Precautions And Quality Control Points For Sheet Metal Welding Processes
Sheet metal fabrication finds extensive application across mechanical engineering, automotive, aerospace and other sectors. As a core process, welding directly impacts product structural integrity, sealing performance and dimensional accuracy. Rigorous control over materials, techniques, operations and post-processing is essential during welding. The following analysis examines four key dimensions.
I. Pre-welding Preparation: Dual Verification of Materials and Equipment
1. Material Inspection and Pre-treatment
Raw materials must comply with the material grade, thickness, and flatness requirements specified in drawings. For instance, the flatness deviation of welded structural steel must not exceed 2/1000. Any deformation must be rectified through mechanical straightening or heat treatment to restore flatness. Prior to welding, oil and rust residues must be thoroughly removed. Automotive sheet metal components may undergo solvent cleaning or sandblasting to prevent impurities causing porosity or slag inclusion defects. Prepare weld grooves according to joint type. Thick plates require V-groove preparation with angles controlled between 60°-70° to ensure uniform penetration depth.
2. Equipment and Parameter Configuration
Select welding machines suited to material properties:
- CO₂ welding is recommended for low-carbon steel sheets (<3mm thickness) due to low cost and high efficiency.
- TIG/MIG welding is required for stainless steel or aluminium-magnesium alloys to prevent oxidation. Welding current, voltage, and speed must correspond to material thickness. For example, CO₂ welding 3mm low-carbon steel requires 180-220A current, 22-24V voltage, and 30-40cm/min speed. Regularly inspect accessories like welding nozzles and conductive rods to ensure good conductivity. TIG tungsten electrodes must be ground to a 30° sharp angle for stable arc formation.
II. Process Implementation: Synergy Between Operational Standards and Structural Design
1. Selection of Welding Methods
Arc welding suits single-piece or small-batch production, offering simple equipment though weld appearance requires grinding. Resistance spot welding is employed for thin-plate structures like automotive bodies and metal cabinets; a 50mm spacing and 6mm diameter ensures load-bearing capacity. Laser welding is used for precision instruments, featuring a heat-affected zone <0.5mm with negligible deformation, though equipment costs are high.
2. Structural Design Principles Weld layouts must be symmetrical, avoiding intersections and overlaps. Long welds on box-type components should be distributed across different planes. Non-load-bearing structures may employ intermittent welding (e.g., 8–10mm welds spaced every 50mm) to minimise distortion. Vertical parallelism tolerance for component opening edges must not exceed 0.5mm to ensure assembly accuracy.
3. Operational Protocol Implementation Welders must wear welding helmets, gloves, and protective clothing. CO₂ welding spatter rates may reach 15%, necessitating spatter suppressants or shielding plates. Welding sequence progresses from the centre outward. For large enclosures, weld longitudinal seams first, followed by transverse seams, concluding with circumferential seams. For localised adjustments, use a small hand hammer for light tapping; direct hammering of flat surfaces is prohibited.
III. Quality Control: Comprehensive Inspection from Appearance to Internal Integrity
1. Weld Appearance Standards: Welds must exhibit a fish-scale pattern, free from bulges, undercut, or cracks. For aerospace components, weld bead height must be controlled between 0-1mm. Post-weld burrs shall be ground smooth using emery cloth, with burr height around automotive sheet metal welds ≤0.2mm. Flat welds on box-type workpieces must not protrude above the base material; after filling with putty, welds must be completely concealed.
2. Internal Defect Inspection Critical components undergo ultrasonic or X-ray testing. Pressure vessel welds require 100% inspection with a defect rate ≤0.5%. Tensile tests validate weld strength, while bend tests assess ductility. Structural steel welds must withstand 90% of the base material's tensile strength.
IV. Post-Processing: Balancing Protection and Functionality
1. Surface Treatment
Workpieces requiring electroplating or bluing undergo uniform sandblasting post-welding to remove oxide layers. Military-grade welds must be sandblasted with 80-mesh grit to achieve surface roughness Ra3.2. Rotating components are wrapped in rubber pads or foam to prevent impact damage.
2. Deformation Correction: Minor distortions shall be rectified using presses. Deformations of chassis side panels <2mm may be corrected by jacking. Weld distortion in thick plates requires localised heating to 600-650°C followed by controlled cooling to eliminate residual stresses.