Claims:We claim:

1. A apparatus for in-situ stress relieving process comprising:
a pneumatic hammer (1001) comprising of hammer pin (100) fitted into one end of a hammer body (700) and a hammer plug or cap (600) fitted into second end of hammer body (700); and
a pneumatic fixture (1002) comprising of at least two guide rods (800), a piston cylinder arrangement (900), a housing (1400) for said guide rods (800) and piston cylinder (900), an end fixtures (1000) connected to said guide rods (800), a base plate (1100), a clamp (1500), an offset plate (1200) to offset the pneumatic hammer (1001) from the centre of piston cylinder (900) and a mounting block (1300), wherein said pneumatic hammer (1001) is attached with a CNC machine and follow the toolpath of metal deposition controlled by software.

2. The apparatus as claimed in claim 1, wherein said hammer body (700) further comprises a central body (200), an inner cylinder (400), an outer cylinder (500) and a piston (300).

3. The apparatus as claimed in claim 1, wherein said piston (300) is fitted inside the bore of said inner cylinder (400).

4. The apparatus as claimed in claim 1, wherein said hammer body (700) is mounted inside said mounting block (1300).

5. The apparatus as claimed in claim1, wherein said hammer plug or cap (600) comprises a spring valve, wherein the spring valve is pressed in a ring type arrangement.

6. The apparatus as claimed in claim1, wherein said inner cylinder (400) and the said piston (300) further comprises a plurality of ports (401) and (301) for regulating the air supply.

7. The apparatus as claimed in claim 1, wherein said inner cylinder (400) comprises a circular slot (402) and tolerance located below the plurality of ports to allow the air pass.

8. The apparatus of claim 1, wherein said base plate (1100) is fixed with head of CNC machine.

9. The apparatus of claim 1, wherein the housing (1400) is clamped at lower end of said base plate (1100).

10. The apparatus of claim 1, wherein the housing (1400) has three vertical grove for allowing said two guide rods (800) and piston cylinder to move vertically up and down.

11. The apparatus of claim 1, wherein the end fixtures (1000) is attached to upper end of said guide rods (800).

12. The apparatus of claim 1, wherein the clamp (1500) is attached to upper end of said base plate (1100) and aligned with said end fixtures (1000) and said housing (1400).

13. The apparatus of claim 1, wherein one end of the piston cylinder (900) is connected to said clamp (1500) and second end of the piston cylinder (900) is fixed in the housing (1400).

14. The apparatus of claim 1, wherein horizontal upper face of said offset plate (1200) is connected with lower end of said two guide rods (800) and said piston cylinder (900) further vertical face of said offset plate is connect with said mounting block (1300).

, Description:FIELD OF THE INVENTION
The present disclosure relates generally to an apparatus for in-situ property improvement of metallic objects more specifically to a Pneumatic hammer for relieving stress during Additive Manufacturing (AM) processes.

BACKGROUND OF THE INVENTION

The metallic objects manufactured by Additive Manufacturing (AM) processes exhibit inherent porosities and residual stresses. These inherent porosities are because of the incomplete fusion between deposited layers which leads to an anisotropic behavior of the objects. Residual stresses are developed by the expansion and contraction of the deposited material. These stresses influence the fatigue life, dimensional stability, corrosion resistance, life of component and brittle fracture.
The post processing of the objects is one of the popular property improvement methods found in the existing prior art. Hot Isostatic Pressing (HIP) is one of the processes found to be very useful for such application. But this process is very expensive and time consuming and also demands a huge space for its installation. Post heat treatment can also be used to eliminate the residual stress from the components. The process has drawbacks such as ineffective for localized effect, possibility of dramatic shape change and special attention required to protect machine parts on which it is retrofitted. The requirement is of an in-situ property improvement process which can be used after deposition of each layer without disturbing the other capabilities of the existing system. However, both the processes mentioned above cannot be used as an in-situ technique. There are some other cold working processes namely laser peening, ultrasonic peening, shot peening, vibratory and magnetic stress relieving and cold rolling. But these processes are complicated to implement and require a change in the entire setup to perform the operation.
In view of the foregoing, there is a need for an apparatus for in-situ stress relieving process for deposition based AM processes such as Hybrid Layered Manufacturing (HLM), Laser Engineering Net Shaping (LENS), Micro Plasma Powder Deposition and Wire + Arc Additive Manufacturing (WAAM).

OBJECT OF THE INVENTION

1. It is the primary object of the present disclosure to provide a pneumatic hammer for in-situ stress relieving process.
2. It is another object of the present disclosure to provide an in-situ process to eliminate/reduce the residual stress and anisotropy nature found in the Additive Manufactured or deposited objects.
3. It is yet another object of the present disclosure to provide a cost-effective in-situ process for deposition based AM processes

SUMMARY OF THE INVENTION

In one aspect of the present disclosure a pneumatic hammer for in-situ stress relieving process in HLM (Hybrid layered Manufacturing) is provided. The hammer pin has been changed and a retractable fixture has been developed to retrofit the hammer unit on CNC machine. The hammer (1001) comprises hammer pin (100) fitted into one end of a hammer body (700) and a hammer plug or cap (600) also fitted into second end of hammer body (700). The hammer body (700) further comprises a central body (200), an inner cylinder (400), an outer cylinder (500) and a piston (300). The piston (300) is fitted inside the bore of inner cylinder (400). The piston (300) is free to move up and down impacting the hammer pin (100) at the bottom of its stroke. A retractable fixture (1002) has been developed to retrofit the pneumatic hammer (1001) on CNC machine. The compressed air requires to operate piston (300) is supplied to the hammer through a connection at the top of hammer plug (600).The hammer has a spring valve in the cap (600) which is pressed with a ring type arrangement. The inner cylinder (400) and piston (300) has ports which regulates the air supply either to the chamber above the piston or the below the piston. The pressure on both the sides of the piston (300) changes continuously as the piston moves up and down.

Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1(a) shows an apparatus for a pneumatic hammer in accordance with an aspect of present disclosure.

Figure 1(b) shows a retractable pneumatic fixture in accordance with an aspect of present disclosure.

Figure 2(a)-2(e) show the working principle of pneumatic hammer in accordance with an aspect of present disclosure.

Figure 3 pneumatic hammer (1001) in position on the CNC machine in accordance with an aspect of present disclosure.

Figure 4a shows the position of pneumatic hammer during face milling or deposition in accordance with an aspect of present disclosure.

Figure 4b shows the position of pneumatic hammer during hammering process in accordance with an aspect of present disclosure.

Figure 5a shows the toolpath of hammering in accordance with an aspect of present disclosure.

Figure 5b shows an example of the raster toolpath in accordance with an aspect of present disclosure.

Figure 6 shows the average value of residual stresses in accordance with an aspect of present disclosure.

Figure 7 shows the results of the densities and percentage of porosity test in accordance with an aspect of present disclosure.

DETAILED DESCRIPTION OF THE INVENTION
The present disclosure describes an apparatus for in-situ stress relieving process and a method thereof. The following description with reference to accompanying drawings is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements.
Figure 1(a) shows a pneumatic hammer (1001) for in-situ stress relieving process in HLM (Hybrid layered Manufacturing). The pneumatic hammer along with welding guns for the stress relieving process are retrofitted on the tool post without interfering with any other functionality of the 3-axis CNC machine. The hammer (1001) comprises hammer pin (100) fitted into one end of a hammer body (700) and a hammer plug or cap (600) also fitted into second end of hammer body (700). The hammer body further comprises a central body (200), an inner cylinder (400), an outer cylinder (500) and a piston (300). The piston (300) is fitted inside the bore of inner cylinder (400). The piston (300) is free to move up and down impacting the hammer pin (100) at the bottom of its stroke. Figure 1(b) shows a retractable pneumatic fixture (1002) that has been developed to retrofit the pneumatic hammer (1001) on CNC machine. The pneumatic fixture (1002) has been described in detail in figure 4.The compressed air requires to operate piston (300) is supplied to the hammer through a connection at the top of hammer plug (600). The hammer has a spring valve in the cap (600) which is pressed with a ring type arrangement. The inner cylinder (400) and piston (300) has ports which regulates the air supply either to the chamber above the piston or the below the piston. The pressure on both the sides of the piston (300) changes continuously as the piston moves up and down.

Figure 2(a)-2(e) show the working principle of pneumatic hammer (1001) for one hammering cycle. The one hammering cycle consist of impact between the hammer pin (100) and piston (300), opening and closing of the cylinder port (701) and piston port (301) due to the moving piston. Figure 2a show the initial position of pneumatic hammer, when the valve (601) placed in cap is remaining close until the push button will be pressed by a ring type arrangement. When the valve is closed, no air circulation take placed in hammer body (700) and the piston (300) remain at the lower position. When the valve (601) is opened, the air passes from the gap between the inner cylinder (400) and outer cylinder (500) and enters into inner cylinder (400) from the inner cylinder ports (401) as shown in Figure 2 (b). Now the inner cylinder (400) contains a small circular slot (402) and tolerance below the ports to allow the air pass. When the air fills the slot, it generates a force on the ‘bottom’ side of the piston (300) as shown in Figure 2 (b). The generated force accelerates the piston (300) in the upward direction as shown in Figure 2 (c). The piston (300) accelerates in the upward direction until the piston port opens in the inner cylinder (400) as shown in Figure 2 (d). As the piston port opens, air travels the path from piston to top of the piston in inner cylinder (400). The air fills the cylinder and generates pressure on the top of the piston, wherein the pressure tries to force the piston in the downward direction as shown in Figure 2 (d). The downward arrow shows air flow and upward arrow shows force on piston. The piston slides downward with force due to the pressurized air until the piston port opens in the bottom side of the cylinder where the pressure is at atmospheric level. Therefore the air exhaust from the cylinder as shown in Figure 2 (e). At the same moment the impact between the piston (300) and hammer pin (100) takes place. Due to impact the piston rebounds and starts moving upwards and the cycle continues.
Figure 3 shows the pneumatic hammer (1001) in position on the CNC machine with milling cutter and deposition torch. While using this pneumatic hammer for HLM process, the deposition, face milling and stress relieving are three consecutive operations performed for each layer. The starting point of the process is a CAD model of the desired object. A near-net shape is manufactured. The near-net shape obtained from HLM is subsequently machine finished. The machining tool-path is generated using commercial CAM software systems. The toolpaths for deposition are obtained by in-house developed software. Thus HLM involves interlinked software and hardware components. The hammering unit should be retracted in upward direction during face milling or deposition to avoid any collision between the hammer and the work piece. The pneumatic controlled piston-cylinder fixture (1002) has been developed for this purpose. Figure 4 (a) shows the position of pneumatic hammer during face milling or deposition and figure 4 (b) shows the position of pneumatic hammer during hammering process. The pneumatic fixture (1002) extends the hammer (1001) during hammering process and retracts the hammer (1001) during deposition/milling process. The pneumatic fixture (1002) comprises at least two guide rods (800) to constrain the rotation and guiding the vertical motion, a piston cylinder arrangement (900) of 300 mm stroke to retract and extend the hammer unit (1001), a housing (1400) for said guide rods (800) and piston cylinder (900), an end fixtures (1000) attached to upper end of said guide rods (800), a base plate (1100) to hold the entire fixture (1002) on the head of CNC machine, a clamp (1500) ) is attached to upper end of said base plate (1100) and aligned with said end fixtures (1000) and said housing (1400), an offset plate (1200) to offset the hammer from the center of the piston cylinder (900) and a mounting block (1300) to hold the pneumatic hammer (1001). The horizontal upper face of said offset plate (1200) is connected with lower end of said two guide rods (800) and said piston cylinder (900), further vertical face of said offset plate is connected with said mounting block (1300). The housing (1400) is clamped at lower end of said base plate (1100) having three vertical grove for allowing said two guide rods (800) and piston cylinder (900) to move vertically up and down. The end fixtures (1000) is attached to upper end of said guide rods (800). The end fixture (1000) moves vertically along said guide rod (800) as said piston cylinder (900) retracts and extends. One end of the piston cylinder (900) is connected to said clamp (1500) and the second end is fixed in the housing (1400).
Process Parameters
There are various factors used to control the hammering process. By controlling these parameters controlled hammering process can be carried out.
a. Air pressure
Air pressure in the hammer can lead to change in the force / impact velocity. Hence it is very important to find out the optimal value of pressure in the hammer for different materials. The value of the air-pressure can vary from 0-7 bar in the present setup.
b. Standoff distance
This is the distance between the hammer pin (100) and the workpiece. When the pin just touches the workpiece the z-value can be set zero at this location. Until the hammer pin (100) will not come with inner piston (200) the vibration will not start. The maximum distance by which the hammer can go down is -20 mm. While doing experiments, the hammer goes down by a distance of -19 mm and then it can come up to -13 mm. It has been observed that if the hammer goes up more than -13 mm the vibration of hammer is not stable. Hence the distance between -13 to-19 is called standoff distance of the hammer. Appropriate value of standoff distance can be given through the CNC program.
c. Hammer speed
The hammer speed can affect the percentage of the surface intended or overlapping between two consecutive hammer marks in the same direction. Hammer speed is set to CNC machine through program or manually.
d. Pitch
The distance between two consecutive passes of toolpath is called pitch. It can be determined by the diameter of the indentation mark of hammering on the layer. The pitch is required as an input parameter while writing the CNC program of hammering.
Figure 5 (a) shows the toolpath of hammering on a deposited layer after face milling. The toolpath for hammering over a layer can be similar to as an area filling toolpath for deposition. This flexibility is one of the major advantages of residual stress relieving process in present disclosure. Zig-zag (Raster) toolpaths are found to be suitable for this application as there is a consistent pitch between two consecutive passes. Figure 5 (b) shows an example of the raster toolpath for hammering. From the CNC program, it can be observe that the value of standoff distance is 15mm and the pitch is 2mm.
Results
The effect of pneumatic hammering on the object manufactured through Hybrid Layered Manufacturing process has been evaluated by comparing:
1) Variations in residual stress
2) Variation in microstructure

Variations in residual stress
The surface and sub-surface compressive residual stress was measured using x-ray diffraction techniques. The sample manufactured through HLM process with and without hammering of layers. The sub-surface residual stress testing was done at 10 mm distance from the top layer. The residual stress were recorded at various location on same layer to account the fluctuation in residual stresses in the same layer. Figure 6 represent the average value of residual stresses. The observed results implies that surface residual stress before hammering are tensile in nature and thus leading to surface cracks. However the hammering process cause the complete reversal of residual stress to compressive. This residual stresses helps in arresting cracks. Similarly stress pattern was observed for sub-surface residual stresses.
Variation in microstructure
Microstructure is evaluated through density and porosity variation. The change in crystallographic packing varies the material density. Density influences the mechanical strength, hardness, electrical conductivity, magnetic and gas permeability. Density variation depends on the hammer speed, air pressure and step-over. The density of deposited object is compared against the density of solid block of same material (2.69 gm/cm3). During density measurement of the objects, open and closed pores were taken into consideration. The open pores refers to surface pores and sub-surface pores that connected to surface. The closed pores may refer to sub-surface pores that do not connect to the surface. The objects was hammered in two ways. One way hammering was done along the deposition direction and other way in a direction perpendicular to deposition direction (step-over). Figure 7 shows the results of the densities and percentage of porosity test. It was observed that the density of the sample before hammering was lower than density of the solid block material. The density of hammered sample was almost equal to the original density of the material. The sample which hammered along the deposition path experiences lower percentage of open porosities than that of sample hammered across the deposition path. The porosity and density is directly correlated to fatigue strength. Hence reduction in porosity and increase in density of sample with respect to non-hammered sample will contribute for improvement in fatigue strength.
The above description along with the accompanying drawings is intended to describe the preferred embodiments of the invention in sufficient detail to enable those skilled in the art to practice the invention. The above description is intended to be illustrative and should not be interpreted as limiting the scope of the invention. Those skilled in the art to which the invention relates will appreciate that many variations of the described example implementations and other implementations exist within the scope of the claimed invention.