Inspection of heat treatment quality of magnesium alloy materials:

    (1) Hardness test

    Hardness testing has the advantages of fast speed, simple operation, and being able to be directly conducted on heat treated workpieces without the need for specialized sample preparation. Brinell and Rockwell scale hardness tests are commonly used, but sometimes Rockwell surface hardness tests are also used for thin section magnesium alloy workpieces. Magnesium alloys with larger grains and lower hardness should be measured using a Brinell hardness tester to obtain better test results. The strength of magnesium alloys usually increases with the increase of hardness. However, due to the dispersed strength indicators corresponding to hardness, strength cannot be calculated using hardness. The measured hardness values are only used as a reference for evaluating the quality of magnesium alloy heat treatment.

    (2) Tensile testing

    The Tensile testing can more accurately measure the heat treatment quality of magnesium alloys, but special tensile specimens are required during the test. Although the samples obtained from magnesium alloy castings after machining are more representative of the true performance of the castings, they are generally cast separately without machining. Testing is usually carried out in accordance with ASTM standards to ensure consistency of test results.

    (3) Microscopic examination

    The quality of heat treatment of magnesium alloy can be measured by examining the microstructure of the metallographic sample made of heat treated magnesium alloy and comparing it with standard microstructure photos. The inspection mainly includes: coarse compounds in cast alloys, pores and molten pores in cast alloys after improper solution treatment, grain size of cast and deformed alloys, and coarse compounds in extruded, forged or rolled alloys.

    Analysis of heat treatment defects in magnesium alloy materials:

    The five common defects that are prone to occur during heat treatment of magnesium alloys are oxidation, overburning, bending and deformation, abnormal grain growth, and uneven performance.

    (1) Oxidation

    If protective gas is not used during heat treatment of magnesium alloy workpieces, local oxidation may occur and even ignite in the furnace fire. Usually, (0.5-1.5) Vol.% SO2 or (3-5) Vol.% CO2, or CO2 shielding gas containing (0.5-1.5) Vol.% SF6, or inert gas is introduced into the heat treatment furnace to avoid oxidation of magnesium alloy workpieces. Inert gases are less commonly used due to their high cost. In addition, it is necessary to ensure the cleanliness, dryness, and sealing of the furnace.

    (2) Overburn

    When the heating speed is too fast, the heating temperature exceeds the solid solution treatment temperature limit of the alloy, and there are many low melting point substances in the alloy, magnesium alloy workpieces are prone to overheating. Usually, segmented heating or heating from 260 ℃ to the solution treatment temperature takes more than 2 hours, and the furnace temperature fluctuation is controlled within± Methods such as reducing the zinc content to the specified lower limit within the range of 5 ℃ to avoid overburning of magnesium alloy workpieces.

    (3) Bending and deformation

    The absence of fixtures or brackets during the heat treatment process, lack of support for the workpiece, and uneven heat distribution can all lead to bending and deformation of magnesium alloy workpieces. In order to reduce or eliminate the bending and deformation of magnesium alloy workpieces, it is necessary to pay attention to the following aspects. For workpieces with thin sections and long spans, support is required; For workpieces with complex shapes, fixtures or forming brackets should be used; For workpieces with uneven wall thickness, wrap the thin-walled parts with asbestos. At the same time, it is necessary to place the workpieces in the furnace reasonably to ensure good circulation of the atmosphere and uniform distribution of heat in the furnace. Residual stress in castings can be eliminated through annealing treatment. In addition, the heating speed should be kept moderate during the heat treatment process.

    (4) Abnormal grain growth

    The use of cold iron during layer by layer solidification leads to rapid local cooling. If internal stress is not pre relieved during subsequent heat treatment, it is easy to cause abnormal grain growth in magnesium alloys. Stress relief treatment before heat treatment, attention to selecting appropriate cold iron during casting, and intermittent heating method during solution treatment can effectively avoid abnormal grain growth of magnesium alloys.

    (5) Uneven performance

    The main reasons for uneven performance of magnesium alloy workpieces are uneven furnace temperature, insufficient thermal cycling in the furnace, or inaccurate furnace temperature control, insufficient solid solution treatment time for thick section workpieces, and uneven cooling rate of workpieces. The main measures to prevent uneven performance of magnesium alloys include: calibrating furnace temperature with standard thermocouples; The thermocouple used to control the furnace temperature should be placed in a place with uniform furnace temperature requirements; When installing the furnace, it is necessary to ensure sufficient thermal circulation inside the furnace; Regularly check the temperature control device of the heating furnace to ensure its normal operation; For thick section workpieces, extend the solution treatment time appropriately to obtain a completely uniform and consistent structure; Perform secondary heat treatment if necessary.

    The above is a sharing from QMAXIS metallographic sample preparation experts in the United States, hoping to provide you with some reference. If you need to conduct material testing, please fill out the following form, detailing your requirements, and request relevant quotations and information. After submission, our service engineer will contact you as soon as possible and provide you with relevant services wholeheartedly.