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materials for sheet metal parts, surface treatments and defects

Sheet metal fabrication is generally a comprehensive cold working process for metal sheets (usually below 6mm) that involves manual or die stamping to produce plastic deformation, forming the desired shape and size, and further forming more complex parts through welding or a small amount of mechanical processing. Sheet metal has not yet had a relatively complete definition, but according to a definition in a foreign professional journal, it can be defined as: sheet metal is a comprehensive cold working process for metal sheets (usually below 6mm), including cutting, punching/cutting/composite, folding, welding, riveting, splicing, forming (such as car bodies), etc. Its prominent feature is that the thickness of the same component is consistent.

Selection of materials

1.galvanized steel

Galvanized steel is mainly divided into two types: electroplated galvanized sheet (EG/SEC) and hot-dip galvanized sheet (GI) - the differences will be discussed later.

2. stainless steel

2.1. Ferritic stainless steel:

It has a high content of Cr, good properties, and high-temperature antioxidant properties.

2.2. Austenitic stainless steel:

Typical grades such as/Cr18Ni9 and/Cr18Ni9T1 are non-magnetic, have good corrosion resistance, good temperature strength and high-temperature oxidation resistance, good plasticity, good impact toughness, and no notch effect. They have excellent weldability and are therefore widely used. This type of steel generally has low strength, low yield strength, and cannot be strengthened by heat treatment. However, after cold pressing and processing, it can increase its tensile strength and improve its elasticity, but the strength obtained by cold drawing at high temperatures is easily reduced. Not suitable for withstanding high loads.

2.3. Martensitic stainless steel:

Typical examples include 2Cr13, GX-8, which have magnetism, excellent shock absorption, good thermal conductivity, high strength and yield limit, and good comprehensive mechanical properties after heat treatment strengthening. Due to its high carbon content, it needs to be reworked after welding to eliminate stress. High temperature cooling can easily form bainite, so it should be slowly cooled after forging and immediately tempered. Mainly used for carrying components.

3. tinplate

Tinplate (SPTE) is a low-carbon steel material coated with tin (Sn) through electroplating; Some people believe that the tin plated sheet used to manufacture canned goods at that time was imported from Macau, Guangdong Province (English name Macau can be read as "Makou"), so it was called "Makou Iron". There are also other claims, such as in the past, China used this tin plated sheet to manufacture the lamp heads of kerosene lamps, which were shaped like tinplate, hence the name "tinplate". The name "tinplate" is not precise, therefore, it was officially named "tinplate" during the 1973 China Tin Plating Conference, and the official document no longer uses the name "tinplate".

Features: Maintaining good plasticity and formability of low-carbon steel; The general material thickness does not exceed 0.6mm.

Purpose: To shield magnetic interference shielding film and produce fewer parts;

4 spring steel

Medium carbon steel containing manganese (Mn), chromium (Cr), silicon (Si) and other alloy steels;

Characteristics: The material can undergo significant elastic deformation, which can be used to absorb impact or shock absorption, and can also store energy to enable the mechanical components to complete their actions.

5.Copper and copper alloys

Features: Good conductivity, thermal conductivity, corrosion resistance, good glossiness, easy plastic processing, easy electroplating and coating.

5.1. Purple copper (containing over 99.5% Cu)

Low material strength and good plasticity; Excellent conductivity, thermal conductivity, and corrosion resistance; Used on wires, cables, and conductive equipment.

5.2. Brass

Copper zinc alloy, mechanical properties are related to zinc content; Generally, the zinc content does not exceed 50%.

Features: Good ductility and stamping ability, used in electroplating, with good resistance to seawater and atmospheric corrosion. But the body is prone to local corrosion.

5.3. Bronze

A general term for a type of copper based alloy metal mainly composed of copper tin alloy.

Characteristics: It has better wear resistance than pure copper and brass, good processability, and corrosion resistance.

5.4. Beryllium copper

Copper alloy containing beryllium (Be);

Features: High strength, hardness, elasticity, wear resistance; High conductivity, thermal conductivity, and cold resistance; No ferromagnetism.

Usage: There are many electromagnetic shielding materials available;

In addition, aluminum alloy will also be used.

Common surface treatments for sheet metal

1. Electroplating

The technology of using electrolysis to deposit metal coatings with good adhesion and different performance matrix materials on mechanical products. Electroplated layers are more uniform than hot-dip coatings and are generally thinner, ranging from a few micrometers to several tens of micrometers. Through electroplating, decorative protective and various functional surface layers can be obtained on mechanical products, and worn and machined workpieces can also be repaired. In addition, it has different functions according to various electroplating needs. for example

1) Copper plating: used as a base to enhance the adhesion and corrosion resistance of the plating layer.

2) Nickel plating: used as a base or for appearance, to enhance corrosion resistance and wear resistance, among which chemical nickel has a higher wear resistance than chrome plating in modern technology.

3) Gold plating: improves conductive contact impedance and enhances signal transmission.

4) Palladium nickel plating: improves conductive contact impedance, enhances signal transmission, and has higher wear resistance than gold.

5) Tin lead plating: enhances welding ability and is quickly replaced by other alternatives (due to lead content, most are now changed to bright tin plating and matte tin plating).

2.  powder coating

1) A single coating can obtain a thicker coating, for example, coating with a thickness of 100-300 μ m. Using ordinary solvent coatings requires about 4-6 coats, while using powder coatings can achieve this thickness in one coat. The corrosion resistance of the coating is very good.

2) Powder coating does not contain solvents, has no three waste pollution, and improves labor hygiene conditions.

3) Adopting new processes such as powder electrostatic spraying, with high efficiency, suitable for automatic assembly line painting; The powder has a high utilization rate and can be recycled.

4) In addition to thermosetting epoxy, polyester, and acrylic, there are also a large number of thermoplastic grease resistant materials that can be used as powder coatings, such as polyethylene, polypropylene, polystyrene, fluorinated polyether, nylon, polycarbonate, and various fluorine-containing resins.

3. Electrophoresis/E-coated

Electrophoretic paint film has the advantages of full, uniform, flat, and smooth coating. The hardness, adhesion, corrosion resistance, impact resistance, and permeability of electrophoretic paint film are significantly better than other coating processes.

Detailed features:

(1) Using water-soluble coatings and water as the dissolving medium, a large amount of organic solvents are saved, greatly reducing air pollution and environmental hazards, ensuring safety and hygiene, while avoiding the risk of fire hazards;

(2) High coating efficiency, minimal coating loss, and coating utilization rate of up to 90% to 95%;

(3) The coating thickness is uniform, the adhesion is strong, and the coating quality is good. Various parts of the workpiece, such as the inner layer, depressions, welds, etc., can obtain a uniform and smooth coating film, solving the problem of coating complex shaped workpieces with other coating methods;  

(4) High production efficiency, construction can achieve automated continuous production, greatly improving labor efficiency;

(5) Complex equipment, high investment costs, high power consumption, high temperature requirements for drying and curing, complex management of coatings and paints, strict construction conditions, and the need for wastewater treatment;  

(6) Only water-soluble coatings can be used, and the color cannot be changed during the coating process. The stability of the coating is difficult to control when stored for too long. (7) Electrophoretic coating equipment is complex and technologically advanced, suitable for production with fixed colors.

4. Immersion molding

Immersion molding technology can provide flexible protection for rigid objects, with characteristics such as insulation, voltage resistance, flame retardancy, temperature resistance, waterproofing, dust prevention, and anti-static.

C. Summary of Common Defects in Sheet Metal Parts

For sheet metal parts, surface quality should be very important. Here, based on my own experience, I will briefly describe various problems that are prone to occur

1. Concave convex: The unevenness is caused by the stamping process of the stamping mold.

2. Scratch: Scratches on parts caused by other reasons, which may be caused between parts or external factors.

3. Corrosion: Improper protection can cause product corrosion, or due to prolonged shelf life, materials that do not meet requirements, or rust proof oil that does not meet requirements.

4. Indentation: Defects caused by mold repair or damage to the product.

5. Wrinkle: Wrinkles on the surface of a product.

6. Deformation: When the product undergoes deformation, most of the contact is caused by later collisions or material rebound.

7. Cracking: caused by the stretching process or stress concentration.

8. Misalignment: The welding position is offset during welding.

9. Blocking hole: For positions with nut holes, welding misalignment causes the nut holes to be blocked.

10. Clearance: The fit between two parts is not ideal, there is a gap in between.

11. File mark: After defects appear on the surface of the product, they are repaired with a file, resulting in later marks.

12. Polishing mark: The mark left by a product after polishing.

Welding and the defects: 

virtual welding, missed welding, welding deviation, welding penetration, welding slag, solder joint depression, solder joint distortion, solder joint cracks, solder joint burrs, solder joint spacing.

1. Virtual welding: Two parts are not fused together, and after destructive testing, the fusion surface is too small or there is no fusion. Usually caused by equipment parameters. When other parts of the welding equipment come into contact with the welding components, it causes a short circuit and results in virtual welding; Caused by factors such as excessive or insufficient current and voltage; There are gaps between components, resulting in small clearances, etc; Voltage fluctuations can also cause virtual soldering, which is related to factors such as cable aging and operating position.

2. Leakage welding: For sheet metal parts, it is normal to weld multiple points on top, so leakage welding points often occur. Standardized assignments should be carried out.

3. Welding deviation: The position of the welding point is not aligned and deviates from the predetermined position.

4. Welding through: Due to equipment parameters (such as excessive current) or a large distance between two parts during welding, current breakdown may occur; Oil stains, rust, or other substances between welded parts can cause breakdown.

5. Welding slag: formed by sparks splashing in all directions during welding

6. Concave welding point: After spot welding, the welding point is uneven, forming a concave point.

7. Welding point distortion: After spot welding, the welding point is uneven and deformed. If the cause of this defect is that the contact surface of the welding tongs is not perpendicular to the surface of the welded workpiece, it is prone to occur at the edge welding points.

8. Solder joint crack: A small crack that occurs at a solder joint.

9. Solder joint burrs: There are burrs at the solder joint, and the welding tongs need maintenance. Excessive current causes splashing; Edge welding points are prone to burrs; Excessive pressure can also easily cause burrs; The welding posture should be accurate.

10. Solder joint spacing: The distance between solder joints.