Metal Fabrication Industry Advances From Shaping to Finishing

Consider the precision instruments in your hands or the critical components within a car engine. How do raw materials transform into final products with specific shapes, performance characteristics, and surface properties? The manufacturing of metal parts is a complex process that integrates multiple techniques. This article explores the intricate stages of metal component production, from initial shaping to advanced surface treatments, revealing the underlying science and technology. We will examine various manufacturing processes and discuss how to select the optimal combination of techniques to achieve the best performance and cost efficiency.

Manufacturing metal components typically involves a series of processes, broadly categorized as primary and secondary operations. Many parts require a combination of both. During production, unfinished components are referred to as "work-in-progress" (WIP), awaiting further processing.

• Primary Processes: These shape materials into forms close to the final dimensions and geometry. They establish the part's fundamental structure and material distribution.
• Secondary Processes: These modify the WIP's surface, material properties, or apply coatings. When primary processes alone cannot meet design requirements, secondary operations are employed. After primary processing, the WIP becomes the "substrate." For example, in a part made of sintered alumina with a metal coating, the alumina serves as the substrate. In a galvanized steel screw, the steel is the substrate.

Primary processes form the core of metal component manufacturing, defining the part's basic structure. Below are key types of primary operations:

Molding and casting involve injecting molten material into a mold, allowing it to solidify, and then ejecting the shaped part. These methods apply to metals, polymers, and glass. For plastics, common techniques include injection molding and blow molding; for metals, die casting, sand casting, and investment casting are prevalent.

• Plastic Injection Molding: Thermoplastic pellets enter a hopper and feed into an injection machine. A rotating screw conveys the material forward while friction and heating zones melt it. Once sufficient molten plastic accumulates, the screw injects it into the mold cavity. After cooling, the mold opens, and the part ejects.
• Die Casting: Molten metal is forced into a mold cavity. Upon solidification, the mold opens, and the part is ejected.

All molding and casting processes require control over material composition and melt temperature. Additional variables like injection pressure, mold temperature, ejection timing, and mold lubrication may also be critical.

This process compacts metal or ceramic powder in a mold under pressure, then sinters it in a high-temperature furnace to fuse particles into a solid part. Hot pressing and hot isostatic pressing combine compaction and sintering.

Ideal sintered parts exhibit controlled porosity, engineered through compaction and sintering parameters to achieve desired properties.

These processes shape solid metals or polymers via mechanical deformation. Starting materials include sheets, tubes, rods, or blanks, sometimes heated for easier forming. Metal parts may be stamped, drawn, forged, or extruded; polymers are shaped via compression molding or thermoforming.

• Compression Molding: Plastic parts form from powder, pellets, or preforms. As the mold closes, compression generates shear, while heated mold halves soften the material to fill cavities. Continued heat and pressure cure the plastic.

This subtractive process removes material from sheets, blocks, or bars to refine cast or molded parts, achieve tighter tolerances, or alter aesthetics. Techniques include machining, chemical etching, and laser beam processing, applicable to metals, polymers, and ceramics.

• Machining: Encompasses grinding, milling, and drilling.
• Chemical Etching: Creates fine features on thin metal sheets or removes unwanted sections.
• Laser Beam Processing: Drills or cuts metals, polymers, and ceramics.

Lamination assembles individual material layers into multi-layer structures, often for composites. Layers are pressed together with or without adhesives, sometimes under heat.

Secondary processes modify WIPs and fall into three categories:

• Material Modification: Alters properties across the part's cross-section.
• Surface Modification: Changes surface characteristics.
• Coating Deposition: Applies or grows coatings on surfaces.

Heat treatment alters metal microstructure to enhance strength, ductility, or magnetic properties. Controlled heating and cooling cycles vary by material and desired outcomes.

• Steel Alloys: Heated in ovens or furnaces, then cooled at rates affecting microstructure. Slow cooling occurs in air; rapid cooling uses oil or water quenching.
• Aluminum, Copper, and Nickel Alloys: Strengthened via solution treatment (heating and rapid cooling) followed by precipitation hardening (aging at lower temperatures).

Chemical, mechanical, or thermal methods refine surface composition, texture, or chemistry to improve wear resistance, fatigue life, friction, or bonding capability.

• Surface Heat Treatment: Processes like induction, laser, or flame hardening create durable surface layers over a ductile core.
• Thermochemical Processes: Carburizing, nitriding, or carbonitriding diffuse elements into surfaces to form hard layers.
• Mechanical Processes: Shot peening (improves fatigue resistance), sandblasting (cleans/roughens), or grinding (finishes surfaces).
• Chemical Cleaning: Removes contaminants using acids, alkalis, or solvents.

Thin layers (from nanometers to micrometers) enhance wear, corrosion resistance, or aesthetics beyond substrate capabilities. Examples include:

• Electroplating: Immerses parts in conductive solutions; current deposits metal ions (e.g., copper, gold, nickel) onto surfaces.
• Conversion Coatings: Grow via chemical reactions (e.g., phosphating on steel, chromating on aluminum).
• Anodizing: Electrochemically oxidizes aluminum, magnesium, or titanium surfaces.
• Painting/Powder Coating: Applies polymer-based liquids or dry powders, cured via heating.
• Vacuum Deposition: Sputters or evaporates metals (e.g., aluminum, titanium) in vacuum chambers.
• Thermal Spraying: Projects molten droplets (metals, ceramics) onto surfaces via flame, arc, or plasma methods.

Some components undergo multiple secondary processes. For instance, sandblasting may precede painting to clean and roughen surfaces. Pre-coating materials (e.g., zinc on steel sheets) before forming can reduce costs compared to post-forming coating.

Beyond bulk shaping, deposition, etching, or chemical conversion techniques build intricate structures, particularly in electronics (e.g., integrated circuits, MEMS). Here, substrates provide mechanical support while integrating into functional designs.