Aluminium die casting parts in automotive: Non-destructive testing (NDT) technology for internal defects

Non-destructive testing (NDT) technology

This article briefly describes the NDT techniques such as radiographic inspection, ultrasonic inspection, penetrant inspection and industrial CT inspection, compares the advantages and characteristics of each technique in the detection of internal defects of aluminum die-cast parts for automobiles, and evaluates the inspection and results.

Table of Contents

Introduction of pressure die casting parts defects

Aluminium die-casting parts are widely used in automobile, hardware, electromechanical, communication, aviation and other fields, especially in the global automobile production and sales growth and automotive lightweight energy-saving environmental protection trend. The demand for aluminium die-casting parts is a continuous rapid growth trend. At present, die-casting aluminium alloy can be roughly divided into four categories: Al-Si system, Al-Si-Mg system, Al-Si-Cu system, and Al-Mg system. The addition of alloying elements increases the heat resistance, corrosion resistance and strength of the material, and the manufacturing process is simple, good product performance, high dimensional accuracy, surface finish, and can be made to manufacture large, thin-walled, complex die casting, suitable for mass production, widely used in Automobile motor shell, power controller shell, structural parts, gearbox shell, battery pack shell, inverter shell and other parts manufacturing.

Auto die casting parts
diecast part auto

Pressure casting shoots the adjusted aluminium liquid into the mould cavity at high speed through a very thin gate to make it solidify quickly, inevitably producing casting defects unique to pressure casting other than conventional casting defects. According to the book, Pressure Casting Defects, Problems and Countermeasures Examples, prepared by the Pressure Casting Research Committee of the Foundrymen’s Society of Japan, pressure casting defects are divided into eight categories:

  • A flying wing (flying edge), succulent
  • B holes
  • C cracks, cold compartment
  • D surface defects
  • E under-pouring, shape defect
  • F dimensional errors
  • G inclusions (involved), segregation
  • H other (leakage)

Holes, cracks, and inclusions are common internal defects of aluminium die-casting parts. The existence of hole defects will not only reduce the force capacity of the die-casting unit area but also make the local area of the die-casting due to stress concentration and the formation of fracture, resulting in a decline in the mechanical properties of the workpiece; aluminium liquid in the solidification shrinkage encountered resistance to thermal cracking defects and cold cracking defects caused by post-solidification stress, will be on the rigidity and strength of the die-casting Causes adverse effects; aluminium mixed with impurities caused by the appearance of inclusions class defects generally bean-shaped, lumpy or long, such as the shape of the defect and the die casting base material is different. 

According to above the defecting types in the use of die castings, there are serious safety risks, so the internal quality of aluminium die casting production and inspection of the primary control factors, the pursuit of fewer defects or zero defects, reduce manufacturing costs, aluminium die-casting enterprises is of great significance.

Non-destructive testing technology for internal defects

The detection of defects such as holes, cracks and inclusions in aluminum die-casting parts for automobiles is usually performed by non-destructive testing methods, which detect the type, number, shape, location, size, distribution and changes of defects inside the workpiece. The main NDT techniques are traditional radiographic testing (RT), ultrasonic testing (UT), penetration testing (PT) and the new industrial computed tomography (ICT). Radiographic testing, ultrasonic testing and industrial CT testing are mainly for internal defects of the inspected workpiece, and penetration testing is mainly for defects near the surface of the inspected workpiece.

X-ray inspection technology

X-ray inspection, also known as radiographic flaw detection, is a method that uses the characteristics that rays have a certain penetrating ability to materials and materials have attenuation to rays to check the internal defects of materials. There are many types of rays that are easy to penetrate the material: X-rays, γ-rays, and neutron rays 3, of which X-rays are widely used for internal defect detection of aluminum die-casting parts. The traditional X-ray flaw detector is sensitive to the volume type defects of aluminum die-casting parts (such as holes and inclusions), the detection rate is very high, can more intuitively display the size and shape of the internal defects of the workpiece, the plane distribution of the defect image is true, the size measurement is accurate. The sensitivity of the thin-walled workpiece flaw detection is high. However, if the camera angle is not appropriate for area-type defects (such as cracks), it is easier to miss detection. Although the high cost radiation detection, and X-rays is harmful to humans, to take appropriate protective measures, in the case of not being equipped with industrial CT costing millions, most aluminium die-casting companies prefer X-ray flaw detection machines to detect internal defects in the product.

x-ray

Ultrasonic Inspection Technology

Ultrasonic inspection is a non-destructive testing method that uses ultrasonic waves to inspect the internal defects of aluminum die-cast parts. The height and location of the echo signal displayed on the fluorescent screen can determine the defect’s size, location and general nature. Ultrasonic inspection is more sensitive to area-type defects (such as cracks), less sensitive to volume-type defects (such as holes, inclusions) and near-surface defects. Ultrasonic inspection has greater penetration capacity, lightweight equipment, safe operation, and easy-to-achieve automated inspection. The disadvantage is that it is not easy to check the shape of complex workpieces, requiring a certain degree of finish on the surface of the inspected workpiece. It generally needs to have a coupling agent filling the gap between the probe and the inspected surface to ensure adequate acoustic coupling. In addition, ultrasonic inspection requires a certain degree of experience in the inspection personnel to operate and judge the test results. Ultrasonic inspection techniques include traditional ultrasonic inspection, electromagnetic ultrasonic inspection, phased-array ultrasonic inspection, etc., of which phased-array ultrasonic can achieve rapid detection and high detection efficiency. Wang Yajun uses ultrasonic phased-array technology to study the defect detection of aluminum alloy die-castings, build a test platform for defect detection of aluminum die-castings, and detect different types of defects.

Penetrant inspection technology

Penetrant inspection, also known as penetrant flaw detection, is a nondestructive detection technique for defects near the surface of aluminum die-cast parts, which is powerless against deep internal defects. The method is simple, practical and efficient and is a required nondestructive testing method. The defect display of penetration detection is intuitive, can roughly determine the nature of the defect, detection sensitivity is high, low cost, for the shape of complex cracks, loose inclusions and other defects can also be a comprehensive detection, easy to use on-site. To avoid making other nondestructive testing left behind by the contaminants blocking the defect discontinuity, penetration testing should be the first implemented. Because penetration testing generally requires chemical detection agents such as penetrant, cleaning agent and developer, which negatively impacts human health and the environment.

Industrial CT inspection technology

With more and more critical, complex aluminum die-casting parts in the automotive application, the internal quality of aluminum die-casting parts put forward higher requirements, need to control the internal defects strictly, the traditional ray detection and ultrasonic detection can achieve internal defect detection. Still, in recent years the rise of industrial CT detection technology has been more advantageous. Industrial CT inspection is a high-precision advanced non-destructive testing technology that can be the three-dimensional reconstruction of the workpiece without touching and destroying the workpiece. The reconstruction results can visually distinguish the internal details of the workpiece under test, such as the internal components of the workpiece to be tested cut structure, material, the presence of defects at the cut surface, as well as the internal defects of the workpiece shape, size, location and other information, and the target information inside the workpiece is clear It is the best means of non-destructive testing and evaluation of aluminum die-casting parts, which other interfering objects will not block.

Industrial CT inspection technology

Reference to national standards for non-destructive testing of internal defects

ASTM International

ASTM E155-20 Standard Reference Radiographs for Inspection of Aluminum and Magnesium Castings

ASTM E1030-2005 Standard Test Method for Radiographic Examination of Metallic Castings

ASTM E505-2001(2006) Standard Reference Radiographs for Inspection of Aluminum and Magnesium Die Castings

ASTM E1814-96(2002) Standard Practice For Computed Tomographic (CT) Examination Of Castings

ASTM A609/A609M-1991(2002) Practice For Castings, Carbon, Low-Alloy, And Martensitic Stainless Steel, Ultrasonic Examination Thereof

ISO - International Organization for Standardization

ISO 11971:2020 Steel and iron castings — Visual testing of surface quality
ISO 4993-2015 Steel and iron castings — Radiographic testing
ISO 9916:1991 Aluminium alloy and magnesium alloy castings — Liquid penetrant inspection

DIN - German Institute for Standardization

DIN EN 1371-1-2012 Founding – Liquid penetrant testing – Part 1: Sand, gravity die and low pressure die castings
DIN EN 12680-1-2003 Founding – Ultrasonic examination – Part 1: Steel castings for general purposes

European standard 

EN 12680-1-2003 Founding – Ultrasonic examination – Part 1: Steel castings for general purposes
EN 1371-2-1998 Founding – Liquid penetrant inspection – Part 2: Investment castings

AFNOR - Association française de normalisation

NF A04-162-1-2003 Foundry products. Steel castings. Ultrasonic inspection
NF A04-191-2-2015  Giessereiwesen – Eindringprüfung – Teil 2: Feingussstücke

JIS standard

JIS G0585-2002 Radiographic inspection for steel castings
JIS G0581-1999 Methods of radiographic examination for steel castings

Conclusion

Non-destructive testing techniques for aluminum die-casting parts for automobiles have their characteristics. X-ray inspection is more sensitive to volumetric defects (such as holes and inclusions) and has a high detection rate, but it is easier to miss area defects (such as cracks). Ultrasonic inspection is more sensitive to area-type defects (such as cracks), less sensitive to volumetric defects (such as holes, inclusions) and near-surface defects. Infiltration detection of the complex shape of the crack, loose inclusions and other defects can also be a comprehensive detection and easy to use in the field. Industrial CT inspection can visually distinguish the internal defects of the workpiece shape, size, location and other information, is the best means of nondestructive testing and evaluation of aluminum die-casting parts, and with the rapid development of big data, cloud computing technology, many companies have begun to study the intelligent detection of defects methods to rely on a large amount of actual data as a defect identification, improve efficiency while ensuring the quality of detection, which is also the future of defect detection technology This is the inevitable trend of the future development of defect detection technology.

Aluminum die-casting internal defect nondestructive testing is mostly a general method class standard, which requires the preparation of inspection process protocols and testing and result evaluation according to the acceptance criteria specified in contracts, technical conditions, drawings or other agreements, etc. The customers and manufacturer need to  clear internal defects inspection methods and acceptance indicators, the formation of a complete system to ensure product quality better. 

Finding a great die-casting manufacturer is important to satisfy the requirements. 

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