A Beginner's Guide to Metal Casting

Metal material forming methods are an important part of part design and are highly concerned by manufacturers. Today we will talk about metal casting from a beginner’s perspective, hoping to inspire you.

Metal casting overview

Casting definition

Casting: Casting is pouring liquid metal that meets the requirements into a casting cavity that is suitable for the shape and size of the part through a series of transfer tools, so that the liquid metal can move in the gravity field or external force field (such as electromagnetic force, centrifugal force, vibration inertia The production method of filling the casting mold cavity under the action of force, etc., and waiting for it to cool and solidify to obtain a blank or part is usually called liquid metal forming or casting.

Characteristics of casting

• Use liquid metal
• Can manufacture workpieces with complex shapes
• Not limited by the weight of the workpiece
• Lower cost than other molding methods

Metal Casting process

1. Mold manufacturing
2. Shape (core), boxing
3. Smelting, pouring
4. Post-processing

Typical casting process

1. Wax injection
2. Wax correction
3. Tree assembly
4. Impregnation
5. Dewaxing
6. Melting
7. Pouring
8. Shelling
9. Cutting
10. Recycled Material
11. Shot blast cleaning
12. Welding
13. Grinding
14. Heat treatment
15. Passivation
16. Inspection
17. Packaging

Metal casting classification

According to material

• Iron alloy casting
• Non-ferrous alloy casting (non-ferrous metals)

According to the casting method

• Sand mold
• Metal type
• Pressure
• Investment mold
• Shell type

According to the stress state of pouring and solidification

• Free casting
• Pressure casting
• Gravity casting
• Vacuum casting
• Centrifugal casting

Characteristics of typical casting methods

Sand casting characteristics

• The primary raw material of the cavity is made of sand
• Castings with complex shapes can be made
• Low mold cost and low production cost
• .Short production cycle
• Not limited by alloy type
• The precision of castings is relatively low

Metal mold casting characteristics

• The cavity material is metal
• It can produce more complex-shaped blanks
• Mold cost is relatively high, and mass production cost is high
• The production cycle is short
• Limited alloy types, low melting point metals
• Casting precision is higher than sand casting

Pressure casting characteristics

• Liquid metal is formed under high speed and high pressure, and the cavity material is metal
• It is affected by pressure during filling and solidification
• The mold cost is high and the production cycle is long
• The casting has high precision and dense structure
• Alloy-restricted, low melting point metal
• Except for special die-casting, heat treatment is not allowed

Characteristics of investment casting (precision casting)

• The mold material has low melting point paraffin, etc
• The cavity material is silicate material
• The shape of castings can be complex and suitable for small parts
• Castings have high precision
• Applicable to all casting alloys
• The cost is high, there are many molds and the manufacturing cycle is long

The difference between casting and forging

Introduction to metal casting materials

Cast iron

Gray cast iron (grey cast iron)

Gray cast iron is cast iron with a large amount of flake graphite dispersed in a pearlite (or ferrite) matrix. Slow cooling during pouring can promote graphitization and gray cast iron can be obtained.

Its brand name is followed by "HT" followed by two sets of numbers. For example: HT20-40 (the first number indicates the lowest tensile strength, the second set of numbers indicates the lowest bending strength).

White cast iron (white iron)

White cast iron is an iron-carbon alloy with no or almost no graphite in the structure, that is, it mainly exists in the form of cementite.

It has high hardness (generally above HB500), but is brittle. It is often used as anti-wear parts, such as agricultural tools, grinding balls, coal mill parts, shot blasting machine blades, mud pump parts, cast sand pipes and chilled rollers. outer layer etc.

Vermicular graphite cast iron (vermicular iron)

Gray cast iron is obtained after creep treatment of molten iron, and the precipitated graphite is in the shape of worms.

Ductile iron (ductile iron)

Gray cast iron is obtained after spheroidizing the molten iron. The precipitated graphite is in the shape of spheroids, which is referred to as ductile iron.

Common ductile iron grades (national standards) (each country has standards) are: QT400-15, QT450-10, QT500-7, QT600-3, QT700-2.

Malleable cast iron

It is obtained after annealing treatment of white cast iron. The graphite is distributed in a floc-like form, referred to as tough iron.

Cast steel

Classification of cast steel:

Cast non-ferrous alloys

Cast non-ferrous alloys are mainly divided into aluminum alloys, copper alloys, zinc alloys, titanium alloys, and magnesium alloys.

Cast aluminum alloy

Cast aluminum alloy is made by adding various elements on the basis of pure aluminum. Commonly used cast aluminum alloys can be divided into 5 categories according to alloy series.

In Al-Si series alloys, the mass fraction of silicon is generally 6% to 13%, and they belong to hypoeutectic and eutectic alloys. Such as ZL101, ZL101A, ZL107, ZL111, etc.

In Al-Cu series alloys, the mass fraction of copper is generally greater than 4%. Such as ZL201, ZL201A, ZL205, ZL207, etc.

In Al-Mg series alloys, the mass fraction of magnesium is generally greater than 5%. Such as ZL301, ZL303, ZL305, etc.

The solubility of Al-Zn series zinc in aluminum is very large. When more than 10% zinc is added to aluminum, the strength of the alloy can be significantly improved. This type of alloy can be used directly without heat treatment. Such as ZL401, ZL402, etc.

AI-RE series alloys have high high-temperature strength and good thermal stability and can be used for parts working at temperatures of 350 to 400°C. However, its mechanical properties at room temperature are poor and there are not many applications at present.

Common heat treatment states of cast aluminum

T1 - Artificial aging as cast (the alloy has no significant heat treatment effect or the casting performance requirements are not high)

T2 - As-cast annealing (the alloy has no significant heat treatment effect or the casting performance requirements are not high)

T4-Quenching (solid solution) high plasticity, low strength

T5 - Quenching + Incomplete Aging, which requires both strength and certain plasticity.

T6 - quenching + complete aging, high strength, high hardness, low plasticity

T7-Quenching + Stabilized Tempering Castings with stable dimensions, heat-resistant aluminum alloy

Cast copper alloy

Brass

Brass is a copper alloy mainly composed of zinc and alloying elements. In cast brass, other alloying elements are added to form manganese brass, aluminum brass, silicon brass, lead brass, etc.

Bronze

Copper alloys that do not contain zinc as the main plus element are collectively called bronze, such as tin bronze, aluminum bronze, lead bronze, beryllium bronze, etc.

Cast magnesium alloy

Commonly used cast magnesium alloys are divided into three categories according to alloy series: Mg-Al series, Mg-Zn-Zr series, and Mg-Re-Zr series.

Cast zinc alloy

Cast zinc alloy is divided into pressure-casting zinc alloy and gravity-casting zinc alloy.

How castings solidify

• Layer-by-layer solidification: Layer-by-layer solidification has constant sources of feeding, good quality, and dense tissue. For example gray cast iron
• Volume coagulation (paste coagulation): Basically solidifies at the same time, and the tissue is not dense. Such as ductile iron
• Intermediate solidification: between layer-by-layer and volumetric solidification. In castings, some areas are layer-by-layer solidification and some areas are volumetric solidification.

Common defects of castings and their treatment methods

The national standard GB/T5611-1998 "Casting Terminology" divides casting defects into eight categories according to their appearance characteristics:

• Multiple meat defects
• Hole defects
• Cracks and cold insulation defects
• Surface defects
• Incomplete defects
• Defects such as shape and weight errors
• Inclusion defects
• Defects such as unqualified performance, composition, and structure

Hole type defects

Mainly include pores, pinholes, shrinkage cavities, shrinkage, and porosity.

Pinholes are a type of stomata. Porosity mainly refers to holes that appear inside or on the surface of a casting, with a circular, oval, waist-round, pear-shaped, or needle-shaped cross-section, isolated or distributed in groups.

Shrinkage porosity is a type of shrinkage cavities, which refers to small dispersed shrinkage cavities.

Methods to prevent shrinkage cavities, shrinkage, and porosity: improve casting process design; improve casting structure design; strengthen alloy refining; reduce pouring temperature and pouring speed, and extend pouring time.

Remedies for shrinkage cavities, shrinkage porosity, and looseness: welding repair; impregnation; important parts can be subjected to hot isostatic pressing.

Cracks and cold insulation defects

Mainly include cold cracking, hot cracking, white spots (cracks), cold insulation, etc.

Methods to prevent cold cracks, hot cracks and white spots: improve the structural design of castings, strive for uniform wall thickness and smooth transition, sufficiently large rounded corners in the casting cavity, reasonable process design, and minimize the shrinkage resistance of the castings; appropriately reduce the pouring temperature; increase the mold temperature; shorten mold opening and core pulling time; strictly control harmful impurities.

Remedial measures for cold cracks, hot cracks, and white spots: Castings that allow welding repair for cold cracking should be stress-relieved after welding; hot cracking castings should generally be scrapped, and if welding repair is allowed, the metal in the defective area should be completely excavated. Carry out welding repairs, and stress relief treatment should be carried out after welding.

Cold insulation definition: Cold insulation is a penetrating or non-penetrating gap in a casting with rounded edges, which is caused by poor fusion when the filling metal flows merge.

Surface defects

Mainly include mold drawing, flow marks, wrinkled skin, shrinkage, poor filling, bubbles, etc.

Causes of bubbles: The temperature of the mold or soup is too high; the filling speed is too high, and too much gas is involved in the metal; the paint generates a lot of gas, and the amount is too much; the exhaust is not smooth; the mold is opened too early.

Measures to improve bubbles: cool the mold or soup stock to the appropriate temperature; reduce the injection speed to avoid eddy air entrainment; use paint with small gas generation, and use a thin and even amount; clean and add overflow grooves and exhaust channels; adjust the mold retention time.

Incomplete defects

The main problems include insufficient watering, insufficient watering, damage, etc.

Methods to prevent damage: improve the casting structure, avoid weak structures, avoid excessive wall thickness differences and sharp corner transitions; carefully clean, store, and transport castings; correctly design risers; remove risers in the correct direction and method; improve alloy mechanics performance.

Shape and weight errors

It means that the shape, size, and weight of the casting do not comply with the casting drawing or technical conditions. Mainly include size and weight errors, deformation, wrong shape (wrong box), wrong core, eccentric core (drift core), shifting, etc.

Measures to improve deformation: improve the structure of the casting to make the wall thickness uniform; determine the optimal mold opening time to strengthen the rigidity of the casting; enlarge the casting slope; carefully pick and place the casting; reasonably stack and remove the gate; reasonably arrange the push rod position.

Improvement measures for the wrong shape: reasonably set up the inner gate; adjust and tighten the insert; replace the guide post and guide bushing.

Inclusion defects

It is a general term for various metallic and non-metallic inclusions in castings and is one of the most common defects. Mainly divided into metallic inclusions and non-metallic inclusions. Common non-metallic inclusions mainly include slag inclusions and blisters.