Technical Field of the Invention

The present invention relates to a method of measuring the density of a powder bed formed by an additive manufacturing (AM) machine during a build operation. It also relates to a method of detecting a defect in an additively manufactured article.

Background to the Invention

In a known AM process an AM machine produces articles from a powdered material, such as a metal or alloy. The machine deposits a layer of powder on a build platform and the powder is subsequently selectively fused or otherwise solidified, typically with a laser or electron beam, to form an article or articles. The process is repeated so that articles are formed layer by layer.

Obtaining a predictable and consistent build quality depends, amongst other things, on the machine depositing each layer of powder with a desired and consistent density. Deviation from a desired density or density range may lead to defects in a manufactured article, which may impair the article's ability to perform its function. For safety critical components, for example blades for an aircraft jet, turbofan or turboprop engine, this could have very serious consequences.

It has been proposed, therefore, to measure the density of the powder bed formed by an AM machine as a way of ensuring that the machine is functioning correctly and thus that articles manufactured by the machine are of satisfactory quality. Measuring powder bed density presents difficulties, however. Disturbing the powder is apt to alter its density - the very property it is desired to measure - as well as to cause the powder to degrade - such as by oxidation if the measurement involves exposing the power to air - and thus limiting its useful life. Both are undesirable. It is an object of embodiments of the present invention to provide for the measurement of powder bed density in an AM machine whilst mitigating the problems discussed above. It is a further object of embodiments of the present invention to help detect a defect in an additively manufactured article.

Summary of the Invention According to an aspect of the present invention there is provided a method of measuring the density of a powder bed formed by an additive manufacturing (AM) machine during a build operation, the method comprising the steps of: causing the machine to form a powder bed and build a capsule from powder of the powder bed so that part of the powder bed is encapsulated within the capsule; determining the internal volume of the capsule; determining the mass of the encapsulated powder; and calculating the density of the part of the powder bed encapsulated by the capsule using the determined volume and mass. As the capsule is formed during the build operation it does not cause any disturbance to the powder bed which might affect its density. After the build is complete the capsule may be removed from the machine. This may be in the same way as any other article manufactured by the machine and allows unfused powder in the machine to be recycled in the usual way. Subsequent measurement of the capsule and/or encapsulated powder may be done away from the machine and the encapsulated powder recycled, discarded or retained as desired. The method enables an accurate measurement of the density of the encapsulated part of the powder bed to be made. This is likely to approximate to the density of the powder bed formed outside the capsule. However, for improved measurement, the machine may be caused to build more than one capsule during the build operation. So, for example, the machine could build capsules at different positions across and/or above the build platform in order to measure powder bed density at different positions within the powder bed and so determine the consistency with which the powder bed is formed by the machine.

The machine may be caused to build one or more articles (other than capsules) concurrently with building the/or each capsule. Thus the/or each capsule can yield density data specific to a build operation for an article or articles produced by the machine, rather than just giving a general indication of machine performance. Where capsules are built concurrently with one or more articles the capsules may located at opposites sides of the one or more of the articles and/or one or more capsules may be built on one or more of the articles. This enables powder bed density to be measured at points immediately around the/or each article being built, and thus most likely to be indicative of the powder bed density where the/or each article is formed.

After the build operation the/or each capsule may be opened, encapsulated powder emptied out of the capsule and weighed to determine the mass of the encapsulated powder. Encapsulated powder may be removed before, but more typically after, removing the capsule from the AM machine. Alternatively or additionally the or each capsule may be removed from the AM machine and weighed, the capsule opened and encapsulated powder emptied out and the empty capsule or parts thereof weighed thereby to determine the mass of the encapsulated powder. This latter approach avoids the need to ensure that all of the encapsulated powder is retained when the capsule is opened.

The internal volume of the capsule may be determined by any suitable method. In one arrangement it is derived from the instructions provided to the AM machine to build the capsule. However, it is generally preferred that the volume is measured. A suitable method for determining the internal volume of the capsule is pycnometry, in particular gas, especially Helium, pycnometry.

Conveniently the external volume of the/or each capsule is measured, the capsule opened and encapsulated powder emptied out, and the volume of the body of the now open empty capsule or parts thereof measured thereby to determine the internal volume of the capsule. These steps can by performed by pycnometry, by placing the complete and subsequently opened and empty capsule (or parts thereof) into a pycnometer.

An advantage of using pycnometry is that in the event of a defect in a capsule which allows gas to pass through the capsule into or out of the volume it encloses the measured external volume of the capsule will be affected. Typically a pycnometer will not be able to determine an external volume at all. But it is also possible that it will measure a smaller volume than for a completely sealed capsule. So the result of an attempted measurement of the external volume of a capsule may be indicative of a defect in the capsule.

According to another aspect of the present invention there is provided a method of detecting a defect in a capsule built by an additive manufacturing machine comprising the steps of: causing the machine to build a capsule encapsulating powder; placing the capsule into a pycnometer and attempting to measure the volume displaced by the capsule; and determining from the output of the pycnometer if there is a defect in the capsule. The method may include the steps of concluding that there is a defect if the pycnometer is unable to resolve the volume of the capsule and/or if the pycnometer measures a volume that is more than a predetermined threshold less than the theoretical volume displaced by the capsule. The theoretical volume displaced by the capsule may be determined by measuring the capsule or from instructions provided to the additive manufacturing machine. The capsule may be built concurrently with one or more articles and the presence of a defect in the capsule may thus indicate the risk of a defect in one or more of the/or each article.

For either aspect of the invention, the/or each capsule may comprise one or more sidewalls which diverge from each other as they rise from a base and subsequently converge towards one another towards a top. The/or each capsule may have the form of two opposed cones or pyramids. Such structures are inherently self-supporting and thus convenient to build with an AM machine. The/or each capsule may have a circular or substantially circular cross-section. Alternatively the/or each capsule may have a polygonal cross-section.

A zone or line of weakness may formed in the/or each capsule at which the capsule will preferentially break when subjected to a force. This facilities opening of the capsule to enable encapsulated powder to be emptied out.

A line of weakness may be formed by a region of reduced wall thickness. The line of reduced thickness may extend substantially or wholly around the capsule and is preferably formed towards one end of the capsule.

The/or each capsule may comprise a part shaped to be received by or connect to a tool or apparatus thereby to facilitate removing the capsule from a build plate or article, and/or opening the capsule to release encapsulated powder. The tool could be a spanner, wrench, socket, screw driver, key or other suitable tool. The apparatus could be a fatigue device, a tensile device, an impactor or other suitable apparatus.

A respective part may be provided at each opposite end of the/or each capsule. The additive manufacturing machine may be an additive layer manufacturing machine.

Detailed Description of the Invention

In order that the invention may be more clearly understood one or more embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a schematic view of an additive manufacturing machine; Figure 2 is a part cut away side view of the build plate of the machine of figure 1 showing a powder bed, article and capsules formed by the machine;

Figure 3 is a part cut away plan view of the powder bed of figure 2;

Figure 4 is a side view of one of the capsules of figure 2; Figure 5 is a plan view of the capsule of figure 4;

Figure 6 is a cross-sectional view of the capsule of figures 4 and 5 taken along the line VI- VI; and

Figure 7 is a perspective view of the capsule of figures 4 - 6 with a part broken away using a spanner enabling powder inside the capsule to be removed. Referring to the drawings, a conventional additive manufacturing (AM) machine, generally 1, comprises a build platform 2 disposed on a moveable support 3 disposed in a recess in a surface 4 of the machine. A powder dispensing head and/or wiper blade 5 is arranged to move across and deposit a layer of powder (typically a metal powder) onto the build platform 2 to form a powder bed 14. A laser 6 is arranged to scan a beam 9 across the surface of the powder deposited on the build platform to selectively fuse the powder in a deposited layer to form an article 7 to be built. Following the fusing step the moveable support is lowered slightly into the recess and the process repeated: a fresh layer of powder is deposited and selectively fused. The process is repeated to build up the article 7 being built layer by layer until it is complete. Once complete the finished article is removed from the machine and unfused power can be re-used in another build process. In order to ensure a sufficient and consistent build quality of a manufactured article the AM machine must lay down each layer of powder with a density falling within desired limits. By measuring the density of powder laid down it is possible to confirm if the machine is functioning correctly and thus if a manufactured article has, or is likely to have, desired integrity.

To enable powder density to be measured without having to interfere with the powder bed, which could affect density and/or lead to contamination or other degradation of the powder and thus limit its useful life, the AM machine is arranged to build one or more capsules 10 during the build process. Where more than one capsule is built the capsules may be distributed across the build platform and be distributed at different positions above the build platform. Each capsule is hollow and encapsulates a part of the powder bed laid down during the build process. Preferably capsules are built adjacent opposite sides of an article being built and may also be built on an article being built, as shown in figures 2 and 3. Measuring powder bed density and build integrity at multiple points around an article being built enables variations of density and likely issues with build integrity within the powder bed to be identified and a determination made as to the likelihood of there being a density variation in the bed where the article is formed or a build integrity problem in the article.

It will be appreciated that figures 2 and 3 show the powder bed cut away sufficiently to reveal the capsules 10 and article 7.

When the build is complete the density of powder encapsulated in each capsule is measured by determining the volume of the capsule and determining the mass of powder encapsulated within the capsule by opening, removing and weighing the powder encapsulated by the capsule.

Conveniently the volume of the/or each capsule is determined by pycnometry. The capsule is removed from the AM machine (detaching it from the build platform or an article if necessary) and placed into a pycnometer to determine its overall volume. The capsule is then opened and the encapsulated powder removed. The capsule or parts of the capsule are then placed into a pycnometer to determine their overall volume. The difference between the two volumes thus represents the internal volume of the capsule and so the volume occupied by the encapsulated powder as laid down during the build process. The encapsulated powder is weighed and its mass divided by the measured volume of the capsule to determine the density of powder bed as laid down during the build process. Alternatively the mass of the encapsulated powder may be determined by weighing the capsule together with encapsulated powder, removing the encapsulated powder, weighing the empty capsule or parts thereof and subtracting its mass from that of the full capsule.

As will be understood by those skilled in the art, a pycnometer measures the volume of an article by gas displacement and a pycnometer will typically perform several operation cycles before arriving at a volume determination. Where there is a build integrity issue with a capsule, such as a crack, this may allow the passage of gas through the capsule into and out of the volume it encloses. In most cases such a defect in the build of a capsule only permits a very gradual flow of gas into and out of the enclosed volume with the result that a pycnometer is not able to resolve a volume measurement for the capsule. In the event of a defect in the capsule that allows a greater flow of gas into and out of the capsule a pycnometer may measure a much smaller volume than would be normally be expected. Inability to resolve a volume measurement or measurement of a smaller than expected volume is indicative of a defect in the capsule and thus of the possibility of a defect in an article built concurrently with the capsule. Referring now to figures 4 to 7 in particular the capsules 10 will be described in more detail.

Each capsule has the general form of two, opposed, hollow cones connected together at their bases. This is a convenient structure to build in an AM machine is it is self-supporting. At each end of the capsule there is formed a six sided head 11 that may conveniently be gripped by a spanner 12. Adjacent one of the heads 12 a line of weakness 13 is formed completely around the sidewall of the capsule by a thinning of the sidewall to facilitate opening of the capsule. In use a spanner or other tool is used to apply a torque to the head 11 adjacent the line of weakness whilst the rest of the capsule is held still, for example with another spanner holding the head at the opposite end. This torque causes the capsule to preferentially break along the line of weakness 13. Similarly, where a capsule has been formed on the build platform or an article being built a spanner may be used to apply a torque to the head 11 adjacent the build platform or article to break the capsule off the platform or article.

The invention provides a convenient and reliable way of measuring the density of a powder bed formed by an AM machine during the build of one or more articles. By distributing capsules throughout the powder bed it is possible to determine if the machine is producing a powder bed of consistent density across the build platform. As the capsules are built concurrently with one or more articles the measured density is directly applicable to assessing the build quality of the articles. Determining the volume of the or each capsule using a pycnometer is particularly advantageous as it will either not yield a result, or a sensible result, if the capsule is not hermitically sealed, thus revealing if there is a defect in the build of the capsule and therefore that there could be a similar defect in the build of the manufactured article(s).

The above embodiment is described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.