ADDITIVE LAYER MANUFACTURING METHODS

An apparatus and method for performing an ALM process is described. A first energy beam source (1) provides an energy beam (1b) which selectively melts a substrate powder (3) into a melt pool. A second energy beam source (2) provides an energy beam (2b) to heat condition substrate powder proximate to the melt pool. The path of the second energy beam (2b) is controlled by a controller (6) to oscillate independently of the path followed by the first energy beam (1b). The method may be applied to control and optimise heating and cooling rates of the sintered substrate during the ALM process enabling its microstructure to be controlled to suit the end use of the product and reduce the occurrence of residual stresses and consequent crack propagation.

FIELD OF THE INVENTION

The invention relates to the manufacture of components using additive layer manufacturing methods. In particular, the invention provides novel methods which result in improved fracture resistance of the finished component.

BACKGROUND OF THE INVENTION

Additive layer manufacturing (ALM) methods are known. In these methods a component is built up layer by layer until the 3D component is defined. In some ALM methods, the layers are laid down from a continuous extrusion of material. In other methods, layers are created by selective treatment of layers within a mass of particulate material, the treatment causing cohesion of selected regions of particulates into a solid mass. In other methods, a liquid mass is selectively treated to produce solid layers. Specific examples of ALM methods include (without limitation); electron beam melting (EBM), direct laser deposition (DLD), laser engineered net shaping (LNS), selective laser melting (SLM), direct metal laser sintering (DMLS) and selective laser sintering (SLS).

As will be appreciated, one of the advantages with ALM manufacturing techniques is that it can provide near net-shape components resulting in little waste which require subsequent additional machining. One exception to this may be the inclusion of supporting features or geometries which enable the components to be made. One particular application of ALM methods is in the formation of components for use in a gas turbine engine. It will be appreciated that, as well as accurate dimensional tolerances, such components must have excellent and consistent mechanical properties to prevent failure of the component.

Control of heating and cooling cycles in many known ALM technologies is limited. The rate of heating and cooling of the substrate can impact significantly on the microstructure of the end product. For example, mechanical deficiencies in an ALM manufactured component can arise when residual stresses result from rapid cooling rates in the heated powder. In high temperature alloys, these residual stresses can result in propagation of cracks within the component during subsequent heat treatments and/or when in use in a high temperature application. It is known to heat treat components manufactured by ALM processes to mitigate the effects of residual stresses.

European Patent number EP724494B proposes the use of a main sintering beam and a defocused “heating up” beam to control heating in the region of the main sintering beam as it travels and generate a pre-determined temperature gradient adjacent the main sintering beam. The defocused beam follows the same path as the main sintering beam.

International Patent Application publication number WO 2015/120168 proposes the use of multiple energy beams which are arranged to follow one another. The energy beams provide different amounts of energy and are used to control the rate of melting and solidification in the region of the melt pool as the powder is sintered. The beams are controlled to move in unison, their paths defined together to create a thermal gradient adjacent the path followed by the main sintering beam as it travels. Hot spots produced by the beams are controlled to travel together a fixed, predefined distance apart from one another.

STATEMENT OF THE INVENTION

The present invention provides a method for performing an ALM process comprising;

melting a substrate into a melt pool with a first energy beam, and

heat conditioning the substrate with a second energy beam, wherein

the second energy beam is controlled independently of the first energy beam to move in an oscillating motion across or around the path of the first energy beam.

There may be multiple second energy beams controllable independently of each other or as a collective to perform desired thermal conditioning steps.

The first and second energy beams may emit two different wavelengths. The first energy beam may emit a higher wavelength than the second energy beam. Optionally, the shape of the second energy beam may be adjusted to more accurately control heat management. For example, for larger areas, the energy beams may be focused to a rectangular or other shape most suitable to the application. Beam shaping may be achieved by means of shaped optical fibre elements. Beam shaping may be achieved by means of beam shaping optics, apertures, gratings, reflectors and other such optical elements. Optionally, deformable beam shaping optics may be controllably deformed to vary the beam shape of the energy beams as it travels.

The second energy beam may be controlled to oscillate in a periodic manner. Oscillation may be achieved by scanning in two dimensions across the plane in which the substrate is laid down. For example, the second energy beam may be controlled to follow a sinusoidal path which periodically crosses over the path of the first energy beam. The shape of the oscillation is not critical to achieving the benefits of the invention, alternative waveforms might, for example, be triangular or rectangular. In more complex embodiments, the second energy beam may be controlled to oscillate in three dimensions rather than just two. For example, the second energy beam may be controlled to move in a substantially helical pattern around the path of the first energy beam. As for the two dimensional embodiments, the precise shape of the oscillation is not critical.

Either or both of the first and second energy beam sources may be provided by a laser. An alternative source to a laser is an electron beam. The energy beams need not be provided by the same form of energy source. It will be appreciated that the second energy beam requires a greater degree of controllability to provide the desired oscillations. For example, the laser may be an IR laser with appropriate focusing optics. Alternatively, the laser may be a direct laser diode combined with a suitable refractive or reflective focusing element. In another option, a line or matrix of laser diodes of varying wavelengths may be employed to provide the second energy beam.

A controller may be programmed to selectively control illumination of the direct laser diodes in a pre-defined sequence whereby to achieve a desired control of the temperature gradient within the process zone in which the first energy beam is operational. For example, a controller might employ a micro-electro-mechanical system (MEMS) to adjust the direction of energy emission from an energy source. In some embodiments, a MEMS may be employed to move a reflective or refractive element relative to the energy source, or alternatively to move the source relative to the substrate.

With knowledge of the material of the substrate powder and geometry of the workpiece to be produced, the controller can be pre-programmed to control the heating and/or cooling rate of the substrate so as to reduce residual stress build up in the region of the melt pool and provide a more equi-axe grain structure in the finished workpiece. To achieve this, the second energy beam may be scanned at varying speeds and profiles to optimise cooling as the shape, cross section or the like of the workpiece defined by the path of the first energy beam changes.

The path and other characteristics of the second energy beam may be controlled to pre-heat and post-heat substrate adjacent the melt pool. In addition, the second energy beam may be controlled to thermally control already processed substrate distant from the melt pool. For example, the second energy beam may be controlled to revisit already solidified material of the workpiece to recondition the already solidified material. This control may be pre-programmed or may be part of an adaptive control system which monitors the condition of the already processed workpiece, identifying faults in the already processed material and responding to an identified fault by redirecting the second energy beam to the region of the identified fault to perform a reconditioning step.

In another aspect, the invention provides an apparatus for performing an ALM process comprising;

a first energy beam source for providing an energy beam to selectively melt a substrate powder into a melt pool;

a second energy beam source for providing an energy beam to heat condition substrate powder proximate to the melt pool; and

a controller for controlling oscillation of an energy beam emitted by the second energy beam source independently of the path followed by a beam emitted by the first energy beam source.

Optionally, the apparatus is configured to provide multiple first energy beams. The controller may be configured to operate a single second energy beam to oscillate about the paths of multiple first energy beams.

Optionally, the second energy beam source comprises a laser. Optionally, the second energy beam source comprises multiple focused IR lasers, preferably high intensity broad wavelength lamps. The lamps may be selected to emit a suitable range of wavelengths suited to heat conditioning the powder substrate used in the ALM process. In an alternative, the second energy beam source comprises an array of laser diodes emitting a range of wavelengths of energy. The array may comprise a line of diodes, alternatively, the array is a matrix of diodes. The diodes may collectively be selected to emit a suitable range of wavelengths suited to heat conditioning the powder substrate used in the ALM process.

The controller may be programmable to define a path of the (or each) second energy beam. Where there are multiple second energy beams, the controller may control the multiple second energy beams independently of one another. The controller may incorporate adaptive optics. For example, the controller may be configured to control operation of a MEMS which in turn may reposition and/or deform a reflective or refractive element relative to the energy beam source. Alternatively, the MEMS may be controlled to move the source itself.

Specific characteristics of the second energy beam may further be controlled by selective use or adjustment of beam shaping optics. For example, the beam shaping optics are deformable and are controllably deformed by the controller to alter characteristics of the second energy beam.

Where an array of diode lasers is employed, the individual lasers may be switched on and off by the controller according to a pre-defined pattern.

For example, the substrate powder may comprise a ferrous or non-ferrous alloy or a ceramic. The workpiece may form the whole or part of a component for a gas turbine engine.

In another aspect, the invention comprises a gas turbine engine incorporating one or more components manufactured in accordance with the method of the invention.

DETAILED DESCRIPTION OF DRAWINGS AND SOME EMBODIMENTS

As can be seen inFIG. 1, an apparatus suitable for performing the ALM process of the invention comprises a first energy beam source1with associated optics la for controlling the characteristics of an energy beam1bemitted by the source1. Also provided is a second energy beam source2with associated optics2afor controlling the characteristics of an energy beam2bemitted by the source2. Both beams1b,2bare focused on a bed3of a powdered substrate which is provided in sequential layers onto a plate4. The first energy beam1bis configured to locally melt powder in the bed3which, as it cools, consolidates to form a workpiece5. The second energy beam2bis configured to heat powder in the locality of the powder melted by the first energy beam1bwhereby to control the rate of cooling of the melted powder and powder adjacent thereto.

Movement of the first energy beam1bis controlled using prior known methods. For example, scanning optics could be used and whose path is pre-programmed using CAD/CAM data which defines the shape of the work piece. In another alternative, the first energy beam is held in a stable position whilst the bed carrying the substrate powder is moved relative to the first energy beam.

The apparatus further comprises a controller6associated with the second energy beam2b. For example the controller is configured to move the second energy beam source2. In addition or alternatively, the controller may be configured to adjust the optics2a. Adjustment may involve repositioning of the optics2a, or in the case of deformable optics, controlled deformation. A MEMS (not shown) may be operated by the controller to adjust the optics or reposition the second energy beam source2.

FIG. 2shows an example of paths followed by a first energy beam1band a second energy beam2bin performing an embodiment of an ALM process in accordance with the invention. As shown, the first energy beam1bfollows a linear path which broadly would coincide with a melt pool created in the substrate powder. The second energy beam2bfollows an oscillating path which swings periodically from one side of the first energy beam1bto the other. The second energy beam2balso travels just ahead of the first energy beam1b.The second energy beam2bintroduces less energy to the substrate than the first energy beam1bover a greater area and so reduces the thermal gradient between the melt pool and surrounding powders. The cooling rate in this region and hence the local microstructure can be controlled, reducing the formation of residual stresses and consequent crack propagation in the finished component.

WhilstFIG. 2shows the concept of the invention in a simplistic, two-dimensional form, it will be appreciated that heat transfers through the substrate powder in three dimensions.FIG. 3illustrates oscillation of the second energy beam2bwith respect to the first energy beam1bin a plane orthogonal to that illustrated inFIG. 2as time T progresses. As can be seen inFIG. 3, the second energy beam2boscillates in an up and down as well as side to side motion with respect to the direction of travel of the first energy beam1b.Thus the second energy beam2bnot only influences the thermal gradient in material adjacent the melt pool in the plane in which the first energy beam1bis travelling, but simultaneously influences the thermal gradient in already processed powder in planes below the plane in which the first energy beam1bis currently travelling. Thus, as well as pre-heating powder about to be melted by the first energy beam1b, the second energy beam2b, controls the rate at which already processed powder is cooled.

The path followed by the second energy beam2bwith respect to the path of the first energy beam1bmay be follow a consistent pattern or may incorporate variations.FIG. 4aillustrates a simple, consistent pattern where the second energy beam2bis programmed essentially to follow a helical path around the path of the first energy beam1b.FIG. 4bshows an alternative where the helical path periodically increases and decreases in diameter.FIG. 4cshows an alternative where the helix in a helical path gradually increases in diameter to a maximum and decrease gradually to a minimum over a period of time. In practice, more complex three dimensional patterns can be defined for the second energy beam2bchanging the quantity of heat, the rate of heating and the area heated by the second energy beam to address changes in the path of the first energy beam1b. For example, where the first energy beam1bis sintering an outer wall of a component, the path of the second beam2bmay be primarily directed to controlling the cooling rate of substrate powder which will become part of the body of the component, avoiding cooling of powder in the same layer which is not to be sintered and likely to be recycled on completion of the process.

Changes in the second energy beam2bpath may also reflect critical parts of the component geometry, particularly attending to controlling the heating and cooling rate in regions which have a high susceptibility to residual stress, for example small radii or angled sections.

By way of example, the following describes specific parameters which might be used for the first and second energy beams when performing an ALM process in accordance with the invention to manufacture a component from a high temperature alloy suited to use in a gas turbine engine.

The energy beam sources may each comprise lasers having a power range from about 100 W to 2 kW. The energy output by a beam is a function of the exposure time and the power of the beam. The required energy output varies from one material to another. It will be within the knowledge and ability of the skilled addressee to select appropriate energy beam powers and exposure times to provide the required energy output for a known substrate material.

The first energy beam laser is operable in a velocity range of from about 0.2 m/s to about 3 m/s. Typically it operates at a constant velocity of about 1 m/s. The second energy beam laser is arranged to either lead or follow or both, the first energy beam laser at a controlled velocity which may be significantly different to the first beam velocity, to achieve the process requirement i.e. preheating or the control of cooling rate or both.

The second energy beam velocity could be in the range from about 1 m/s to about 7 m/s. The velocity for the second beam may be slower than for the first beam depending on the application requirements.

Referring back toFIG. 3, the first energy beam laser1bis travelling at an absolute velocity whilst the second energy beam oscillates between ahead of the first energy beam and behind the first energy beam. The second energy beam travels a maximum distance d ahead of the first energy beam pre-heating the substrate) and a maximum distance d′ behind the first energy beam (post-heating the substrate and controlling its rate of cooling). The distances d and d′ are typically between 1 mm and 20 mm and may be the same or different. Depending on the specific properties of the substrate material, in some cases it may be beneficial to traverse the second energy beam further in one of the post-heating or pre-heating direction. Again, with knowledge of the substrate material, it will be within the ability of the skilled addressee to determine (perhaps through trials or calculation) optimum distances d and d′ for a specific application of the process.

As previously stated, the frequency of the oscillation of the second energy beam may be periodic and follow a consistent pattern. This is most likely where the first energy beam is sintering a straight line at the centre of the component geometry where the impact of the first energy beam on the substrate and component is consistent. However, a periodic oscillation is rarely optimal for the entire ALM process. Hence the pattern followed by the second energy beam will be varied and adapted, for example, to address significant changes in the geometry or thickness of the component whose shape is defined by the path followed by the first energy beam.

Where the second energy beam is controlled to oscillate periodically, the frequency of the oscillation is typically from about 1 oscillation to about 30 oscillations per second.

Various approaches might be taken to control the second energy beam path. Optional control strategies include;Deriving a path from mathematical modeling of the specific applicationReferring to a previously collated database of parametersUsing a real time monitor of the temperature of the material being processedAny combination of the above strategies

In one advantageous embodiment shown schematically inFIG. 5, apparatus for performing the ALM process of the invention includes a temperature measuring system which feeds back temperature data to the controller. The controller may then be configured adaptively to control the path and/or other parameters of a second energy beam to ensure optimal heating and cooling rates for identified regions of the sintered powder.

In common with the embodiment ofFIG. 1, the embodiment ofFIG. 5includes; a first energy beam source51with associated optics51afor controlling the characteristics of an energy beam51bemitted by the source51. Also provided is a second energy beam source52with associated optics52afor controlling the characteristics of an energy beam52bemitted by the source52. Both beams51b,52bare focused on a bed53of a powdered substrate which is provided in sequential layers onto a plate54. The first energy beam51bis configured to locally melt powder in the bed53which, as it cools, consolidates to form a work piece55. The second energy beam52bis configured to heat powder in the locality of the powder melted by the first energy beam51bwhereby to control the rate of cooling of the melted powder and powder adjacent thereto.

The apparatus further comprises a controller56associated with the second energy beam52b. For example the controller is configured to move the second energy beam source52. In addition or alternatively, the controller may be configured to adjust the optics52a. Adjustment may involve repositioning of the optics52a, or in the case of deformable optics, controlled deformation. A MEMS (not shown) may be operated by the controller to adjust the optics or reposition the second energy beam source52. A thermal imaging device57is arranged to monitor temperatures in the powder bed53during the ALM process. Data from the thermal imaging device57is input to the controller56which then adaptively controls the path, oscillation and/or other parameters of the second energy beam52bto optimise heating and cooling of processed powder.

A variety of known temperature measurement systems are known which could be adapted into a control system as described above. For example (without limitation), the device may be a thermal imaging device, a thermal camera, a radiation detector (e.g. infra-red)or an array of suitably positioned thermocouples. Such a temperature measurement system may measure a temperature of the targeted material directly, or may measure temperatures adjacent (including in a space above the deposited substrate) the targeted material. In the latter case, the controller may perform calculations to determine the temperature at the targeted material using known characteristics of the material.

The temperature measurement system may be configured to map various zones of the powder bed. This could be used advantageously where multiple second energy beams are employed. For example, one of the second energy beams could be controlled to travel with the first energy beam controlling the heating and cooling rate of powder in the region of the melt pool whilst another is controlled to effect thermal gradient management in already sintered zones. Such a temperature measurement system may comprise multiple temperature measuring devices.

Additional energy beams may be employed which may be moved in an oscillating manner. For example, such additional energy beams may be focused on defined zones of the powder bed and their beam shape/intensity controlled to maintain a desired thermal profile in that zone. Optionally any of the energy beams may each have an associated temperature measuring device, the energy beam and device being configured and controlled to manage thermal profiles in a defined zone.

With reference toFIG. 6, a gas turbine engine is generally indicated at60, having a principal and rotational axis61. The engine60comprises, in axial flow series, an air intake62, a propulsive fan63, an intermediate pressure compressor64, a high-pressure compressor65, combustion equipment66, a high-pressure turbine67, and intermediate pressure turbine68, a low-pressure turbine69and an exhaust nozzle70. A nacelle71generally surrounds the engine60and defines both the intake62and the exhaust nozzle70.

The gas turbine engine60works in the conventional manner so that air entering the intake62is accelerated by the fan63to produce two air flows: a first air flow into the intermediate pressure compressor64and a second air flow which passes through a bypass duct72to provide propulsive thrust. The intermediate pressure compressor64compresses the air flow directed into it before delivering that air to the high pressure compressor65where further compression takes place.

The compressed air exhausted from the high-pressure compressor65is directed into the combustion equipment66where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines67,68,69before being exhausted through the nozzle70to provide additional propulsive thrust. The high67, intermediate68and low69pressure turbines drive respectively the high pressure compressor65, intermediate pressure compressor64and fan63, each by suitable interconnecting shaft.