Patent Application
20150266285
This invention relates to a field of additive manufacturing and in particular, to a method and an apparatus for controlling a grain size of a component manufactured using an additive manufacturing process.
In additive manufacturing techniques, a heat source is used to melt a specified amount of metal, which is in the form of a powder or wire, onto a base material. By repeating the process, layers of melted metallic powder are each arranged sequentially upon a preceding layer, resulting in the formation of a desired component. Additive manufacturing (AM) techniques can include selective laser melting (SLM), electron beam melting (EBM), laser metal forming (LMF), laser engineered net shape (LENS), or direct metal deposition (DMD). The invention is related mainly to the SLM additive manufacturing technique.
In the SLM technique, a laser beam scans a layer filled with metal or plastic powder, thereby melting and solidifying the powder in the areas of contact with the laser beam. The beam diameter of the laser is small, typical in the range of 100-300 um, thereby resulting in a small melt pool size. This leads to rapid solidification once the beam moves to another point on the layer. The time for solidification of a melted powder layer is limited, and the grain size in a solidified component is very small.
Components used in turbines or turbomachines need to operate at extremely high temperatures. Components with small grain size, such as those manufactured using AM techniques, deteriorate quickly due the effects of creep, stress rupture and thermo mechanical fatigue (TMF) and the like.
Further, materials such as Nickel and Cobalt based superalloys resist grain growth during service and retain the grain size developed during the manufacturing process. It is therefore necessary to control the grain size of the component manufactured using AM techniques.
It is therefore, an object of the invention to achieve controlled grain growth in components manufactured using additive manufacturing (AM) techniques.
The aforementioned object is achieved by manufacturing a component for a turbomachine according to a method disclosed herein, and by a corresponding apparatus disclosed herein for construction of the component.
In accordance with the invention, the grain size of a component, manufactured using an additive manufactured process, is controlled using a method which comprises constructing a first fused layer of the component by fusing a plurality of layers of a fusible material using a heat source, wherein the first fused layer has a thickness T1. Batch of the plurality of layers of fusible material is in powdered form before applying heat using the heat source. The fusible material is at least one of a metallic powder or a powdered alloy.
In accordance with the invention, stress is introduced through the first fused layer of the component. In an exemplary embodiment, the stress is introduced by deforming the first fused layer of the component. Subsequently, the stress is introduced into all of the plurality of the layers, such as the first fused layer, constituting the component. The process of heating the material layer to consolidate the powdered material and inducing stress components in the layer is repeated until the component is generated.
In accordance with the invention, after the component is generated, by assembling stress induced layers, the component is heated to a temperature above a recrystallization start temperature (Rxst) to control the grain size of the component.
When the component is heat treated at a temperature above the recrystallization start temperature (Rxst), the distorted grain structure of the cold-worked material undergoes recrystallization and grain growth takes place within the stress induced layers of the component. The degree of recrystallization and resultant grain size can be controlled by varying the amount of residual stress stored in the component, the heat treatment temperature, duration of the heat treatment and thickness of the plurality of layers constituting the component.
The advantage of the invention is that, the grain size of the component can be controlled by varying the thickness of the layers that are used to construct the component and the amount of compressive residual stress induced in the layers during the construction the component. The post heat treatment grain size of the component with stress induced layers is significantly larger than a component manufactured using an AM process without inducing strain.
Further, the control of the grain size of the component can be achieved by choosing the right material to construct the component. The grain size can also be controlled by the level of stress induced the layers while constructing the component. Furthermore, the grain size also depends on the time and temperature at which the component is heated for initiating recrystallization and grain growth.
In an embodiment of the invention, the stress is introduced in the component by mechanical deformation of the component. In some embodiments, the grain size is a function of the thickness T1 of the plurality of layers constituting the component.
In accordance with an embodiment of the invention, the fusible material is at least one of a nickel based superalloy and a cobalt based superalloy. The component is designed to be used in turbomachinery where the component is exposed to extreme temperatures. The nickel and cobalt based superalloys are capable of withstanding extreme heat.
In an aspect of the present invention, an apparatus for generating a component having a controllable a grain size using an additive manufacturing process is disclosed. The apparatus includes a construction unit, wherein the construction unit generates a first fused layer of the component by fusing a plurality of layers of a fusible material using a heat source, wherein the first fused layer has a thickness T1.
In a further aspect of the present invention, the apparatus comprises a stress inducing unit, wherein the stress inducing unit introduces stress through the first fused layer. The stress inducing unit is configured to introduce stress into the component which aids in controlling the grain size of the component. Furthermore, the apparatus includes a heat treatment unit for heating the component at a temperature above recrystallization start temperature (Rxst) to control the grain size of the component. The heat treatment unit may vary the temperature in order to modify the grain size of the component.
The figures illustrate in a schematic manner further examples of the embodiments of the invention, in which:
At step 3, stress is introduced through the first fused layer of the component. In the preferred embodiment, the stress is introduced by deforming the first fused layer of the component. In some embodiments, deforming of the first fused layer of the component is performed using techniques such as, ultrasonic peening and laser peening. In the preferred embodiment, the stress introduced is compressive residual stress. The stress components may be introduced uniformly throughout the first fused layer. In some embodiments, the stress components may be introduced along the three dimensional structure of the first fused layer at various degrees. Further, the grain size of the component is a function of the level of stress induced within a plurality of layers constituting the component
At step 5, the steps 2 and 3 are repeated until the component is generated by AM process. The component is generated layer by layer, by fusing the metallic powder and introducing stress in the layer.
At step 7, after the component is generated, the component is heated to a temperature above a recrystallization start temperature (Rxst) to control the grain size of the component. The recrystallization start temperature (Rxst) depends on the fusible material used to construct the component. In some embodiments, the temperature to which the component is heated is varied based on the desired grain size. In the case of gamma prime strengthened nickel based superalloys the recrystallization temperature is above the gamma prime solution temperature.
In the preferred embodiment, the construction unit 12 includes a heat source. The construction unit 12 generates a first fused layer of the component by fusing a plurality of layers of a fusible material 17 using a heat source, wherein the first fused layer has a thickness T1. The plurality of layers of fusible material may be a portion of the layers of fusible material in a powder bed, which is used for constructing the component 10. The powder bed is further explained in
The apparatus 9 includes the stress inducing unit 14 which introduces stress through the first fused layer 18. The stress inducing unit 14 introduces stress in the first fused layer 18 by deforming the first fused layer 18. Similarly, the stress inducing unit 14 introduces stress in all the layers constituting the component. Further, the stress inducing unit 14 may be configured to induce different levels of stress in each of a plurality of layers forming the component 10. The stress inducing unit 14 induces compressive residual stress within the layers based on the desired grain size of the component 10. Further, the grain size is a function of a level of stress induced within the plurality of layers constituting the component.
Further, the construction unit 12 and the stress inducing unit 14 assembles the component 10 layer by layer, wherein each layer is formed by fusing a plurality of layers of fusible material and inducing stress into the fused layer, as illustrated in
Thereafter, the heat treatment unit 16, heats the component to a temperature above recrystallization start temperature (Rxst) to control the grain size of the component. The heat treatment unit 16 also accepts temperature values from a user and accordingly heats the component 10 to that temperature value. In an exemplary embodiment, apparatus 9 is configured to accept a grain size value from a user and sets one or more parameters of the construction unit 12, stress inducing unit 14 and the heat treatment unit 16 to achieve the desired grain size.
In some embodiments, the heat treatment unit 16 accepts a grain size value from the user and sets the temperature value so as the achieve user desired grain size. In some exemplary embodiments, the stress inducing unit 14 and the heat treatment unit 16 are configured to operate based on the grain size value.
Further, the apparatus 28 includes stress inducing unit 14, which is an ultrasonic peening tool which induces compressive residual stress into the layer of the component generated by fusing a plurality of layers of the fusible material on the powder bed 36.
Once the construction of the component 10 is finished, the component is heated by the heat treatment unit 16, wherein the component is heated to a temperature above the recrystallization start temperature (Rxst) to control the grain size of the component.
Though the invention has been described herein with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various examples of the disclosed embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the embodiments of the present invention as defined.
