107 having a pair of parallel sides 107a, 107b and a perpendicular surface 107c. As the spray deposition continued, the sprayed deposit 105 builds up on the object surface 106 eventually bridges the width of channel 107 leaving a void 108, as shown in Figure 1(b) caused by the shadowing effect of the build up of deposit 105.

[0022] It has been found, surprisingly, that by initial spraying with droplets of larger size than used conventionally in spray deposition metallurgical techniques, the effects of shadowing and bridging, in situations where the surface topography of the object surface 106 is varied, are modified substantially. Furthermore, external and internal deposited edges (such as edges 105a to 105d) are formed with superior integrity than would otherwise be the case where droplets of conventional size are used. It has been found that sprays having droplets distributed about gaussian mean of 200 microns and above (preferably 350 microns and above) provide significant process advantages. It is believed that the use of larger droplets may provide enhanced process performance because one or either of the following occur:

(a) droplets have increased momentum and remain liquid longer with more opportunity for flow driven by the momentum;

(b) "splashing" of particles on impingement with substrate or deposit lessens "line of sight" problems.

[0023] Using the apparatus shown in Figure 2 the improved performance of the process according to the invention can be achieved. The apparatus of Figure 2 comprises arc spray guns 1,2 mounted on a 6-axis industrial robot 10, producing atomised metal sprays 3,4 which impinge upon pattern or substrate 5. Pattern or substrate 5 sits on a rotating table 6, and is provided with a varied topography object surface 7. A computer control arrangement 8 is used to control manipulation of the robot 10, and also coordinate and control process parameters of the respective sprays 3,4 produced by guns 1,2 (such as, for example, the gas spraying pressure, and wire feed rate/current of the respective guns 1,2). The apparatus is completely enclosed within a dustproof acoustic chamber 9, connected to an appropriate dust and fume extraction system (not shown).

Example

[0024] Both spray guns 1,2 were used to spray low carbon steel (from stock feed wire). For the particular model of arc spray guns used, an initial spray droplet size of approx. 350 microns could be achieved by means of propelling compressed gas at a pressure of 2.6 bar. These conditions were maintained for a period of approximately 120 seconds, for both spray guns. This length of time was sufficient to ensure that all internal and external edges of the object surface of the substrate were covered by deposited spray having droplets of average diameter 350 microns approx. Spraying parameters for both arc spray guns were then adjusted by increasing the pressure of the propelling gas to 3.5 bar and simultaneously decreasing the current supply in order to decrease the rate of generation of molten metal in the arc. These conditions were used to produce a finer droplet size below 150 microns for building up the remainder of the deposit, and to control stresses according to WO-A-96/09421.

[0025] The deposit was subsequently released from the substrate 5 and found to have improved edge integrity and less extensive shadowing or bridging defects than would have been the case when conventionally spraying with droplet sizes of less than 150 microns throughout the process.

Claims

1. A process for producing a metallic tool, mould, die or other body of significant thickness or a coating, the process comprising directing a spray comprising molten metallic droplets carried by a propelling gas toward an object surface of a substrate or pattern so as to build up a metallic deposit or coating comprising the mould, tool, die, body or coating on the object surface of the substrate or pattern, wherein at one or more predetermined stages during spraying droplets of a size distributed about a Gaussian mean of 200 microns and above are sprayed and at one or more other stages droplets having a size below 150 microns are sprayed and wherein the larger droplets are initially sprayed onto the object surface of the substrate or pattern.

2. A process according to claim 1 comprising spraying droplets of a size distributed about a Gaussian mean of 350 microns and at one or more other stages droplets having a size below 150 microns are sprayed.

3. A process according to any preceding claim, wherein a metallic tool, mould, die or other body of significant thickness is produced on a substrate or pattern including topographical relief features, the switching between spraying relatively larger and smaller droplets being dependent upon the nature of and/or the location of the topographical relief features.

4. A process according to claims 1 or 2, wherein the relatively larger droplets are sprayed onto the topographical relief surface, the spray subsequently being switched to spray the relatively smaller droplets.

5. A process according to any preceding claim, wherein the droplets are produced by arc spraying.

6. A process according to any preceding claim, wherein when spraying droplets of the relatively smaller size, spray parameters are tailored to control internal stresses in the deposited metal.

7. A process according to claim 6, wherein the spray parameters are tailored to effect phase change or reaction in the deposited metal.

8. A process according to claim 7, wherein the spray 15 parameters are tailored to effect martensitic phase change in the deposited metal.

9. A process according to any preceding claim, wherein at one stage in the deposition process substantially only droplets of relatively larger mean size are sprayed.

10. A process according to any preceding claim, further comprising spraying of relatively large droplets from one spray source onto the object surface of the substrate or pattern and introducing a further spray of relatively smaller metallic droplets from a second spray source.

11. A process according to claim 10, wherein the further spray of relatively smaller metallic droplets is operated concurrently with the first spray.

12. A process according to any preceding claim, wherein a change between spraying of relatively larger and smaller droplet size sprays is operated by pre-programmed control means.

Ansprüche

1. Ein Prozeß zur Erzeugung eines metallischen Werkzeugs, einer Form, eines Stempels oder eines anderen Körpers wesentlicher Stärke oder einer Beschichtung, wobei der Prozeß umfaßt einen geschmolzene metallische Tröpfchen umfassenden Sprühnebel, getragen von einem Treibgas, direkt in Richtung einer Zielfläche eines Substrats oder Musters zu sprühen, um eine die Form, das Werkzeug, den Stempel, den Körper oder die Beschichtung auf der Zielfläche des Substrats oder Musters umfassende metallische Ablagerung oder Beschichtung aufzubauen; worin in einem oder mehreren vorherbestimmten Stadien während des Sprühens um einen Gaußschen Mittelwert von 200 Mikron und darüber verteilten Tröpfchen versprüht werden; und worin in einem oder mehreren vorherbestimmten anderen Stadien Tröpfchen versprüht werden, die eine Größe unterhalb 150 Mikron besitzen; und worin die größeren Tröpfchen anfänglich auf die Zielfläche des Substrats oder Musters aufgesprüht werden.

2. Ein Prozeß gemäß Anspruch 1, der es umfaßt Tröpfchen einer um einen Gaußschen Mittelwert von 350 Mikron verteilten Größe zu versprühen; und in einem oder mehreren Stadien Tröpfchen, die eine Größe unterhalb von 150 Mikron besitzen.

3. Ein Verfahren gemäß einem der vorstehenden Ansprüche, in dem ein metallisches Werkzeug, eine Form, ein Stempel oder ein anderer Körper wesentlicher Stärke auf einem topographische Relief- oder Entlastungsmerkmale einschließenden Substrat oder Muster erzeugt wird, und die Umschaltung zwischen dem Versprühen relativ großer und kleinerer Tröpfchen von der Natur und/oder der Lage der topographischen Entlastungsmerkmale abhängt.

4. Ein Prozeß gemäß den Ansprüchen 1 oder 2, in dem die relativ gesehen größeren Tröpfchen auf die topographischen Relief- oder Entlastungsmerkmale aufgesprüht werden, und der Sprühnebel nachfolgend umgeschaltet wird, um die relativ gesehen kleineren Tröpfchen zu versprühen.

5. Ein Prozeß gemäß irgendeinem der vorstehenden Ansprüche, in dem die Tröpfchen durch Lichtbogensprühen erzeugt werden.

6. Ein Prozeß gemäß irgendeinem der vorstehenden Ansprüche, in dem Sprühparameter zugeschnitten sind um die inneren Belastungen in dem abgelagerten Metall zu steuern, wenn man die Tröpfchen relativ kleinerer Größe versprüht.

7. Ein Prozeß gemäß Anspruch 6, in dem die Sprühparameter zugeschnitten sind um einen Phasenwechsel oder eine Reaktion in dem abgelagerten Metall zu bewirken.

8. Ein Prozeß gemäß Anspruch 7, in dem die Sprühparameter zugeschnitten sind um einen martensitischen Phasenwechsel in dem abgelagerten Metall zu bewirken.

9. Ein Prozeß gemäß irgendeinem der vorstehenden Ansprüche, in dem während eines Stadiums des Ablagerungsprozesses im Wesentlichen nur Tröpfchen einer relativ größeren mittleren Größe versprüht werden.

10. Ein Prozeß gemäß irgendeinem der vorstehenden Ansprüche, der weiterhin das Versprühen relativ großer Tröpfchen von einer Sprühquelle auf die Zielfläche des Substrats oder Musters umfaßt, und einen weiteren Sprühnebel von relativ gesehen kleineren metallischen Tröpfchen von einer zweiten Sprühquelle einzubringen.

11. Ein Prozeß gemäß Anspruch 10, in dem der weitere Sprühnebel von relativ gesehen kleineren metallischen Tröpfchen gleichzeitig mit dem ersten Sprühnebel betrieben wird.

12. Ein Prozeß gemäß irgendeinem der vorstehenden Ansprüche, indem ein Wechsel zwischen dem Versprühen von Sprühnebeln relativ gesehen größerer und kleinerer Tröpfchengrößen durch vorprogrammierte Steuervorrichtungen getätigt wird.

Revendications

1. Procédé de production d'un outil métallique, d'un moule, d'une matrice ou d'un autre corps d'épaisseur significative ou d'un revêtement, le procédé consistant à diriger un jet pulvérisé composé de gouttelettes de métal fondu portées par un gaz propulseur vers une surface cible d'un substrat ou d'un modèle de façon à réaliser un dépôt ou un revêtement de métal constituant le moule, l'outil, la matrice, le corps ou le revêtement sur la surface cible du substrat ou du motif, dans lequel des gouttelettes dont la taille est répartie autour d'une moyenne Gaussienne égale ou supérieure à 200 microns sont pulvérisées à un ou plusieurs instants prédéterminés durant la pulvérisation et des gouttelettes dont la taille est inférieure à 150 microns sont pulvérisées à un ou plusieurs autres instants, et dans lequel les plus grosses gouttelettes sont initialement pulvérisées sur la surface cible du substrat ou du modèle.

2. Procédé selon la revendication 1 consistant à pulvériser des gouttelettes ayant une taille répartie autour d'une moyenne Gaussienne de 350 microns et à pulvériser à un ou plusieurs autres instants des gouttelettes ayant une taille inférieure à 150 microns.

3. Procédé selon l'une des revendications précédentes, dans lequel est produit un outil métallique, un moule, une matrice ou un autre corps d'une épaisseur significative sur un substrat ou un modèle, y compris des détails de relief topographique, le basculement entre la pulvérisation de gouttelettes relativement plus grosses ou plus petites dépendant de la nature et/ou de l'emplacement des détails de relief topographique.

4. Procédé selon les revendications 1 ou 2 dans lequel les gouttelettes dont la taille relative est plus importante sont pulvérisées sur la surface du relief topographique, et dans lequel les gouttelettes dont la taille relative est la plus faible sont ensuite pulvérisées.

5. Procédé selon l'une des revendications précédentes, dans lequel les gouttelettes sont produites par pulvérisation à l'arc.

6. Procédé selon l'une des revendications précédentes, dans lequel, lors de la pulvérisation des gouttelettes dont la taille relative est la plus faible, les paramètres de pulvérisation sont adaptés afin de maîtriser les contraintes internes dans le métal déposé.

7. Procédé selon la revendication 6, dans lequel les paramètres de pulvérisation sont adaptés pour provoquer un changement de phase ou une réaction dans le métal déposé.

8. Procédé selon la revendication 7, dans lequel les paramètres de pulvérisation sont adaptés pour provoquer un changement de phase martensitique dans le métal déposé.

9. Procédé selon l'une des revendications précédentes, dans lequel, à un moment du processus de dépôt, sensiblement toutes les gouttelettes pulvérisées sont d'une taille moyenne relative plus importante.

10. Procédé selon l'une dés revendications précédentes, comprenant en outre la pulvérisation de gouttelettes relativement grosses par une source de pulvérisation sur la surface cible du substrat ou du modèle et l'introduction d'une pulvérisation supplémentaire de gouttelettes relativement petites par une deuxième source de pulvérisation.

11. Procédé selon la revendication 10, dans lequel la pulvérisation supplémentaire de gouttelettes relativement plus petites est effectuée en même temps que la première pulvérisation.

12. Procédé selon l'une des revendications précédentes, dans lequel le basculement entre la pulvérisation de gouttelettes relativement plus grosses et la pulvérisation de gouttelettes relativement plus petites est effectuée par un moyen de commande préprogrammé.

Drawing