**3. Simulation and Analysis of the Welding Process**

Welding deformation is the most important factor affecting welding quality. Welding deformation will lead to a manufacturing delay, economic cost, and reduced productivity. Excessive deformation may seriously damage manufacturing in extreme cases, leading to failure [25]. At the same time, high welding residual stresses in the weld can adversely affect the safety and performance of welded components [26,27]. In this study, Simufact Welding software is used to simulate the welding process of duckbill welding parts, and the influence of deformation and the stress of weldments under a single-sided single welding torch and bilateral symmetrical double welding torch, two welding forms, and two welding process parameters, is analyzed.

#### *3.1. Heat Source Model*

In welding simulation, a reasonable heat source model is very important for the accurate calculation of post-weld deformation and welding stress [28]. To realize the simulation calculation, the commonly used heat source models are the classical Gaussian distribution heat source model and the double ellipsoid heat source model [29,30]. The Gaussian model can obtain better calculation accuracy for planar high-energy beam welds in simulation calculations. The double ellipsoid heat source model is more close to the actual welding situation of a fillet weld, so this study chooses the double ellipsoid heat source model for calculation.

The heat flux density expression of the front part of the double ellipsoid heat source is:

$$q\_f(x,y,z) = \frac{6\sqrt{3}f\_t q\_0}{abc\_f \pi \sqrt{\pi t}} \exp(-\frac{3x^2}{c\_f^2} - \frac{3y^2}{a^2} - \frac{3z^2}{b^2}).\tag{1}$$

The heat flux distribution expression of the second half of the double ellipsoid heat source is:

$$q\_b(x,y,z) = \frac{6\sqrt{3}f\_t q\_0}{abc\_b \pi \sqrt{\pi t}} \exp(-\frac{3x^2}{c\_b^2} - \frac{3y^2}{a^2} - \frac{3z^2}{b^2}).\tag{2}$$

In the formula: *a*, *b*, *cf*, and *cb* are oval shape parameters of the heat source; *q*<sup>0</sup> is the heat input power, and *q*<sup>0</sup> = *η*UI; and *ff*, *fb* are the heat flux distribution coefficients of the ellipsoid before and after the heat source, *ff* + *fb* = 2.

#### *3.2. Establishment of Welding Model*

The solid model of duckbill welded parts was established by SolidWorks, and then the model was imported into Hypermesh for hexahedral meshing. The number of finite element mesh nodes was 37,394, and the number of finite elements was 27,997. The divided model was imported into Simufact Welding for assembly and configuration, as shown in Figure 4. In this study, the weldment material is Q235, and the energy input per unit length of the weld (line energy) is calculated according to Equation (3).

$$Q = \eta \frac{IL}{v} \tag{3}$$

**Figure 4.** Meshing model of duckbill welding parts.

In the formula: *Q* is the line energy; *I* is the welding current; *U* is the welding voltage; *v* is the welding speed; and *η* is the welding thermal efficiency. As the weld of duckbill weldment is fillet weld, the welding heat is relatively concentrated. In this study, the welding thermal efficiency is taken as 0.8 in the simulation process [31].

#### *3.3. Welding Simulation Results and Analysis*

3.3.1. Effect of the Unilateral Single Welding Torch and Bilateral Symmetrical Double Welding Torch on Welding Deformation and Stress

Figure 5 shows the deformation of the duckbill welding parts under the single welding torch and the bilateral symmetrical double welding torch. By comparing and analyzing their total displacement cloud diagrams, the following conclusions were obtained: The area of deformation was larger under the condition of the single welding torch. This is because the two sides of the workpiece are uniformly heated and uniformly contracted at the same time by using the bilateral symmetrical double welding torch to reduce the distribution of welding deformation. The maximum displacement difference under the two conditions is 0.09 mm.

Figure 6 shows the equivalent stress diagram under the condition of the single welding torch and the bilateral symmetrical double welding torch. It can be seen from the figure that under the two conditions, the equivalent stress decreases rapidly from the center of the weld generation area, and then tends to be gentle until it is close to zero. A large stress is generated in the weld zone, which is one of the main reasons for the deformation of the static duckbill. After welding, the weldment is cooling, and the volume shrinkage around the weld is caused by the decrease in temperature. However, the weldment is constrained to prevent its shrinkage, so large tensile stress is generated in the weld area. Under both conditions, the maximum stress difference produced by the duckbill component is 7.28 MPa, but welding a duckbill component with a single torch takes more time than with a bilateral symmetrical double torch. Therefore, this study finally chose the welding method of the bilateral symmetrical double welding torch.

**Figure 6.** Equivalent stress diagram under the condition of the single welding torch and bilateral symmetrical double welding torch. (**a**) Single welding torch; (**b**) bilateral symmetrical double welding torch.

3.3.2. Effect of Welding Form on Welding Deformation and Stress

Figure 7, respectively, shows the use of continuous welding and spot welding under the two forms of total displacement cloud. From Figure 7, it can be seen that the displacement areas of the two were mainly distributed at the top of the static duckbill, and the deformation of the rest was relatively small. This is because the deformation of the fixed part is smaller than that of the free part. The position and deformation of the fixed part will be greatly limited under the action of the clamping device, so the thermal deformation

is reduced during the welding cycle. The maximum displacement of continuous welding is 0.98 mm, and that of spot welding is 0.26 mm. This is because in the weld, continuous welding, compared to spot welding, outputs greater thermal energy.

Figure 8 is the equivalent stress diagram of continuous welding and spot welding. It can be seen from Figure 8 that the stress distribution of spot welding is smaller than that of continuous welding, and the difference in their maximum stress value is 121.89 MPa. Their stress distribution is similar, the stress distribution appears to diffuse from the weld to the distance and then weaken, but it is obvious that the stress distribution of continuous welding is wider and wider. This study finally chose the welding form of spot welding.

**Figure 8.** Equivalent stress diagram under continuous welding and spot welding. (**a**) Continuous welding; (**b**) spot welding.

#### 3.3.3. Effect of Welding Process Parameters on Welding Deformation and Stress

Figure 9 is the total displacement diagram of the duckbill welded parts when the welding speed is 4 mm/s and 10 mm/s. It can be seen from the figure that the total displacement difference between the two welding speeds is 0.13 mm, but at the welding speed of 4 mm/s, the deformation area is relatively larger. This is because the deposition amount of the wire metal on the unit-length weld is inversely proportional to the welding speed, and the melting width is inversely proportional to the square of the welding speed. Therefore, when the welding speed increases, the energy decreases, the penetration depth and width decrease, and the deformation area is relatively reduced.

**Figure 9.** Total displacement diagram at the welding speed of 4 mm/s and 10 mm/s; (**a**) 4 mm/s; (**b**) 10 mm/s.

Figure 10 is the equivalent stress diagram under the two welding speeds of 4mm/s and 10 mm/s. As can be seen from the figure: 4 mm/s welding speed under the maximum equivalent stress is larger and the equivalent stress of a wider range of areas. Welding speed is directly related to the size of the welding productivity, and to obtain the maximum welding speed, should be on the premise of quality assurance as far as possible, according to the specific circumstances of the appropriate adjustment of welding speed, to ensure that the weld height and width are the same. In this study, the welding speed is finally selected as 10 mm/s.

**Figure 10.** Equivalent stress diagram at the welding speed of 4 mm/s and 10 mm/s; (**a**) 4 mm/s; (**b**) 10 mm/s.

#### **4. Design of Duckbill Welding Robot for Cotton Seeder**

### *4.1. Structure Composition and Working Principle*

The duckbill welding robot of the cotton planter is mainly composed of a girdle feeding mechanism, static duckbill feeding mechanism, hinge feeding mechanism, support table, welding fixture, welding actuator, and control system, as shown in Figure 11.

Working process: Firstly, the girdle feeding mechanism completes the girdle feeding, and then the hinge and the static duckbill feeding structure completes the feeding work in turn. After the three welding parts of the girdle, the hinge, and the static duckbill are all loaded, the workpiece enters the position to be welded, the clamping cylinder works to clamp the workpiece, and the welding actuator moves and performs welding. After the welding is completed, the welding platform is opened, and the weldment falls to the ground.

**Figure 11.** Structure diagram of duckbill welding robot for cotton seeder. 1. Girdle feeding mechanism; 2. static duckbill feeding mechanism; 3. hinge feeding mechanism; 4. support platform; 5. welding fixture; 6. welding actuator.

### *4.2. Design of Girdle Feeding Mechanism*

According to the analysis of the assembly requirements of the duckbill parts, the feeding mechanism needs to meet the following requirements: (1) the hinge and the girdle should be vertical; (2) the static duckbill and the hinge are symmetrically distributed in the transverse center when they are matched with the girdle; (3) the static duckbill should avoid shielding girdle under the mouth. According to the above assembly requirements and the structural parameters of duckbill welding parts, the feeding structure is designed. The feeding mechanism realizes the sequential feeding action of welded parts through the cooperation of an inductive proximity switch, electromagnet, and cylinder.

The structure size of the girdle feeding mechanism is 800 mm × 68 mm × 22 mm. It adopts a modular design and is installed on the support platform through the aluminum profile pillar. The working process is as follows: When the inductive proximity switch detects that there is a girdle in the storage chute, the electromagnet is energized and absorbs the second girdle, and the cylinder shrinks. The first girdle falls freely to the girdle waiting area due to gravity, and finally, the mini cylinder pushes the girdle into the welding area. The girdle feeding mechanism is shown in Figure 12.

**Figure 12.** Structure diagram of the girdle feeding mechanism. 1. Welding area; 2. girdle blanking waiting for area; 3. cylinder; 4. inductive proximity switch; 5. electromagnet; 6. storage chute; 7. girdle; 8. mini cylinder.

#### *4.3. Design of Static Duckbill and Hinge Feeding Mechanism*

To save space, the static duckbill feeding mechanism and the hinge feeding mechanism adopt an integrated design, and the assembly relationship of the parts is shown in Figure 13. The static duckbill and the hinge feeding mechanism are equipped with fixed plates to fix inductive proximity switches, electromagnets, and cylinders.

**Figure 13.** Structure diagram of static duckbill and hinge feeding mechanism. 1. Block cover; 2. cylinder; 3. inductive proximity switch; 4. electromagnet; 5. static duckbill; 6. arc feeding plate; 7. girdle; 8. connecting plate; 9. storage chute; 10. fixed plate 2.

The width of the storage chute of the hinge feeding mechanism is bent according to the dimensions of the hinge, and the bending angle is 90◦. To ensure that the hinge is perpendicular to the girdle during blanking, the lower end of the storage chute adopts a circular arc design, and its arc inner diameter is 100 mm. To prevent the hinge from sliding out of the arc guide rail when feeding, the guide bars are symmetrically distributed on both sides to guide and limit displacement. The verticality of the hinge is ensured by limiting the outer side of the guide bar and the arc guide rail. The guide bar is shown in the partially enlarged section view in Figure 13.

The main component of the static duckbill feeding mechanism is an arc feeding plate, and the arc feeding plate is connected with the hinge storage chute through a connecting plate. When the static duckbill is feeding, the contact with the feeding plate is strip contact, and the contact area is small, which greatly reduces the friction when sliding. When sliding, the static duckbill slides along the outer edge of the arc feeding plate. To prevent it from sliding out directly at the outer arc position, a block cover is placed at the lug of the hinge storage chute. The feeding accuracy of the static duckbill will directly affect the welding quality. Therefore, there are multiple through holes on the arc feeding plate and the connection plate, respectively, and the porous coordination ensures structural stability. The static duckbill and hinge feeding mechanism structure diagram is shown in Figure 13.

#### *4.4. Design of Welding Fixture*

As shown in Figure 14, the welding fixture is mainly composed of three parts: girdle clamping mechanism, hinge clamping mechanism, and static duckbill clamping mechanism. The girdle clamping device is positioned by a limit block and clamped by a girdle pusher. The girdle first slides down from the girdle storage chute to the girdle waiting area, and the girdle push plate sticks out. According to the four-point positioning principle, the transverse and longitudinal positioning and clamping of the girdle are completed.

**Figure 14.** Structure diagram of welding fixture. 1. Limit block; 2. limit block; 3. limit block; 4. welding workbench; 5. static duckbill clamping device; 6. girdle push plate; 7. static duckbill; 8. hinge; 9. hinge clamping device; 10. girdle; 11. limit block.

The hinge clamping mechanism is composed of a guide bar and a hinge push plate. The guide bar is close to the side wall of the storage chute, symmetrically distributed on both sides, and plays a guiding and limiting role to the hinge. The guide bar is shown in the partially enlarged section of Figure 13. After the hinge is loaded onto the welding platform, the hinge is pushed out to complete the positioning of the hinge.

The static duckbill clamping mechanism is mainly composed of a cylinder and clamp push plate. The arc feeding plate supports and guides the static duckbill. After the static duckbill slides down to the welding workbench, the clamp push plate is pushed out to complete the horizontal and vertical positioning of the static duckbill.

The bottom of the welding workbench is composed of two welding bottom plates and two cylinders. The welding workbench can open and close under the action of the cylinder.
