Zinc Alloy Material Testing

Zinc alloy material inspection mainly conducts scientific testing and analysis on the chemical composition analysis, mechanical property testing, metallographic structure analysis, physical property testing, casting property evaluation and corrosion resistance 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 chemical composition of the zinc alloy is the basis of determining its properties. This testing aims to precisely determine the contents of the main alloying elements (such as the aluminium, copper and magnesium), as well as strictly control the limits of the harmful impurity elements (such as the lead, cadmium, tin and iron), ensuring that the material meets the designation requirements and avoiding the casting defects or property deterioration caused by the composition deviations.

Mechanical property testing: This testing is the key of evaluating the load-bearing and deformation resistance abilities of the zinc alloy. Mainly including the testing of the indicators such as the hardness (Brinell, Rockwell or Webster hardness), tensile strength, yield strength, elongation after fracture and impact toughness, to verify whether the strength, plasticity and toughness of the material meet the design requirements under the actual service conditions.

Metallographic structure analysis: The micro-structures of the zinc alloy are observed through the optical microscopes or scanning electron microscopes. The testing items include the grain sizes and morphologies, the distributions of the second phases (such as the ε phase and η phase), the eutectic structure morphologies, as well as whether there are the internal defects such as the shrinkage porosity, blowholes and inclusions, thereby correlating and analysing the mechanical properties and process quality of the material.

Physical property testing: This testing involves the basic physical characteristics of the zinc alloy. Mainly including the determination of the density, melting point, thermal expansion coefficient, electrical conductivity and thermal conductivity. These data are crucial for the product design, mold development, casting process parameter setting and the selection of the specific application scenarios (such as the conductive parts).

Casting property evaluation: For the die-casting zinc alloys, their fluidity and shrinkage need to be evaluated. Through the fluidity testing (such as the spiral specimen lengths), the mold filling abilities of the liquid metals are evaluated; through the measurement of the linear shrinkage rates and volumetric shrinkage rates, the compensation basis is provided for the mold design, reducing the deformation and cracking tendencies of the castings.

Corrosion resistance testing: This testing evaluates the corrosion resistance of the zinc alloy in the specific environments. The common methods include the neutral salt spray testing (such as ASTM B117), damp heat testing and specific medium immersion testing, etc. The durability is judged by observing the corrosion morphologies on the sample surfaces and determining the mass losses or corrosion rates.

Dimension and appearance testing: The geometric dimensions, geometric tolerances and surface qualities of the zinc alloy castings or profiles are inspected. The tools such as the callipers and coordinate measuring machines are used to check the key dimensions; the surfaces are visually inspected or inspected with the aid of the magnifying glasses to check whether there are the appearance defects such as the cracks, cold shuts, drawing marks, burrs and flow marks.

Testing Scope

Die-casting zinc alloys: Mainly testing the widely applied high-aluminium zinc alloys such as the ZA and ZAMAK series (such as ZnAl4 and ZnAl4Cu1). The testing emphases are their casting fluidity, dimension stability, mechanical properties and surface qualities, ensuring that they are suitable for the mass production of the complex thin-wall parts such as the automobile parts, electronic housings and hardware parts.

Gravity casting zinc alloys: Covering the zinc alloy materials for the large or thick-wall castings. The testing scope focuses on the creep resistance, high-temperature strengths of the material, as well as the problems possibly caused by the slow cooling rates such as the coarse micro-structures and composition segregation, evaluating their applicability to the mechanical parts, mold inserts and other products.

Wrought zinc alloys: Referring to the zinc alloy plates, strips or wires that can be formed through the processes such as the rolling, extrusion and forging. The testing scope includes their work hardening characteristics, anisotropy, deep drawing properties, as well as the structure and property uniformity after the annealing, serving the products such as the battery casings and roofing sheets.

Zinc-based wear-resistant alloys: Mainly referring to the high-aluminium zinc-based alloys (such as ZA-27). The testing scope focuses on their wear resistance under the high loads, friction coefficients, load-bearing capacities and bearing shell compatibility, verifying the feasibility of their material substitution schemes as the wear-resistant parts such as the bearings and sliders.

Zinc alloy solders and brazing materials: The melting temperature ranges, wetting and spreading properties, joint strengths and corrosion resistance of such alloys are tested. Their compositions need to be strictly controlled to ensure the welding process stability, avoiding the joint brittleness or accelerated corrosion caused by the impurity elements, and they are applied to the low-temperature welding in the specific fields.

Zinc alloy scraps and return materials: To ensure the melting quality, the recovered zinc alloy scraps need to be tested. Mainly testing their chemical compositions (especially the impurity element enrichment conditions), contamination degrees (oil contamination and oxide contents) and metal recovery rates, providing the basis for the scientific batching and refining processes.

Functional zinc alloy coatings: The zinc alloy coatings prepared through the methods such as the thermal spraying and electroplating are tested. The testing scope includes the coating thicknesses, porosity, bonding strengths, hardness and cathodic protection effects to the substrates, evaluating their application properties in the anti-corrosion field of the iron and steel structures.

Testing Methods

Inductively coupled plasma atomic emission spectrometry: ICP-OES is the mainstream method of carrying out the qualitative and quantitative analysis of the chemical compositions of the zinc alloy. Its principle is to use the plasma to excite the atoms/ions in the sample to generate the characteristic spectra, and the element concentrations are determined through the spectral intensities. This method has the low detection limits, high precision and can simultaneously determine the multiple elements, with the excellent efficiency.

X-ray fluorescence spectrometry: XRF is a fast non-destructive chemical composition screening method. Its principle is to bombard the sample with the high-energy X-rays and measure the intensities of the excited characteristic X-ray fluorescence to determine the element contents. It is suitable for the fast sorting on the production sites, designation identification and coating thickness measurement, but the analysis ability of the light elements (such as Be) is weak.

Tensile testing method: According to the standards such as GB/T 228.1 or ASTM E8, the universal material testing machines are used to apply the axial tensile loads to the standard specimens until the fracture. Through the recorded force-displacement curves, the core mechanical parameters such as the tensile strength, yield strength and elongation after fracture of the material can be precisely calculated.

Hardness testing method: The appropriate methods are selected according to the material states and expected hardness ranges. The Brinell hardness (HBW) is suitable for the softer or coarse-grained materials; the Rockwell hardness (HRB/HRF) is used for the routine hardness testing with the fast operation; the Webster hardness (HW) is often used for the on-site non-destructive testing of the finished castings.

Metallographic microscopic analysis method: This method requires a series of sample preparation procedures for the zinc alloy samples such as the cutting, mounting, grinding and polishing and etching (commonly using the nitric acid alcohol solutions or chromic acid solutions). Then the micro-structures are observed under the optical microscopes, and the grain sizes, phase compositions and defects are rated according to the relevant standards (such as ASTM E3 and E407).

Scanning electron microscopy and energy dispersive spectroscopy: The SEM-EDS combined technology combines the morphology observation with the micro-area composition analysis. The SEM provides the high-resolution micro morphology images and can detect and observe the fracture characteristics and phase interfaces, etc.; the EDS can carry out the qualitative and semi-quantitative composition analysis of the observed specific micro-areas. It is the powerful tool of the failure analysis.

Salt spray corrosion testing method: According to GB/T 10125 or ASTM B117, the zinc alloy specimens are placed in the closed salt spray test chambers and continuously sprayed with the sodium chloride solution of the specified concentration, simulating the marine or industrial atmospheric environments. Through the regular observation and recording of the corrosion starting times, corrosion product morphologies and ratings, their corrosion resistance grades are evaluated.

Testing Standards

Domestic Standards:

1. GB/T 13818, Die casting zinc alloys

2. GB/T 8738, Zinc alloy ingots for casting

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

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

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

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

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

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. ISO 6506-1, Metallic materials — Brinell hardness test — Part 1: Test method

4. ISO 6508-1, Metallic materials — Rockwell hardness test — Part 1: Test method

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 optical emission spectrometer: This instrument is the core equipment of the chemical composition analysis in the laboratories, composed of the sample introduction systems, RF generators, plasma torches, grating dispersion systems and detectors. Its stability, resolution and detection limits directly determine the accuracy of the analysis results, and need to be regularly calibrated and maintained with the reference materials.

Universal material testing machine: Used to complete the mechanical property testing such as the tensile, compression and bending. The key components of the equipment include the high-precision load transducers, high-rigidity frames, precision displacement measurement systems and computer control systems, which can precisely control the loading rates and collect and process the test data in real time, outputting the standard reports.

Hardness tester series: Including the Brinell hardness testers, Rockwell hardness testers, micro Vickers hardness testers and portable Webster hardness testers. The hardness testers of the different principles are suitable for the different scenarios: the bench-type Brinell, Rockwell and Vickers hardness testers are mostly used for the precise laboratory measurement; the portable Webster or Leeb hardness testers are relied on for the fast production site sampling inspections.

Metallographic microscope and sample preparation equipment: The metallographic analysis system includes the cutting machines, mounting machines, grinding and polishing machines, etching devices and upright or inverted metallographic microscopes. The modern metallographic microscopes are often equipped with the digital cameras and image analysis software, realizing the automatic measurement and statistics of the parameters such as the grain sizes and phase area percentages.

Scanning electron microscope: The SEM is the high-end equipment for the micro morphology observation and micro-area composition analysis of the material. It uses the focused electron beams to scan the sample surfaces and images through the detection of the generated signals such as the secondary electrons and backscattered electrons. The equipped X-ray energy dispersive spectrometers (EDS) can realize the qualitative and quantitative analysis of the elements in the micron-level areas.

Environmental simulation test chamber: Mainly including the salt spray test chambers, constant temperature and humidity test chambers, and high and low temperature alternating test chambers, etc. These equipment can precisely control the environmental parameters such as the temperature, humidity and corrosive medium concentrations, simulating and accelerating the various harsh environmental conditions that the zinc alloy may encounter in the actual use, evaluating their long-term reliability.

Coordinate measuring machine: Used for the high-precision testing of the geometric dimensions and geometric tolerances of the zinc alloy castings or precision machined parts. The CMM collects the three-dimensional coordinate points by contacting the workpiece surfaces with the probes, compares with the CAD models or drawing tolerance requirements through the software, and generates the detailed inspection reports. It is the key equipment of controlling the product dimension consistency.