Patent Application
20080099533
The invention relates generally to solid state welding technology, and more particularly to friction welding.
In recent years, there has been a considerable effort put into designing and building powerful and efficient turbo-machinery such as gas turbine engines. The design involves use of materials having properties such as enhanced high temperature performance and strength, or advantageous strength-to-weight ratios. However, an increased susceptibility to cracking and other defect generation, including unacceptable property degradation, was observed in such materials when joined by conventional welding technology.
Solid state welding or joining processes have been developed as a way of addressing these issues. One of the more successfully employed techniques is friction stir welding. Friction stir welding is regularly used to join metals and metal alloys. The friction stir welding technique overcomes a number of problems associated with other more conventional joining techniques. In a typical friction stir welding process, a rotating, often cylindrical, non-consumable tool such as a pin tool is plunged into a rigidly clamped workpiece at a location containing a joint to be welded. The rotating tool can be traversed along the joint to be welded, held in place as the workpiece is fed past the tool, or any combination of the two. As the weld progresses, the workpiece material within the joint vicinity becomes a plasticized (non-liquid) metal, metal alloy or other material, and workpiece material from all components of the joint transfers across a joint interface co-mingling to form a strong cohesive bond between all workpiece components through a localized solid-state forging and/or extrusion action.
During the friction stir welding process, elevated temperatures are generated in the tool. The high temperatures in the tool, in combination with relatively high pre weld workpiece heating rates and high post-weld workpiece cooling rates, may result in a weld joint of poor quality, such as poor mechanical strength and toughness often but not always attributable to defects, undesirable material structure, and workpiece distortions.
Therefore, a need exists for an improved welding or a joining system that would address problems set forth above.
In accordance with an embodiment of the invention, a method for creating a solid state joint is provided. The method includes providing an adjoining apparatus. The adjoining apparatus includes a tool, a backing plate and a thermal control plate disposed below the backing plate. The method also includes rotating the tool and traversing the tool along a joint to be welded on a stationary workpiece. Alternatively, the workpiece can be fed past a stationary rotating pin tool. Additionally, the rotating tool and workpiece can be mobile. The method further includes manipulating the temperature of the tool and the backing plate in order to control the temperature and rate of change of temperature experienced by the workpiece, and to enable pre-weld, post-weld, and in-situ control over the thermal profile at a weld affected zone at the joint. The method also includes maintaining a user chosen temperature differential between the weld affected zone and the backing plate via the thermal control plate.
In accordance with another embodiment of the invention, a method of operation is provided. The method includes monitoring temperature of a weld-affected zone. The method also includes applying a temperature control via a thermal control plate based upon the temperature that is monitored. The method further includes maintaining the temperature to about 50 to about 80 percent of melting temperature of the workpiece.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
As discussed in detail below, embodiments of the present invention provide a method for controlling microstructure and hence improving properties of a material during a solid state joining technique via thermal management of the material through controlled use of the welding apparatus. Some non-limiting examples of the properties of a material in solid state joints include yield strength, ultimate tensile strength, ductility, impact toughness, fracture toughness, fatigue crack growth resistance, low cycle fatigue resistance, high cycle fatigue resistance, and superplastic formability. In an example, the solid state joining includes a friction stir welding technique. The friction stir welding technique may be used to join one or more similar or dissimilar materials forming a workpiece. Some non-limiting examples of materials include metals, metal alloys, and thermoplastics. The term ‘controlling microstructure’ used herein refers to non-limiting examples such as controlling grain size, phase content, phase morphology and phase spatial distribution, and avoiding harmful phase transitions in materials at solid state joints.
The pin tool 16 may be rotated at varying speeds depending upon the materials 20 and 22 to be welded. In a specific embodiment, the pin tool 16 may be rotated at speeds between about 50 rpm and about 2000 rpm. The rotating speeds of the pin tool 16 are also dependent upon thickness of the workpiece 18 to be friction stir welded. Typically, higher speeds are used with thinner sections and lower rotational speeds are used with thicker sections. The pin tool 16 may partially protrude out of a tool holder 26. The tool holder 26 includes a shoulder 28 and an annular spindle 30. In a particular embodiment as shown in
The spindle 30 may also have an inside diameter slightly larger than the diameter of the pin tool 16 in order to prevent any restriction. The length of the spindle 30 may be long enough in order to allow a sufficient length of pin tool 16 to be provided so as to produce a continuous weld. The spindle 30 may also include one or more channels 32 to provide a flow for a temperature controlling media. In a particular embodiment as shown in
The pin tool 16 is plunged into the workpiece 18 and traversed along the joint 24 to be welded. The pin tool 16 provides a combination of frictional heat and thermo-mechanical working in order to accomplish a weld. As the pin tool 16 is traversed along the joint 24 to be welded, the joint vicinity becomes plasticized (non-liquid) and workpiece material from all components of the joint transfers across the joint interface 24, co-mingling to form a strong cohesive bond between all workpiece components through a localized solid-state forging and/or extrusion action.
The thermal management system 14 includes a backing plate 36 and a thermal control plate 38. The backing plate 36 forms a welding table on which the workpiece 18 is disposed. In an example, the backing plate 36 may include a steel plate. In a particular embodiment, a hard metal backing sheet 40 may also be disposed between the workpiece 18 and the backing plate 36. Some non-limiting examples of the hard metal backing sheet 40 include a sheet made of a tungsten alloy or a molybdenum alloy. The thermal control plate 38 disposed below the backing plate 36 provides cooling or heating to the workpiece 18 before, during, and/or after the weld, in order to control the imposed thermal profile, and hence microstructure of the workpiece 18 in a weld affected zone 42. The term ‘weld affected zone’ used herein refers to area within and around the joint 24 of the weld wherein microstructural properties of the workpiece 18 may be affected. During the welding process, the materials 20 and 22 being bonded may undergo transformations in microstructural properties such as grain size and grain orientation, phase morphology, phase content, and phase distribution. The thermal control plate 38 provides a method of thermal management to enable control over such microstructural properties.
In an illustrated embodiment of the invention as shown in
A thermal control plate 38 as referenced in
In another illustrated embodiment of the invention as shown in
In the aforementioned thermal management system, temperature of the weld affected zone may be controlled as per a characteristic cooling curve in a material-specific CCT diagram, for instance, in order to achieve a desired microstructure. In general, the instantaneous temperature very near the pin tool is substantially different than that away from the pin tool. Consequently, a portion of the workpiece very near the pin tool may be at a substantially different position in time-temperature space along the most desirable cooling curve than a portion away from the pin tool. In order to actively control the microstructure in such cases, it may be necessary to impose various thermal gradients across the backing anvil. Such a requirement may be addressed by enabling segmented thermal control along a length of the backing plate and separately controlling temperature in each of the segments.
In a particular embodiment, the temperature of the workpiece is manipulated before the pin tool is brought in contact with the joint. In another embodiment, the temperature of the workpiece is manipulated when the pin tool is in contact with the joint. In yet another embodiment, the temperature of the workpiece is manipulated after the pin tool has been in contact with the joint. In an example, the peak welding temperature may be limited below the beta-transus temperature of an alpha-beta titanium alloy, in order to prevent grain growth in the weld affected zone. In another example, the peak welding temperature may be limited below the austenitization temperature in steels, in order to avoid formation of a brittle martensite upon cooling. In yet another example, the post-weld cooling rate may be controlled to avoid the formation of deleterious phases within and around the weld affected zone. Further, controlling the temperature may include monitoring and controlling cooling rate of a temperature control media passed through the thermal control plate in accordance with a desirable cooling curve. In another example, controlling the temperature may include monitoring and controlling temperature of the temperature control media. In yet another example, controlling the temperature may also be provided by multiple strip heaters or multiple resistive heaters. The method 110 also includes maintaining a user chosen temperature differential between the weld affected zone and the backing plate via the thermal control plate in step 118. This helps in controlling any microstructural changes in the workpiece. Some non-limiting examples of controlling the microstructural changes may include controlling phase distribution and phase morphology, avoiding harmful phase transitions and controlling grain size of the material in the workpiece.
The various embodiments of a method for controlling microstructure via thermal management described above thus facilitate a way to improve or preserve material properties such as yield strength, tensile strength, ductility, impact toughness, fracture toughness, fatigue crack growth resistance, low cycle fatigue resistance, high cycle fatigue resistance, and superplastic formability of a friction weld and surrounding regions. This method also allows for improved in-situ control of structure and properties in a weld.
Of course, it is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the techniques described herein may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
