Titanium-magnesium Alloy Testing

Titanium-magnesium alloy inspection mainly conducts scientific testing and analysis on the chemical composition analysis, mechanical property testing, metallographic structure analysis, corrosion property testing, non-destructive testing and dimension and geometric tolerance testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Chemical composition analysis: The main component contents of the titanium (Ti) and magnesium (Mg), as well as the ratios of the key alloying elements and trace impurity elements such as the iron (Fe), aluminium (Al), silicon (Si), manganese (Mn) and zinc (Zn) are precisely determined. This item is the fundamental basis of judging the material designations, evaluating the raw material qualities and tracing the production process stability, directly affecting the final properties of the alloy.

Mechanical property testing: Evaluating the behaviors of the titanium-magnesium alloy under the stressed states. The core items include the tensile testing (determining the tensile strength, yield strength and elongation after fracture), hardness testing (Brinell, Rockwell or Vickers hardness), as well as the bending and compression property testing. These data are the direct input parameters of the structural design and safety evaluation.

Metallographic structure analysis: The micro-structures of the alloy are observed through the optical microscopes (OM) or scanning electron microscopes (SEM), including the grain sizes, morphologies, phase compositions (such as the distributions of the α phase and β phase), as well as the second phase precipitation conditions. The structure analysis is the bridge connecting the compositions, processes and macro properties, used to optimize the heat treatment processes.

Corrosion property testing: Targeting the easily corroded characteristics of the titanium-magnesium alloy, the salt spray testing (such as the neutral salt spray NSS), electrochemical testing (such as the polarization curves and electrochemical impedance spectroscopy), as well as the immersion corrosion testing are emphatically carried out. Evaluating their corrosion resistance in the specific environments (such as the marine atmosphere and industrial environments), providing the basis for the selection of the surface treatment processes.

Non-destructive testing: Detecting the internal and surface defects under the premise of not destroying the workpieces. The common methods include the ultrasonic testing (UT, for the internal cracks and inclusions), penetrant testing (PT, for the surface open defects) and X-ray testing (RT, for the internal blowholes and shrinkage porosity). This is crucial for the key components such as the aerospace and medical devices.

Dimension and geometric tolerance testing: The precision equipment such as the coordinate measuring machines (CMM), laser scanners and profilometers are used for the quantitative testing of the geometric dimensions, shape errors (such as the straightness and flatness) and position errors (such as the parallelism and coaxiality) of the finished or semi-finished products, ensuring that they meet the assembly and functional requirements.

Testing Scope

Raw materials and ingots: The comprehensive incoming inspection is carried out on the initial forms such as the purchased titanium-magnesium alloy ingots, bar billets and plate billets. The testing emphases are the chemical composition conformity, macro low-power structures (such as the porosity and segregation), as well as the ultrasonic flaw detection, controlling the quality from the sources.

Semi-finished products (plates, pipes, bars and profiles): The testing is carried out on the intermediate products after the plastic processing such as the rolling, extrusion and forging. In addition to the routine compositions and mechanical properties, the anisotropy, surface qualities (cracks and peeling), as well as the residual stress distributions caused by the processing processes need to be emphatically focused on.

Castings and forgings: Covering the complex-shaped workpieces such as the aerospace engine components, automotive wheel hubs and precision instrument housings. The testing focuses on the internal compactness (X-ray and fluorescent penetrant), mechanical properties of the key parts (sampling testing), as well as the structure uniformity after the heat treatment.

Welded joints and structures: Evaluating the qualities of the titanium-magnesium alloy welding processes. The testing scope includes the chemical composition dilution of the weld areas, tensile and bending properties of the welded joints, metallographic structure evolution of the welds and heat-affected zones, as well as the use of the penetrant or radiographic testing for the welding defects (such as the blowholes, lack of fusion and cracks).

Surface treatment layers: The special testing is carried out on the workpieces after the surface treatments such as the anodizing, micro-arc oxidation, electroless plating and spraying. Including the coating thickness measurement (eddy current method and metallographic method), adhesion testing (cross-cut method and tensile method), coating hardness and corrosion resistance (salt spray testing) evaluation.

In-service equipment and components: The periodic inspection or failure analysis is carried out on the titanium-magnesium alloy components already in use, such as the aviation structural parts, chemical containers and sports equipment. Through the on-site non-destructive testing (such as the portable ultrasonic and eddy current) and sampling analysis, their aging, fatigue damage and corrosion conditions are evaluated.

Testing Methods

Inductively coupled plasma optical emission spectrometry (ICP-OES): Used for the precise determination of the major and trace element chemical compositions in the titanium-magnesium alloy. This method has the advantages of the low detection limits, wide linear ranges and multi-element simultaneous analysis, and is one of the preferred methods of the chemical composition arbitration analysis. It needs to be combined with the appropriate acid dissolution sample preparation processes.

X-ray fluorescence spectrometry (XRF): A fast and non-destructive surface composition analysis method, suitable for the raw material acceptance, production process online monitoring and finished product classification. Divided into the wavelength dispersive type (WDXRF) and energy dispersive type (EDXRF), the former has the higher precision and the latter has the faster speed.

Universal material testing machine testing: The standard method of carrying out the mechanical property tests such as the tensile, compression and bending. The standard specimens need to be strictly prepared according to the standards such as GB/T 228.1 (Metallic materials — Tensile testing), and under the controlled temperature, humidity and loading rate conditions, the accurate stress-strain curves and property data are obtained.

Metallographic specimen preparation and observation: The standard processes include the series of steps such as the sampling, mounting, grinding, polishing and etching (commonly using the Kroll reagents or hydrofluoric acid series etchants), and then the observation and photographing are carried out under the metallographic microscopes. According to the relevant standards (such as GB/T 6394), the grain size rating and phase analysis are carried out.

Salt spray testing: The accelerated testing method of evaluating the corrosion resistance of the titanium-magnesium alloy, mainly according to the standards such as GB/T 10125 (Corrosion tests in artificial atmospheres — Salt spray tests). The specimens are exposed in the closed salt spray chambers, simulating the marine or chlorine-containing environments. Through the regular observation of the corrosion products and the calculation of the corrosion rates, their corrosion resistance grades are evaluated.

Ultrasonic pulse-echo method: One of the most commonly used non-destructive testing methods. The probes transmit the high-frequency ultrasonic waves to the workpieces, and through the reception of the reflected echo signals of the internal defects or discontinuous interfaces, the positions, equivalent sizes and natures of the defects are judged. This method is particularly sensitive to the planar defects such as the internal cracks and inclusions.

Testing Standards

Domestic Standards:

1. GB/T 3620.1, Titanium and titanium alloys — Designation and chemical composition

2. GB/T 5153, Magnesium and magnesium alloys — Designation and chemical composition

3. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

4. GB/T 6394-2017, Determination of estimating the average grain size of metal

5. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

6. GB/T 229-2020, Metallic materials — Charpy pendulum impact test method

7. GB/T 232-2010, Metallic materials — Bend test method

International Standards:

1. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

2. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

3. ASTM B265, Standard specification for titanium and titanium alloy strip, sheet, and plate

4. ASTM B348, Standard specification for titanium and titanium alloy bars and billets

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Inductively coupled plasma spectrometer (ICP-OES/ICP-MS): The core equipment of the chemical composition analysis. The ICP-OES is used for the quantification of the constant and trace elements, and the ICP-MS (mass spectrometry) has the lower detection limits, used for the ultra-trace impurity element analysis. The equipment needs to be equipped with the hydrofluoric acid-resistant sampling systems to process the titanium-containing samples.

Electronic universal material testing machine: Equipped with the high-precision load transducers and extensometers, used to complete the mechanical testing such as the tensile, compression, bending and shear. The modern models usually integrate the digital control systems and data acquisition software, realizing the automatic control of the testing processes and the complex data analysis.

Metallographic microscope and image analysis system: Including the inverted or upright optical microscopes, equipped with the observation modes such as the bright field, dark field, polarized light and differential interference. Combined with the professional image analysis software, the automatic quantitative analysis of the metallographic structures can be carried out, such as the grain size measurement and phase area percentage calculation.

Scanning electron microscope and energy dispersive spectrometer (SEM-EDS): Providing the high-resolution micro morphology observation, and the qualitative and semi-quantitative analysis of the micro-area compositions (EDS). Especially suitable for observing the fracture morphologies (judging the fracture modes), analysing the inclusion compositions and observing the fine second phase precipitation.

Salt spray corrosion test chamber: The dedicated equipment for simulating the accelerated corrosion environments. The chambers need to be made of the corrosion-resistant materials (such as the PP plastics), with the precise temperature control, salt water atomization, sedimentation collection and pH value monitoring functions. The test conditions need to strictly meet the relevant standard provisions.

Ultrasonic flaw detector: The key equipment of the non-destructive testing, divided into the analog and digital types. The modern digital flaw detectors are more powerful. Need to be equipped with the probes of the multiple frequencies and types (straight-beam probes, angle-beam probes and dual crystal probes, etc.), and combined with the standard test blocks for the equipment calibration and sensitivity setting.

Coordinate measuring machine (CMM): Used for the precision geometric dimension and geometric tolerance testing. Through the probe contact with the workpiece surfaces to obtain the three-dimensional coordinate points, the data such as the dimension, position and shape errors can be obtained through the software processing. The high-precision CMM is the necessary equipment of the dimension acceptance of the complex titanium-magnesium alloy components (such as the impellers and casings).