Common control parameters of friction and stir welding are tool rotation speed, tool immersion depth, and tool travel speed relative to the material. The thesis will then discuss the tool properties and how they can affect the thermomechanical conditions of the weld in the context of the considered models and weld parameter maps.
Rosenthal Solutions
Two main questions dominate modeling efforts today: Is the peak temperature dominated by heat generation from friction or plastic dissipation and is. The dominant question in thermal models is whether the peak temperature is dominated by heat generated from friction or plastic dissipation.
Schmidt and Hattel Heat Generation Model
The material closest to the tool moves at the same speed as the tool, creating a boundary layer of It is defined as the material velocity closest to the pin divided by the tool velocity, Equation 3.
Mendez Heat Generation Model
Colligan Material Flow Experiments
Reynolds Material Flow Experiments
Arbegast Extrusion Model
Ulysse Material Flow Model
Nunes Rotating Plug Model
Pei and Dong Adiabatic Shear Band Model
At these low strain rates, the influence of material temperature on yield stress can be neglected. The material speed near the pin quickly approaches the tool speed causing high stress and strain rates.
Crawford Couette Flow Model
The localized generation of plastic heat continues to plasticize the material and the shear bandwidth of the material traveling at the speed of the pin increases. The first term of equation 8 represents the torque from the shoulder, and the last two terms represent the contribution of the pin rotation.
Nandan Critical Viscosity Model
At higher welding distances, the Couette current becomes more predictive than the visco-plastic model. High temperatures increase the material's ability to flow. Although strain rate contributes to material flow for both models, adiabatic shear bands suggest that strain rate dominates the material flow process and causes a change in the dislocation mechanism and is affected by strain hardening.
Johnson-Cook Model
22] The critical viscosity model suggests more of a balance between strain rate and temperature that is thermally activated by the heat generated by plastic dissipation and not affected by strain hardening. Small shear bands may be created, but they are small in scale and do not dominate the material flow.
Zener-Hollomon Parameter and Hyperbolic-Sine Model
Onion Rings
This thickness is divided by the number of faces on the pin for non-cylindrical tools (three for triangular tools, for example). Onion ring structures start on the receding side and compete with tunnel defects originating from the advancing side of the weld.
Dynamic Recrystallized Zone Shape
Onion rings are dominated by the shoulder and only occur when there is transverse movement. Onion rings are likely formed when excess plastic material escapes the consolidation and is wiped off by the shoulder behind the weld.
Precipitate Depositing Effects
It is more likely that a temperature increase due to an increase in heat generation at the shoulder will reduce the viscosity of the material, allowing it to flow into the weld zone. At high transverse velocities, the shoulder heat does not have time to conduct in the z direction, resulting in an ellipsoid.
Modelling Summary
The key concepts behind the model are that it assumes a stuck condition and that shoulder heat generation can be viewed as a preheating mechanism for the weld, and that the pin generates heat from plastic dissipation that dominates the temperature near the tool. The strength of the weld is highly dependent on the ability of the weld to deposit material in the cavity behind the pin. The material is heated by friction until it flows, and enough plasticized material flows to fill the void behind the pin.
Velocity Field Calculation
If the torque is not known, this thesis assumes that the temperature self-regulates around the solidus temperature. Some reported peak temperatures are reportedly at or above 98% of the solidus temperature and support this hypothesis. Higher strength welds are assumed to maintain a temperature below but near the solidus temperature.
Strain Rate Tensor
29] Equation 14 describes the velocity field used in this thesis in the radial and transverse directions respectively.
Critical Viscosity
Flow Band Width
Current Weld Parameter Metric
31] Rolling on the tool shoulders can increase the temperature near the pin so that the width of the shear band increases, allowing increased plastic flow at lower rotational speeds and the material remaining plastic long enough to be consolidated into the cavity behind the pin. Threaded pin also causes material flow in the Z direction, which increases the stress on the material and can also increase the width of the shear band. Below this limit, the contact pressure between the pin and the material is too low.
Proposed Weld Parameter Metric
There are strong relationships within the same shear band cluster between energy deposition and strength. So far we have discussed optimal welding parameters and an upper limit on energy deposition for a given flow bandwidth. Flow with an energy deposition of 2231 J/mm and a yield bandwidth of 5 mm, the predicted strength is lower than 135 MPa.
Shoulder
Pin
At the same time, the pin transports the material around the tool and deposits it into the cavity. 33] This material is extruded on the retreating side between the pin and the parent material. The Whorl pin has a spiral edge on the pin surface, like an auger, that directs the material flow downward.
Material
Shoulder Scrolls
Pin Threads
The filaments can help reduce energy deposition for a variety of welding parameters by transporting hot material near the heat generation caused by the shoulder down into a cooler material. It has also been suggested that increasing material flow from the yarns can increase the strain rate and flow bandwidth.
Triangular Pin
Machine Setup
Anvil Topo
The maximum deviation of the vehicle was found to be about 0.5 degrees with a dominant frequency of 24.78 Hz. Next, the topography of the anvil and the specimen clamped on the anvil were mapped by lowering the tool until an axial force was detected by the dynamometer. Elevation was recorded at various locations only on the anvil and with specimens until a topography could be interpolated.
Specimen Topo
An image processing program was written to identify the tool from background noise and then track the location of the top and bottom of the tool. There is a distinct low spot on our anvil that translates to the sample when clamped to the anvil. Although this difference may begin to affect welds, it should only affect welds at the highest point, or the extreme end of the weld, and then only slightly.
Tools
It was estimated from previous experimental observations that a height change of about 0.005 inch could affect weld quality. The difference in height of the sample from the lowest point to the highest point is 0.0051 inch. Excessive flash in previous welds started to become a problem about halfway through the weld, or at about 20 inches across in the figure.
Weld Data
Optical Testing Methods
Since the diameter of the tool is known, the image can be processed to determine the width of the shoulder in contact with the material. If the camera can be reliably muted, this could be a way to check the shoulder engagement without knowledge of the system. By using a reference length in photos of the dynamic recrystallized zone, pixels were related to distances and precise widths of flow bands could be measured.
Tension Testing Methods
This is a potentially easy method to check for changes in weld topography or thermal expansion of the material, but vibrations made calculations difficult. This was particularly useful when the recrystallized zone was difficult to see, but post-processing the image could increase the contrast between the recrystallized zone, heat-affected zone, and parent material.
Flow Band Model Verification
The resulting current bandwidth model, depending on the critical viscosity, is strongly dependent on the energy deposition. There are not enough data points to make hard observations, but it appears that higher rotational speeds will ultimately result in less increase in flow bandwidth, but reducing traverse speed will linearly increase flow bandwidth. The relationship for both rotational speed and transverse speed with current bandwidth is described in Figure 19 and Figure 20.
Axial Force
The TPS-t tool has axial force results on the order of half the values for other tools, ranging from as low as 4000 N to 12,000 N. The increase in axial force over most individual welds is about 2000 N, but some of this can also be attributed to rising anvil topography. It is unknown, but interesting, why an absence of a pin would increase the axial force.
Transverse Force
During the second welding, of approx. 18 inch location, the shear force data becomes more erratic, signaling the initial weakening of the joint between the pin and the shoulder. It appears that an internal fracture occurred in the pin during the second weld, but the pin did not actually break off until the third weld. When the transverse forces at the test locations are examined, a similar grouping of tool values is found as was noted in axial forces at the test locations.
Torque
The peak torque would be the torque applied to achieve the material flow stress. The third weld demonstrates torque values similar to the first weld up to the 19-inch point when the pin visibly breaks and the magnitude of the swing torque and peak torque values are greatly reduced to negligible values. The triangular tool exhibits the widest range of torque values from 0 to 12 N-m, but only due to pin breakage.
Yield Strength
The TP 9545 tool resulted in the highest average strength, the TPS 9545 tool showed the largest standard deviation, and the TPS 9545_t tool showed the lowest standard deviation. This trend was stronger for the TPS 9545 and TPS 9545_t tools, while the TP 9545 on the right was stronger than average. The strongest location for all tools except TPS 9545 was in the center, but the strongest location for all tools was the left side.
Tool Features Effect on Transverse Force
Turbulence can result from vortex shedding or the outer layer of recirculated material interacting with stagnant parent material. In contrast, a triangular pin separates plasticized material into orderly packets with smaller vortices and smaller pressure fluctuations in their wake. This lack of turbulence can lead to lower standard deviation in shear forces for triangular pin tools.
Tool Features Effect on Axial Force
As with shear forces, this can be a result of laminar flow, because vortices do not form as with the circular needle. This explains why both the magnitude and the standard deviation of the axial forces are much lower.
Tool Features Effect on Torque
Tool Features Effect on Yield Strength
Overall weld strengths are much lower than a traditional metric of weld parameters would predict. This lower strength may be a result of the material acting more like a fluid under such high heat input conditions. Welding parameters should optimize heat input to minimize this high temperature region of the weld zone.
Tool Features Effect on Dynamic Recrystallized Zone Shape
Since DXZ is not the weakest part of the weld, a smaller DXZ cross-sectional area is associated with lower weld strength. The DXZ of sample TP 9545 is almost vertical and recedes near the top of the weld near the shoulder. The angle changes to be more vertical in about the top quarter of the weld, resulting in more cross-sectional area for DXZ.
Tool Features Effect on Flow Band Width
It is assumed that the lack of rollers actually benefited the TP 9545 tool by not drawing excess, overheated material to consolidate in the weld or be flashed out.
Mechanical Conditions and Weld Parameter Map
Thermal Expansion Controller
It was previously believed that the heat from welding built up at the end of the material and that the temperature caused too much material to plasticize, resulting in flash. In addition, however, more material is gripped by the tool as the material expands axially. The combination of too much energy deposited in the weld and more material than the welding parameters were designed for is likely the cause of excess flash at the end of welds.
Database
I would not consider this model sufficient to prove the effectiveness of the Nandan Critical Viscosity model. Scaling of coupled heat transfer and plastic deformation around the pin in friction stir welding. Periodic variation of torque and its relation to interface sticking and sliding during friction stir welding.
