1.Why can't weld directly?
Melting Point Mismatch
Galvanized Steel: The melting point of steel is approximately ~1500°C, while the melting point of the zinc coating on the surface is approximately ~420°C, and the boiling point is approximately ~907°C.
Aluminum Alloy: Melting point is approximately ~660°C.
Problem: When the heat input is sufficient to melt the steel (>1500°C), the aluminum alloy will have already melted, evaporated, or even burned through. However, when the temperature is controlled within the acceptable range for aluminum alloys (approximately 660°C), the steel is far from reaching a molten state.
Alumina Issues
A dense alumina film with an extremely high melting point (~2050°C) instantly forms on the surface of the aluminum alloy. This film hinders the flow and bonding of molten metal and must be removed before welding (usually by AC TIG or mechanical scraping), but the process is complex.
Zinc Volatilization and Porosity
Before reaching the melting temperature of the steel, the zinc coating (boiling point 907°C) boils and volatilizes, producing a large amount of zinc vapor. These zinc vapors are drawn into the molten pool, forming numerous pores after the weld solidifies, severely weakening the weld's density and strength.
At the same time, the volatilized zinc contaminates the welding torch and tungsten electrode, producing toxic white zinc oxide fumes that pose a significant health hazard to operators.
2.What are the steps involved in a mechanical connection?
Riveting: Connections are made using blind rivets or solid rivets. The process is simple, low-cost, and suitable for automated production.
Bolted Connections: Removable, suitable for structures requiring maintenance or adjustment.
Precautions: Electrochemical corrosion (Gavaniz corrosion) must be considered. Steel (as cathode) and aluminum (as anode) form a galvanic cell in the presence of an electrolyte (such as humid air), leading to accelerated corrosion of the aluminum alloy. The solution is to use insulating gaskets, rubber rings, or sealant to isolate the contact surfaces.
3.What are the advantages and disadvantages of using structural adhesives?
Advantages:
Avoids heat-affected zones and intermetallic compound problems.
Naturally isolates two metals, preventing electrochemical corrosion.
Provides good sealing and vibration damping properties.
Disadvantages: Requires a long curing time, demands high surface cleanliness, and is not easily disassembled.
4.What are the precautions for special welding techniques?
Friction stir welding: This is a solid-state joining technique that uses a high-speed rotating stirring head to generate heat through friction, softening the materials without melting them, and then bonding them together through stirring. It effectively suppresses the formation of intermetallic compounds, but the equipment is expensive and has requirements regarding workpiece shape and accessibility.
Explosion welding: This technique uses the immense pressure generated by an explosion to bond the surfaces of two metals. It is mainly used for manufacturing composite sheets, not for on-site joining.
Bragging:
Bragging aluminum: Using aluminum-based filler metal (melting point ~500-600°C), the heat is insufficient to melt steel, but a bond can be achieved through the adhesion between the filler metal and the aluminum, and between the filler metal and the zinc layer. This method damages the zinc plating and has limited strength.
Bragging steel (e.g., with zinc-based filler metal): The heat would melt the aluminum alloy, making it unsuitable.
5.In practical production applications, what should be prioritized based on product performance requirements, production cycle time, and cost?
Riveting + Sealant (Economical and efficient)
Self-piercing riveting (Suitable for multi-layer boards)
Adhesive bonding (For high requirements of aesthetics and sealing)
Glue-riveting composite (For extremely high requirements of strength and reliability)