Magnesium machining

Using plastic machining the main machining methods are rolling, extrusion, forging, etc., the magnesium ingot is processed into materials the main products are plates, bars, profiles, pipes, forgings, etc..

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Magnesium has active chemical properties, is easy to oxidize, and has poor corrosion resistance. During smelting, casting and heating, protective measures must be taken and safety should be paid attention to. During the processing, the surface of the blank and the obtained product is oxidized, oiled and packaged.


Except for magnesium-lithium alloys, most magnesium alloys have a close-packed hexagonal lattice structure, which has poor plastic deformation ability at room temperature. Magnesium alloys have high plasticity, so hot working or warm working processes are generally used. Magnesium alloys are easy to form coarse grains during processing, which will deteriorate the mechanical properties. During smelting, grain refinement measures must be adopted.

smelting

Reverberatory furnaces and crucible furnaces are commonly used for smelting magnesium and its alloys. The reverberatory furnace has the characteristics of fast melting speed and mechanized operation, which is suitable for large-scale production.

Because magnesium and oxygen have a strong affinity and are easily combustible, the key to smelting technology is to prevent the charge from burning.

The whole heating process must cover the charge with flux. The flux is mainly magnesium chloride and potassium chloride, and a small amount of fluoride is added. In order to prevent the oxidation of magnesium alloy melt, 0.02% beryllium is often added.

Magnesium alloy refining is mainly to remove non-metallic inclusions. Flux refining is usually used, and the dosage is about 1% of the charge. In order to make the slag sink to the bottom of the furnace, the refined melt needs to be left for a period of time before casting. In order to prevent the uneven structure of some magnesium alloys and the appearance of columnar crystals, fan-shaped crystals and coarse grains, a modifier should be added during smelting to improve and adjust the grain structure, and to eliminate a small amount of iron, copper, and manganese metal inclusions with high melting points.
 

casting

The method of semi-continuous casting is commonly used to obtain ingots for plastic working. In order to prevent the burning of magnesium melt and the generation of oxidized slag, casting should be carried out in a protective atmosphere or in a closed system. The protective atmosphere is usually sulfur dioxide gas (but pay attention to the corrosiveness of sulfur dioxide to equipment).

The sulfur dioxide forms a dense magnesium sulfide film on the surface of the flowing liquid, which can prevent the oxidation of the magnesium melt in the crystallizer. Sealed casting can also use argon protection. Electromagnetic stirring during casting can obtain fine-grained structure and can effectively reduce hot cracks.

Under normal circumstances, reasonable selection of crystallizer height, casting speed and suitable cooling speed can avoid ingot cracking, especially when casting slab ingot, the section should be cooled evenly.

The smelting and casting of magnesium alloys must pay attention to safety. Such as the use of wet flux during smelting, or direct contact of molten metal with water during casting, it will cause an explosion.
 

rolling

Plates are usually rolled with flat rolls. Generally used in block production, can also be produced in rolls. The main processes are: hot rolling, rough rolling, medium rolling and finishing rolling. Thick and medium plates (6 to 21 mm) can be directly rolled by hot rolling. Sheets (0.5 to 5 mm) are often produced by multiple heating and warm rolling processes.

Except for MB1 and MB8 magnesium-manganese alloys, the billets must be homogenized before hot rolling to ensure uniform structure and eliminate casting stress, thereby improving process performance. Ingot heating before rolling should be carried out in a circulating ventilation furnace.

The surface needs to be milled before heating, and special attention should be paid to removing burrs to prevent burning caused by burrs. The rolls must be preheated before hot rolling or warm rolling, and the rolls of warm rolled sheets are often preheated by electric induction.

In order to remove oxides on the surface of the roll, an appropriate amount of lubricant or water-based emulsion can be sprayed on the roll surface to avoid contaminating the plate and affecting the surface finish of the plate. Hot-rolled slabs or coils must be cleaned and chemically treated to remove the surface oxide film before sheet rolling. The temperature of sheet warm rolling should be lower than 150℃.


When rolling in coils, without lubrication, once rolled, the processing rate can reach 30-60%. In the case of block production, the pass processing rate is generally less than 30%, and the excessive reduction will cause uneven deformation and affect the surface quality. The total processing rate between the two heating times is 30 to 80%.


The finishing of the plate is generally carried out under the condition of 50 ℃ lower than the annealing temperature, and the plate pressing method is used, and the pressure is 0.03 ~ 0.07kgf/mm2, and good results can be obtained. For MB8 magnesium-manganese alloy, the plate thickness greater than 4mm can be straightened on a multi-roller straightening machine, and the working temperature should be higher than 100 ℃. In order to ensure the surface quality of the plate and improve the corrosion resistance, the finished plate needs to be mechanically cleaned and chemically oxidized.
 

extrusion

The extrusion methods include forward extrusion, reverse extrusion, lubricated extrusion and non-lubricated extrusion, which can produce profiles, bars, pipes and hollow products of various cross-sections.


The heating temperature and time of the billet must be strictly controlled. Generally, the heating temperature should not exceed 450 °C; the heating time should be less than 3 to 4 hours.

The billet should avoid contact with the aluminum alloy during the heating process, especially to prevent burning due to local overheating. Extrusion temperature, speed and deformation rate can significantly affect the properties of the product. Excessive extrusion speed will increase the thermal effect of deformation, promote the formation of coarse grains, and affect the mechanical properties of the product. Extrusion at a lower temperature can obtain higher and uniform mechanical properties of the product.

In order to reduce the anisotropy, the extrusion deformation of the structural material should not be less than 90%, and the extrusion deformation of the forging blank should not be less than 60%. Extruded products should be straightened by heating, and the residual deformation should not be greater than 3%. Small-section extruded products can be directly heated by electricity for stretching and straightening.
 

Forging and Die Forging

The influence of deformation temperature, deformation amount and deformation speed on process plasticity is obvious. Low-strength alloys (MB1) and medium-strength alloys (MB2, MB8) have higher plasticity and a wider range of deformation rates. The plasticity of high-strength alloy (MB15) decreases significantly with the increase of deformation speed. Generally, extruded bars are used as billets to obtain better process plasticity. The mold must be preheated before forging, and the preheating temperature is 150-300 °C.

Die forging is generally carried out on a press, which is divided into two steps: pre-die forging and final forging. Pre-die forging provides forging blank for final forging, and the deformation rate of blank is 40%. Final forging is to make the product reach the required size. In order to work harden the product and obtain a fine-grained structure, the final forging is generally performed at a lower temperature.

Die forging uses a mixture of spindle oil (or mineral oil) and graphite (or process yellow wax) as a lubricant. Die forged products can be surface-strengthened and improved surface finish by shot peening.

heat treatment

There are two kinds of annealing treatment and artificial aging treatment (see desolvation): ① For alloys that cannot be strengthened by heat treatment, the final heat treatment is annealing. Annealing can make the product obtain suitable mechanical properties and reduce anisotropy. The grain size of the processed products of magnesium alloys has a significant effect on the properties. In order to prevent grain coarsening, the general annealing temperature is 350-370 °C, and the faster the heating rate, the better.

Low-temperature annealing of work-hardened magnesium alloys between 160 and 210 °C can improve the elongation and corrosion resistance of the alloys. ② The final heat treatment of heat-treatable-strengthened alloys (such as MB6, MB7, MB15, etc.) is quenching and artificial aging.

The MB15 alloy can also be artificially aged directly after hot extrusion. The quenching temperature is generally 370-420℃, and the quenching medium includes air and water (70-100℃). Quenched in hot water, the mechanical properties of the product are better than quenched in air.

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Source: @cyanbat   Author: cyanbat

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