Casting vs Forging

Metal products can be produced by many ways, the most common ways are Casting and Forging.

What is Casting, what is Forging?

The definition of Casting

Casting is an engineering manufacturing process generally used for mass production in which materials in a molten state are poured into a mold where they solidify. In this process, complex parts can be manufactured economically and rapidly, otherwise, it would involve a lot of time if produced by other methods like shaping or cutting. The casting process can be utilized to produce a large variety of parts that are used in different industries, ranging from small plastic products to industrial supply. The casting profession focuses on the metal smelting process and the process control during the casting process.

The definition of Forging

Metal forging is a metal forming process that involves applying compressive forces to a work piece to deform it, and create a desired geometric change to the material. The forging process is very important in industrial metal manufacture, particularly in the extensive iron and steel manufacturing industry. A steel forge is often a source of great output and productivity. Work stock is input to the forge, it may be rolled, it may also come directly from cast ingots or continuous castings. The forge will then manufacture steel forgings of desired geometry and specific material properties. These material properties are often greatly improved.

What is the difference between Casting and Forging?

1.Shape methods

Forging

1) Free forging. Refers to the processing method of forgings that use simple universal tools or directly apply force to the billet between the upper and lower anvils of the forging equipment to deform the billet to obtain the desired geometry and internal quality. Forgings produced by the free forging method are called Free forgings. Free forging is mainly based on forgings with small production volume. The forging hammer, hydraulic machine and other forging equipment are used to form the blanks to obtain qualified forgings. The basic processes of free forging include upsetting, drawing, punching, cutting, bending, twisting, shifting and forging. Free forging adopts hot forging method.

2) Die forging.Die forging is divided into open die forging and closed die forging. The metal blank is compressed and deformed in the forging die cavity with a certain shape to obtain forgings. Die forging is generally used to produce parts with low weight and large batch. Die forging can be divided into hot die forging, warm forging and cold forging. Warm and cold forging are the future development direction of die forging, and also represent the level of forging technology.

According to the material, die forging can also be divided into ferrous metal die forging, non-ferrous metal die forging and powder product forming. As the name implies, the materials are ferrous metals such as carbon steel, nonferrous metals such as copper and aluminum, and powder metallurgy materials.

Extrusion should belong to die forging, which can be divided into heavy metal extrusion and light metal extrusion.

Closed die forging and closed upsetting are two advanced processes of die forging. Since there is no flash, the material utilization rate is high. With one procedure or several procedures, it is possible to finish the finishing of complex forgings. Since there is no flash, the area of the forged piece is reduced, and the required load is also reduced. However, it should be noted that the blank cannot be completely restricted. To this end, the volume of the blank should be strictly controlled, the relative position of the forging die and the measurement of the forgings should be controlled to reduce the wear of the forging die.

3) Grinding ring. Rolling ring refers to the production of ring parts with different diameters through a special equipment ring rolling machine, and is also used to produce wheel-shaped parts such as automobile wheels and train wheels.

4) Special forging. Special forging methods include roll forging, cross wedge rolling, radial forging, and liquid die forging. These methods are more suitable for the production of parts with special shapes. For example, roll forging can be effective

The pre-forming process greatly reduces the subsequent forming pressure; cross wedge rolling can produce steel balls, transmission shafts and other parts; radial forging can produce large forgings such as barrels and step shafts.

5) Forging die.According to the movement mode of the forging die, forging can be divided into swing rolling, swing rotary forging, roll forging, cross wedge rolling, ring rolling and cross rolling. Swinging, swinging forging and rolling rings can also be processed by precision forging. In order to improve the utilization rate of materials, roll forging and horizontal rolling can be used as the front-end processing of slender materials. Rotary forging, like free forging, is also partially formed. Its advantage is that it can be formed even when the forging force is small compared to the size of the forging. In this forging method, including free forging, the material expands from the vicinity of the die surface to the free surface during processing, so it is difficult to ensure accuracy. The forging force can obtain products with complex shapes and high precision, such as forgings of steam turbine blades with many varieties and large sizes.

Casting

There are many types of casting, which are traditionally divided into as below

1)Ordinary sand casting. It Including three types of wet sand, dry sand and chemically hardened sand

2)Special casting. It can be divided into special casting with natural mineral sandstone as the main modeling material (such as investment casting, mud casting, shell casting in casting workshop, negative pressure casting, solid casting, ceramic casting) Etc.) and special castings with metal as the main casting material (such as metal casting, pressure casting, continuous casting, low pressure casting, centrifugal casting, etc.).

The casting process usually includes:

①Preparation of the casting mold (the container that turns liquid metal into a solid casting). The casting mold can be divided into sand mold, metal mold, ceramic mold, clay mold, graphite mold, etc. according to the materials used, and can be divided into disposable mold and semi-permanent mold according to the number of uses And the permanent type, the quality of the mold preparation is the main factor affecting the quality of the casting;

② melting and pouring of the casting metal, the casting metal (casting alloy) mainly includes cast iron, cast steel and cast nonferrous alloy;

③ Casting treatment and inspection. Casting treatment includes removal of foreign objects on the core and casting surface, removal of risers, shovel grinding burrs and burrs and other protrusions, as well as heat treatment, shaping, anti-rust treatment and rough machining.

The casting process can be divided into three basic parts, namely casting metal preparation, mold preparation and casting processing. Cast metal refers to the metal material used for casting castings in casting production. It is an alloy composed of a metal element as the main component and added with other metal or non-metal elements. It is traditionally called casting alloy. Cast steel and cast non-ferrous alloys.

Metal smelting is not just simple melting, but also includes the smelting process, so that the metal poured into the mold meets the expected requirements in terms of temperature, chemical composition and purity. For this reason, various inspection tests for the purpose of quality control should be carried out during the smelting process, and the liquid metal can be allowed to be poured after reaching the prescribed indicators. Sometimes, in order to meet the higher requirements, the molten metal has to be treated outside the furnace after being released from the furnace, such as desulfurization, vacuum degassing, refining outside the furnace, inoculation, or deterioration treatment. Commonly used equipment for smelting metals are cupola, electric arc furnace, induction furnace, resistance furnace, reverberatory furnace, etc.

Different casting methods have different mold preparation contents. Taking the most widely used sand casting as an example, the preparation of the casting mold includes the preparation of molding materials and the core work of molding. The various raw materials used for modeling and core making in sand casting, such as foundry sand, sand binder and other auxiliary materials, as well as the molding sand, core sand, paint, etc. made from them are collectively called molding materials. Requirements, the nature of the metal, select the appropriate raw sand, binder and auxiliary materials, and then mix them into a molding sand and core sand with certain properties according to a certain ratio. Commonly used sand mixing equipment include roller-type sand mixer, counter-flow sand mixer and blade groove sand mixer. The latter is specially designed for mixing chemical self-hardening sand, continuous mixing and fast speed.

Moulding and core making are carried out on the basis of determining the moulding method and preparing the moulding materials according to the requirements of the casting process. The accuracy of castings and the economic effect of the entire production process mainly depend on this process. In many modern casting workshops, modeling and core-making have been mechanized or automated. Commonly used sand molding core making equipment include high, medium and low pressure molding machines, sand blasting machines, boxless injection molding machines, core shooting machines, cold and hot core box machines, etc.

After the castings are taken out from the casting and cooling mold, there are gates, risers and metal burrs. The sand castings also adhere to sand, so they must go through the cleaning process. The equipment for this kind of work includes shot blasting machine, gate riser cutting machine and so on. Sand casting cleaning of sand castings is a process with poor labor conditions, so when choosing a molding method, you should try to consider creating convenient conditions for sand cleaning. Due to special requirements, some castings have to be post-treated, such as heat treatment, shaping, anti-rust treatment, rough machining, etc.

Casting is a relatively economical blank forming method, which can show its economic efficiency for parts with complex shapes. Such as the cylinder block and cylinder head of the car engine, ship propeller and exquisite artwork. Some difficult-to-cut parts, such as nickel-based alloy parts for gas turbines, cannot be formed without casting methods.

In addition, the size and weight of the cast parts have a wide range of adaptation, and the metal types are almost unlimited; while the parts have general mechanical properties, they also have comprehensive properties such as wear resistance, corrosion resistance, shock absorption, etc., which are other metal forming methods such as forging , Rolling, welding, punching, etc. Therefore, in the machine manufacturing industry, the rough parts produced by the casting method are still the most in terms of quantity and tonnage.

Materials commonly used in foundry production include various metals, coke, wood, plastics, gas and liquid fuels, modeling materials, etc. The required equipment includes various furnaces for smelting metals, various sand mixers for sand mixing, various molding machines and core making machines for modeling and core making, sand falling machines and shot blasting machines for cleaning castings Wait. There are also machines and equipment for special castings and many transportation and material handling equipment.

The casting production has different characteristics from other processes, mainly including wide adaptability, many materials and equipment needed, and environmental pollution. Foundry production will produce dust, harmful gas and noise pollution to the environment, which is more serious than other machinery manufacturing processes, and measures need to be taken to control it.

The development trend of casting products is to require castings to have better overall performance, higher accuracy, less margin and a smoother surface. In addition, energy-saving requirements and society's call for restoring the natural environment are also increasing. To meet these requirements, new casting alloys will be developed and new smelting processes and new equipment will appear accordingly.

While the degree of mechanized automation in foundry production continues to increase, it will develop more towards flexible production to expand its adaptability to different batches and multiple varieties of production. New technologies that save energy and raw materials will be given priority, and new processes and equipment that produce little or no pollution will be given priority. The quality control technology will have new developments in the detection of various processes, non-destructive testing and stress determination.

2. Difference in performance

Forging

During forging, the metal undergoes plastic deformation and has the function of refining grains and continuous fiber cutting. Therefore, it is often used in the manufacture of important parts such as shafts and teeth. Casting has requirements on the material to be processed. Generally, castability of cast iron and aluminum is better. Casting does not have many advantages of forging, but it can produce complex shapes of zero, so it is often used in the manufacture of Mao Pi for support parts with low mechanical properties. For example, machine tool shell, etc.

Casting

Casting is the process of smelting metal into a liquid that meets certain requirements and pouring it into a mold. After cooling and solidification and cleaning, a casting (part or blank) with a predetermined shape, size and performance is obtained. The basic technology of modern machinery manufacturing industry. The cost of the blank produced by casting is low, and it is more economical for parts with complex shapes, especially with complex inner cavities; at the same time, it has a wider adaptability and has better comprehensive mechanical properties. However, there are many materials (such as metal, wood, fuel, molding materials, etc.) and equipment (such as metallurgical furnaces, sand mixers, molding machines, core making machines, sand falling machines, shot blasting machines, etc.) required for casting production, and Will produce dust, harmful gases and noise and pollute the environment.

3. Difference in raw materials

Casting

1). Gray cast iron

It has good casting performance and cutting performance, high wear resistance, shock absorption and low notch sensitivity, and the price is cheap. Therefore, it is widely used. In cast iron production, gray cast iron accounts for more than 80% of output. For example, HT200 is often used to make machine tool beds, boxes and racks that bear large loads and have complex shapes or high precision requirements (the castings need to be stress-relief annealed to reduce the internal stress of the castings), machine tool guide rails and cylinders ( Castings need to be surface-quenched with a hardness of 50-55HRC to increase the surface of the guide rail and the inner wall of the cylinder and the hardness and wear resistance).

2). Malleable cast iron (actually not malleable)

Through graphitization annealing, it can have high strength, great plasticity and toughness, good low temperature toughness, and relatively simple molten iron treatment, stable quality, and easy organization of flow production. Therefore, it is widely used in the automobile and tractor manufacturing industries to manufacture thin-walled, small and medium-sized parts with complex shapes and withstand impact loads. For example, KTH330-08 (black heart) can be used to manufacture machine tool wrenches and car wheel shells that bear moderate dynamic loads and static loads; KTZ650-02 (pearlite) can be used to manufacture high load, wear resistance and certain toughness Important parts, such as crankshafts, connecting rods, gears, etc.

3). Ductile iron (after autumn flower treatment, most or all of the graphite is spherical)

Has good mechanical properties and process performance, and can further adjust the mechanical properties through heat treatment (annealing to eliminate internal stress, normalizing to improve strength and wear resistance, tempering to obtain good comprehensive mechanical properties) Therefore, it can replace carbon cast steel and malleable cast iron, used to manufacture some parts with complex stress, high strength, hardness, toughness and wear resistance. Such as QT500-7AK can be used to manufacture internal combustion engine oil pump gears and flywheels, railway vehicle bearings.

4). Cast steel (carbon steel for casting)

It is generally used to manufacture mechanical parts with complex shapes (it is difficult to manufacture by forging or machining methods) and require high mechanical properties (cast iron whose mechanical properties cannot be achieved). For example, ZG270-500 has higher strength and better plasticity and good casting performance. Therefore, it is used to manufacture steel rolling mill frames, hydraulic press beams, etc. In 1998, the largest cast steel length manufactured by China No. 2 Heavy Machinery Group Corporation was 3.5 m (medium and heavy plate rolling mill stand). The casting blank weighed 375t and used molten steel 530t. The material used was ZG270-500. It is also produced by China's second largest heavy machinery group company and weighs 4t. The blank of 600.MW steam turbine high-pressure outer cylinder block is cast by AG15CrMo.

5). Cast brass (copper alloy)

Cast brass is made of ordinary brass by adding main and other elements and other elements. It has higher hardness and tensile strength, and has a certain plasticity. For example, ZcuZn38 is commonly used to manufacture flanges, valve seats, handles, nuts, etc.

6). Casting road alloy (commonly known as silicon aluminum)

Has good casting performance. Through modification treatment, the mechanical properties of the alloy can be improved. Elements such as copper and magnesium can also be added, followed by quenching and aging treatment to further improve the mechanical properties of the alloy. For example, ZL105 can use complex-shaped parts that work at higher temperatures below 255 ° C: cylinder heads and oil pumps of air-cooled engines (using metal mold casting or sand casting, subject to short-term treatment of aging and low temperature).

7). Other materials

Cast iron of vermicular graphite cast iron, cold shock alloy cast iron, stainless steel and other materials.

Forging

Forging materials are mainly carbon steel and alloy steel of various compositions, followed by aluminum, magnesium, copper, titanium and their alloys. The original state of the material is bar, ingot, metal powder and liquid metal. The ratio of the cross-sectional area of the metal before deformation to the cross-sectional area after deformation is called the forging ratio. Correct selection of forging ratio, reasonable heating temperature and holding time, reasonable initial forging temperature and final forging temperature, reasonable deformation amount and deformation speed are very important for improving product quality and reducing costs.

General small and medium-sized forgings use round or square bar stock as blanks. The grain structure and mechanical properties of the bar are uniform and good, the shape and size are accurate, and the surface quality is good, which is convenient for mass production. As long as the heating temperature and deformation conditions are reasonably controlled, forgings with excellent performance can be forged without large forging deformation.

4. Comparison between Casting and Forging

	Casting	Forging
Definition	A manufacturing process in which molten metal material is poured into molds to create the desired shape	A manufacturing process which makes use of compressing forces  to enhance rigidity
Tensile and Fatigue strength	Lesser	Higher
State of the metal	Involves pouring of molten metal into a mold for cooling	Can be done to cold, warm and hot metal depending on the requirement of the final product.
Metallurgical defects	Can occur	Avoided
Response to heat treatment	Requires close control due to the probability of alloy segregation	Better
Main materials used	Original matter is in the state of ingot, metal powder or molten metal Casting process involves materials of metal, wood, fuel and mold	Include carbon steel, alloy steel, bronze, steel, magnesium and other alloys
Types	Gravity casting (including sand casting and die casting) and pressure casting (including low pressure casting and high pressure casting).	Impression die forging, cold forging, open die forging and seamless rolled ring forging.

We should choose the suitable process according to the part dimension requests, application occasions and other factors, maybe the cost is also an important one of it. The most important thing is that we can produce the parts to meet customers' needs.