Temperature Distribution during Single Pass Multi-Layer Welding in Additive Manufacturing

Thermal Modeling of Weld Deposition Process

Effects of Residual Stresses on the Weld Bead

Boundary Conditions

The model is assumed to be a thin structure with heat transfer in the y-direction only, in which the weld is deposited layer by layer to produce the component. Heat transfer in the x and z axes is assumed to be negligible compared to the transfer in the y axis. A thin-walled structure refers to a geometry whose properties in one dimension differ from those in the other two axes by a large amount.

T is the temperature in Kelvin, ξ is the fixed distance from the source, k is the thermal conductivity, C is the specific heat.

Process Parameters

Simulation

Condition 1 : Fixed Base Plate Temperature

The model deals with the uniform heat transfer from the side walls of the single welded part by convection through air. The bottom of the part was kept at a constant low temperature, which facilitates heat dissipation. The temperature applied to the weld bead on the upper layer is equal to the melting temperature of steel, while the lower layer is equal to the ambient temperature, i.e.

The temperature distribution result obtained as shown in figure 3.3 describes the heat flow in the component body in the direction of the deposited layer. It was observed that the heat flowing in the width direction of the component is negligible compared to the deposition direction. A graph as shown in Figure 3.4 gives a distribution between the temperature of the deposition layers and the time required to cool.

Since the direction of heat flow is mostly unidirectional only, Figure 3.5 shows the heat transfer distribution pattern in the model. It emphasizes heat transfer in the y axis along with transfer in the direction of weld deposition.

Figure 3.3: Temperature distribution in the direction of layer deposition

Condition 2 : Isolated Base Plate

The applied boundary condition is the same as that in Model 1, except for the bottom layer. Because the bottom layer is insulated, heat is retained in the body, which slows the cooling process of the weld. The graph obtained for the average cooling time of the weld bead over the distance traveled by the heat source is as shown in Figure 3.8.

It shows a trend of the graph where a certain increase in the temperature of the component is seen due to the effect of insulated bottom before the component cools down towards reaching an equilibrium with the ambient temperature.

Figure 3.7: The heat source moving in the direction of the weld deposition

Results and Discussion

Condition 1 : Fixed Base Plate Temperature

The graph below in Fig 3.10 depicts the temperature drop of a heated element in relation to the distance traveled by the weld bead. This graph is of the element in the layer immediately below the one on which the weld is being made. Heat flow through conduction is experienced by the layer above it, as well as those below it that have not yet cooled.

Since there is only heat conduction from the successive layers below the element, in addition to the heat transferred through the weld, the following graph is obtained for the cooling of the element. The graph as shown below in Figure 3.11 describes a disturbed temperature graph due to the constant heat addition by the elements that surround this element on all sides. The elements transfer heat from the top and bottom layers to the element being analyzed.

At the same time, heat is also taken away from the element through the process of convection using air as well as conduction to the elements at a lower temperature than the element in question. A very abrupt change in temperature (as shown in Figure 3.12) was observed in the element which is in direct contact with the base layer, which keeps it at a constant low temperature. Although heat input occurs, the heat flux crossing the element drawing heat from the body is much higher than the heat flux supplied to the element per unit of time.

Figure 3.9: Temp( 0 C) vs weld deposited(mm) plot for topmost element

Condition 2 : Isolated Base Plate

The graph below in Fig 3.14 describes the temperature drop of a heated element in relation to the distance traveled by the weld bead. Since there is heat conduction, not only from the successive layers below the element, but also from the heat accumulated due to the isolated base state of the model, a gradual drop in temperature is seen compared to the rapid drop in state 1. The graph as shown below in Figure 3.15 describes a temperature graph showing a drop in temperature for a certain period of time, but due to the constant addition of heat by the elements surrounding this element from all sides and due to insulated base, the temperature of the element increases again.

The graph in Figure 3.16 below shows the drop in temperature of the element at the bottom lowest layer of the component. The weld bead is a function of the. a) amount of current used to produce the arc, (b) material of the wire used,. This was done to control the variation in properties between layers deposited and to control the surface modulations for a better finished product.

The temperature measured during cooling of the weld bead is done at the center of the thin. The experimental analysis shows that the cooling methods affect the geometry of the bead leading to modulations of the surface. A set of values ​​was obtained for the cooling time of the layers with respect to the deposition of the weld bead was obtained for Condition 1 with the base maintained at constant low temperature and for Condition 2 with an insulated base.

A graph as shown in Figure 4.4 shows a trend of the cooling of the welded layers under different conditions. But the time it took for the insulated base to cool was much longer than that of the model with a constant low temperature base. This was because the temperature in the inner body of the workpiece with insulated base was constantly high due to the continuous addition of heat by the weld deposit.

The temperature data obtained for measuring the temperature of the midpoint of the layers through the thermocouple in each layer was much higher compared to the same point of constant low temperature base measured after the same time period. From the experimental analysis, it was found that a cooling pattern exists for both models with a certain percentage of error considering the practical implications and limitations of the problem. It was observed that the percentage of error between the values ​​exists, but the tendency of cooling of the welded component remains almost the same.

The variation of temperature with respect to the weld distance as simulated in the software was found to be consistent with the trend of the values ​​obtained from the experimental analysis. The percentage of error obtained was due to practical difficulties during the welding process carried out in the experimental setup.

Figure 3.13: Figure 3.12: Temp( 0 C) vs weld deposited(mm) plot for the top most element

Conclusion

The performed simulation was compared with the data obtained from the experimental setup for validation. Graphs are obtained for different nodes undergoing heat addition and heat removal simultaneously. The results show that there is a continuous change in the cooling pattern of each junction and thus a mixed grain structure is formed after the weld cools.

It is observed that there are differences in temperatures and structures at the end and start of the weld, but the middle layers usually show a stabilized stress trend. The influence of the complex stress, strain and microstructure distribution results in a different type of component properties.

Future Scope

The figures obtained for the variation of the heat flow with respect to time and distance from the weld pool give us a clear understanding of the flow path of the thermal energy in the models. This pattern can be used to analyze the residual stresses in the component and treat the stresses accordingly to achieve a stronger structure. A study of the buy-to-fly ratio of the component can be performed to optimize the use of raw material.

Buy to fly ratio refers to the ratio between the weight of the raw material used to manufacture the component and the weight of the component produced. This will not only reduce the wastage of raw materials, but will also improve the timing and effort required to manufacture the component, thus saving resources. 2] Jayaprakash S, Suryakumar S, 'Additive Manufacturing Of Complex Shapes Through Weld-Deposition and Feature Based Slicing', International Mechanical Engineering Congress and Exhibition;2015.

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7] Hoye N P, Appel E, Cuiuri D and Li H, "Characterization of metal deposition during additive manufacturing of Ti-6Al-4V by arc wire methods", Twenty-Fourth Annual International Symposium on Solid Form Manufacturing; 2012. 8] Ding D, Pan Z, Cuiuri D and Li H, “Process planning strategy for additive manufacturing of wires and arcs”, International Conference on Robotic Welding, Intelligence and Automation; 2015.