Why can high-quality welded joints be obtained through technological processes?

The core of obtaining high-quality welded joints through process methods lies in the precise control and optimization of welding process parameters. These parameters directly affect the mechanical properties, weld formation quality, and defect occurrence of the welded joint. The specific reasons and implementation methods are as follows:

I. Key Influences of Process Parameters on Welding Quality

**Welding Current**

Function: Determines weld penetration and heat input. Excessive current can lead to overheating of the weld metal, coarse grains, and even cracks; insufficient current easily results in incomplete penetration defects.

Example: In low-carbon steel welding, excessive current increases the brittle and hard microstructure in the heat-affected zone, making it prone to cracking under stress.

**Welding Voltage**

Function: Affects arc length and weld width. Excessive voltage easily leads to porosity and undercut; insufficient voltage results in insufficient weld width and incomplete fusion.

Example: In manual arc welding, excessive voltage can damage the molten pool protection, increasing the risk of porosity.

**Welding Speed**

Function: Controls heat input and weld formation. Excessive speed can easily lead to porosity and slag inclusions; insufficient speed can cause overheating or burn-through.

Example: In CO₂ gas shielded welding, excessive speed results in incomplete metallurgical reaction in the molten pool, leading to porosity from residual gas.

Other Parameters

Electrode Pressure (Spot Welding): Insufficient pressure leads to spatter, while excessive pressure results in incomplete penetration.

Shielding Gas Flow Rate: Insufficient flow rate causes oxidation, while excessive flow rate leads to arc instability.

II. Implementation Approaches for Process Optimization

Parameter Optimization Methods

Establish a quantitative relationship between parameters and mechanical properties through orthogonal experiments, response surface methodology, etc., to determine the optimal combination.

For example, use statistical models to optimize the matching of current, voltage, and speed to reduce defects.

Process Qualification and Verification

Conduct multiple sets of comparative tests according to standards (such as NB/T 47014), and verify the effectiveness of parameters through macroscopic metallographic inspection, bending tests, etc.

Develop a Welding Procedure Specification (WPS) covering all working conditions to ensure parameter stability.

III. Auxiliary Measures for Process Design

Structural Design: Employing symmetrical structures and rigid supports to reduce deformation; closed structures to prevent torsional deformation.

Material Selection: Assessing weldability based on carbon equivalent to avoid welding difficulties caused by high-carbon materials.

Preheating and Heat Treatment: Preheating before welding reduces stress; post-weld heat treatment improves microstructure and properties.

IV. Advantages of the Process Approach

Controllability: Precisely controlling heat input and penetration depth through parameter adjustments avoids defects.

Economy: Optimizing parameters reduces rework costs and improves efficiency.

Adaptability: Flexible process adjustments for different materials and thicknesses meet diverse needs.

In summary, scientifically selecting, optimizing, and verifying process parameters, combined with reasonable structural design and material processing, is an effective way to obtain high-quality welded joints.