Cast Aluminum Material Testing

Cast aluminium material inspection mainly conducts scientific testing and analysis on the chemical composition analysis, mechanical property testing, metallographic structure examination, low-power structure and fracture analysis, dimension precision and geometric tolerance testing and non-destructive 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: Determining the precise contents of the various elements in the cast aluminium material is the basic and key testing item. The main tested elements include the aluminium (Al), silicon (Si), copper (Cu), magnesium (Mg), zinc (Zn), iron (Fe), manganese (Mn), nickel (Ni) and titanium (Ti), etc. Through the analysis, ensuring that the material meets the composition requirements of the specific designations (such as ZL101A and A356.2), which is the premise of guaranteeing the mechanical properties and casting processability of the material.

Mechanical property testing: Evaluates the behavior and abilities of the cast aluminium material under the applied forces. The core testing indicators include the tensile strength, yield strength, elongation after fracture and hardness (Brinell HBW or Rockwell HRB/HRE), etc. These data are directly related to the load-bearing capacity, safety factors and service lives of the parts, and are the important basis of the design material selection and process verification.

Metallographic structure examination: The micro-structures of the material are observed through the optical microscopes or scanning electron microscopes. The emphases are on the analysis of the α-Al matrix morphologies, the morphologies and distributions of the eutectic silicon, the primary silicon sizes, the types and amounts of the intermetallic compounds (such as β-Al5FeSi), as well as whether there are the casting defects such as the porosity, blowholes, shrinkage cavities and slag inclusions. The metallographic examination is the core link of relating the composition, processes and properties.

Low-power structure and fracture analysis: The structure uniformity and defect conditions of the castings or specimens at the macro scale are checked. Including checking the distributions of the porosity, blowholes, shrinkage cavities, cracks, cold shuts and inclusions. The fracture analysis judges the fracture modes (ductile, brittle and fatigue, etc.) and origins through the macro and micro observation of the fracture surfaces, providing the direct evidence for the failure analysis.

Dimension precision and geometric tolerance testing: The equipment such as the coordinate measuring machines (CMM), laser scanners and dedicated gauges are used for the precise measurement of the key dimensions and geometric tolerances (such as the flatness, roundness and coaxiality) of the castings. Ensuring that the castings meet the drawing requirements and satisfy the subsequent assembly needs is the important part of the quality control.

Non-destructive testing: Under the premise of not destroying the castings, their internal and surface defects are probed. The common methods include the X-ray real-time imaging for detecting the internal shrinkage cavities and blowholes; the ultrasonic testing for detecting the internal cracks and inclusions; the penetrant testing (colour or fluorescent) for detecting the surface open defects; the eddy current testing for the near-surface defect detection and electrical conductivity measurement.

Special property testing: The special tests carried out according to the application scenarios. Including the high-temperature lasting strength and creep properties (for the engine parts), fatigue properties (under the cyclic loads), corrosion resistance (such as the salt spray testing), wear resistance, thermal expansion coefficient and thermal conductivity, etc. These tests provide the data support for the reliability of the material under the specific working conditions.

Testing Scope

Cast aluminium alloy designation identification: Covering the cast aluminium designations under the main domestic and foreign standard systems, such as the ZL series in the Chinese GB/T 1173, A356, A360 and A380 in the American ASTM B26/B108, the EN AC-xxxx series in the European EN 1706, and the ADC series in the Japanese JIS H5202. The testing needs to clearly distinguish the materials of the different series (such as the Al-Si, Al-Cu and Al-Mg series).

Automotive parts castings: Including the key safety parts and structural parts such as the engine blocks, cylinder heads, pistons, transmission housings, steering knuckles and wheel hubs. The testing emphases are the high strengths, high densities, good fatigue properties and dimension stability, and usually the comprehensive mechanical property, metallographic and non-destructive testing are required.

Aerospace structural parts and components: Involving the cabin structures, brackets and housings with the extremely high safety requirements. The testing standards are extremely strict. In addition to the routine items, the fracture toughness, low-cycle fatigue properties, strict classification of the internal defects (such as according to the standards such as AMS2175) and long-term service reliability evaluation of the material are particularly emphasized.

Power and electronic equipment housings: Such as the motor casings, inverter housings and heat sinks. The testing focuses on the thermal conductivity, electromagnetic shielding effectiveness, sealing (through the air tightness testing) and environmental corrosion resistance of the material. The dimension precision and surface qualities are also the important assessment points.

General machinery and pump and valve housings: Including the various gearboxes, pump bodies and valves. The testing focuses on the pressure-bearing capacity, wear resistance, medium corrosion resistance (for the chemical pump valves) of the material, as well as the internal defect control levels brought by the casting processes.

Artworks and building components: Such as the sculptures, decorative parts and curtain wall connectors. The testing focuses more on the appearance qualities, weather resistance after the surface treatments, the effects of the chemical compositions on the anodizing colouring effects, as well as the necessary mechanical properties to ensure the firm installation.

Raw materials and process monitoring: The testing scope extends to the composition analysis of the raw materials such as the aluminium ingots, master alloys and return materials used for the casting, as well as the hydrogen content (hydrogen meters) during the melt treatment and the modification/refinement effect evaluation (grain size determination) in the melt treatment processes, realizing the whole-process quality control.

Testing Methods

Spectroscopic analysis method: Used for the fast and accurate chemical composition analysis. Mainly adopting the direct-reading spectrometers (OES) for the multi-element simultaneous quantitative analysis of the bulk samples. For the high-precision requirements or arbitration analysis, the inductively coupled plasma optical emission spectrometers (ICP-OES) or atomic absorption spectrometers (AAS) are used, and the samples need to be dissolved into the liquids.

Tensile testing method: According to the standards such as GB/T 228.1 and ASTM E8/E8M, the universal material testing machines are used to apply the axial static tensile forces to the standard tensile specimens until the fracture. Through the recorded load-displacement curves, the key mechanical parameters such as the tensile strength, yield strength (often taking Rp0.2) and elongation after fracture are calculated.

Metallographic specimen preparation and observation method: Strictly carried out according to GB/T 13298 or ASTM E3. The process includes the sampling, mounting, rough grinding, fine grinding and polishing, followed by the etching with the appropriate etchants (such as the 0.5% HF aqueous solutions) to reveal the structures, and finally the observation and photographing under the optical microscopes and the rating according to the relevant standards (such as GB/T 9438).

Brinell/Rockwell hardness testing method: The Brinell hardness (HBW) testing is carried out according to GB/T 231.1, using the cemented carbide ball indenters of the larger diameters and applying the larger loads, measuring the indentation diameters. The results can better reflect the average hardnesses of the material and are suitable for the as-cast structures. The Rockwell hardness (HRB/HRE) testing is carried out according to GB/T 230.1, using the steel ball indenters and measuring the indentation depths, with the faster operation, suitable for the batch inspection.

X-ray non-destructive testing method: According to the standards such as GB/T 5677 and ASTM E1030. The X-rays penetrate the castings. Due to the different absorption of the rays by the defect parts and the matrix, the differences are formed on the imaging plates or digital detectors, thereby intuitively displaying the positions, sizes and shapes of the internal blowholes, shrinkage cavities and slag inclusions, and the digital ratings are carried out.

Ultrasonic testing method: According to the standards such as GB/T 7233.1 and ASTM E797. The high-frequency sound waves (usually the longitudinal waves) are coupled and transmitted into the castings. When the sound waves encounter the defects or bottom surfaces, the reflections occur. By analysing the times, amplitudes and waveforms of the reflected echoes, the depths, equivalent sizes and properties of the internal defects can be judged, and it is sensitive to the planar defects such as the cracks.

Penetrant testing method: According to the standards such as GB/T 9443 and ASTM E1417. Suitable for detecting the surface open defects of the non-porous materials. The penetrants containing the fluorescent or colour dyes are applied to the surfaces to seep into the defects, and after removing the excess penetrants, the developers are applied. The penetrants in the defects are adsorbed to the surfaces to form the indication marks, thereby being observed.

Testing Standards

Domestic Standards:

1. GB/T 1173, Casting aluminium alloys

2. GB/T 9438, Aluminium alloy castings

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

4. GB/T 231.1, Metallic materials — Brinell hardness test — Part 1: Test method

5. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

6. GB/T 13298, Inspection method of microstructure for metals

7. GB/T 5677, Steel castings — Methods of radiographic testing

8. GB/T 7233.1, Steel castings — Ultrasonic testing — Part 1: Methods for steel castings

9. GB/T 9443, Steel castings — Penetrant testing

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 E8/E8M, Standard test methods for tension testing of metallic materials

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

Direct-reading spectrometer: The core equipment of the chemical composition analysis. Through the arc or spark exciting the samples to generate the characteristic spectra, after the grating dispersion, the intensities of the characteristic spectral lines are detected by the CCD or photomultiplier tubes, and the contents of the various elements are calculated by comparing with the calibration curves established with the standard samples. It has the advantages of the fast analysis speed (tens of seconds), high precision and multi-element simultaneous analysis.

Universal material testing machine: The key equipment of carrying out the mechanical testing such as the tensile, compression and bending. The servo motors or hydraulic systems are used to provide the precise loading, equipped with the high-precision load transducers and extensometers (for the precise deformation measurement). The computer control systems are responsible for controlling the test processes, collecting the data and automatically generating the stress-strain curves and test reports.

Metallographic microscope and image analysis system: The basic tool of the metallographic observation. Including the inverted or upright optical microscopes, equipped with the observation modes such as the bright field, dark field and polarization, as well as the objectives of the different magnifications. The modern systems integrate the high-resolution digital cameras and professional image analysis software, which can carry out the grain size measurement, phase area percentage statistics and automatic defect identification and rating.

Brinell/Rockwell/Vickers hardness tester: The dedicated equipment of the hardness testing. The Brinell hardness testers are composed of the loading mechanisms, indenters and microscopes (used to measure the indentations). The Rockwell hardness testers directly read the hardness values by measuring the indentation depths, with the high efficiency. The Vickers hardness testers use the diamond square pyramid indenters, suitable for the hardness testing of the smaller areas or thin layers.

Digital X-ray real-time imaging system: Composed of the X-ray machines (radiation sources), mechanical motion platforms, flat panel detectors (or image intensifiers), image acquisition and processing computers and professional software. It can realize the multi-angle perspective imaging, image enhancement, automatic defect marking and dimension measurement of the castings, and the defect grade ratings are carried out according to the digital radiograph standards.

Ultrasonic flaw detector: The core components include the pulse generators, receiving amplifiers, probes (transducers) and displays (now mostly the colour LCDs). The digital ultrasonic flaw detectors have the A/B/C scan functions, can store the waveforms and data, support the various intelligent analysis algorithms, and combined with the probes of the different frequencies and angles, adapt to the detection of the castings of the various complex shapes.

Coordinate measuring machine: Used for the precise geometric dimension detection. The detection systems (such as the contact probes or laser scanning heads) are installed on the three mutually perpendicular guide rails. By probing a series of points on the casting surfaces, the computer software fits the geometric features of the measured elements (such as the circles, planes and cylinders), compares them with the CAD models or theoretical dimensions, and outputs the deviation reports.