**1. Introduction**

A failure occurs when an engineering system, a mechanical component, an engineering material, or a process cannot correctly perform its intended or design function. When a system or component fails, it causes unimaginable disruption of operations and services; sometimes, it attracts or incurs a legal tussle and a stringent warranty claim. On several levels, failure can be described as (i) a loss of function, implying that the component or system works but cannot accomplish the designed function; (ii) a loss of operational life, that is, when the component or system performs its function but is unsafe or unreliable to operate; and (iii) an inoperable, meaning that the system or engineering component is completely unusable [1]. The above definitions categorize the failure of an engineering system, component, material, machine, assembly, or process. However, an engineering system or components can experience a mechanical failure in service due to design insufficiencies, maintenance deficiencies, manufacturing and material imperfections, service abuse and overload, and a hostile service atmosphere [2–4].

Structural and engineering components are generally used to produce automobiles, aircraft, buildings, power generation plants, jet engines, military equipment, manufacturing plants and marine equipment. These components are made from different materials, such as gray cast iron (GCI), steel, aluminum alloys, titanium alloys, etc. Engineering components are typically exposed to various externally applied shear and perpendicular stresses [5]. These engineering components habitually function under changeable amplitude recurrent loadings in service, although they are designed considering the level of static stress or constant amplitude fatigue strength [6]. In many cases, failure of components may lead to fatalities, property loss, degrading of the company image or loss of credibility, and many legal issues; therefore, it is of great concern to the industry [4]. Failure of some engineering components and accessories used in various engineering applications, such as railway wheels, automotive brake discs, engine cylinder heads, crankshafts, foundation bolts, chains and hooks, cranes, conveyors, and excavators, have been studied to establish the root cause of failure [6–14].

An automotive brake rotor is a critical component of a brake system designed to reduce the acceleration of or stop vehicles in motion. The fundamental comprehensive responsibilities of vehicle braking systems are to decrease the car's speed, bring the vehicle to a standstill, prevent spontaneous acceleration during downhill driving, and keep the car stationary when stopped [15, 16]. Automotive brake rotors have been generally produced from GCI material for several decades because of their inherent properties. Besides the automotive, many other industrial sectors, such as wind power, agriculture, machines, and tools, use cast iron parts extensively. Several engineering components of complex geometries and thin sections that require a combination of mechanical and thermal properties to achieve the desired function are commonly produced from cast iron materials. The existence of flaky graphite in their microstructure offers good thermal conductivity, while the pearlitic matrix is usually accountable for its mechanical properties. GCI material has the essential attributes required for the functioning of an automotive disc brake. It is a material of choice for brake rotors due to its excellent friction properties, good thermal conductivity and castability, retain strength at elevated temperatures, relative ease of manufacture and thermal stability with excellent damping capacity [11, 17–19]. However, the significant disadvantages of using GCI brake rotors include high density, susceptibility to corrosion and the propensity to noise and vibration issues. Despite these drawbacks, GCI brake rotors remain the most sought in the automotive industry to manufacture brake discs rotors due to their relatively cheap cost, inherent properties, and wellknown production process.

The automotive brake discs are usually produced through a metal casting process technique. A sand mold casting is used to manufacture brake rotors, where four to six brake discs are simultaneously cast in a mold. Cast components may look perfect visually, but occasionally they may contain concealed inadequacies which might not be noticed until the part fails in service. Different factors could influence the failure of an engineering component; these factors may include (i) hidden manufacturing defects/imperfections, (ii) poor design, (iii) the type and size of load the component is exposed to, (iv) inappropriate raw materials, (v) improper repair or maintenance, and (vi) the environmental conditions under which the component served. However, cast components' functionality strongly depends on the soundness of the castings. Cast components are consequently required to be virtually free of imperfections that can impair their quality and lead to early failure of castings, failed machine tools, or poor mechanical properties of cast components. According to the International Committee of Foundry Technical Association, casting defects are classified into

#### *Root Cause Failure Analysis of Castings: A Case Study of a Brake Rotor DOI: http://dx.doi.org/10.5772/intechopen.107950*

seven categories - metallic projections, cavities, discontinuities, defective surfaces, incomplete casting, incorrect dimension, and inclusions or structural anomalies [20]. The operation of the foundry process is a complex multi-step process with varying technical levels. Therefore, the final quality of the cast products can be influenced by the operator's skills, defective pattern design, improper metal melting, adopted quality management system and equipment, defective molding material, incorrect quality of raw materials used, improper mold venting, casting processing problems, and improper service condition and maintenance of the available equipment. If the defects in cast products are not effectively inspected, it will impair the quality of the manufactured products and hence lead to unsatisfied customers due to a faulty product. Generally, the most severe defects that can serve as stress raisers or crack promoters in cast components include pre-existing cracks, internal voids, and nonmetallic inclusions.

Casting inclusions defects can be defined as nonmetallic and occasionally intermetallic phases embedded in a metallic matrix. They are frequently simple oxides, sulfides, nitrides, or their complex compounds in ferrous alloys and can include intermetallic phases in nonferrous alloys. In almost all instances of metal casting, they are considered detrimental to the cast component's performance [11, 21]. Casting inclusions are subsurface defects, which may sometimes be detected during the machining operations or even, in many cases, remains undetected until the component fails in service. Nonmetallic inclusions can adversely influence the mechanical properties of castings because they act as a stress raiser. Some mechanical properties are more sensitive to inclusions than others; for instance, elongation or reduction in area is very sensitive to the presence of inclusions and generally adjusted more significantly than ultimate tensile strength. Therefore, cast products use ductility specifications as standard quality-control indices. Inclusions defects have different chemical roots and negatively influence the cast products' mechanical properties, such as machinability, corrosion resistance, fracture toughness, and formability. Inclusions are classified into indigenous or exogenous, depending on their source [11, 21]. The inclusions derived from external sources, such as slag, dross, ladle lining, eroded and entrapped mold and refractories materials, are classified as exogenous; at the same time, those that are native, innate, or inherent in the molten metal treatment process are known as indigenous inclusions. Exogenous inclusions are also derived from ferroalloys, flux materials, and other starting materials that do not float to the surface of the liquid metal or dissolve in it [22]. Sometimes, exogenous inclusions are visible to the naked eye at the casting surface. They may be seen beneath the peripheral casting surface when the casting is sectioned if they have had insufficient time to float out or settle due to density differences concerning the molten metal. Indigenous inclusions, however, are of micro size and can easily be identified under the microscope, and they are frequently distributed uniformly within the casting's microstructure. These inclusions are the products of the liquid melt reactions with deoxidizers such as silicon, manganese, and aluminum particles or during desulphurization, leaving some residual oxide inclusions in the casting. The distribution of indigenous inclusions in a grain boundary in the microstructure of the cast components can severely impair their mechanical properties [21] and reduce the components' service life.

To unravel how a cast product failed correctly, the failure analysis techniques adopted for the castings must include a systematic understanding of how the component was made and processed. However, the scope of the failure examination depends on the problem's definition. Still, in all failure analyses, the understanding of the reasons (how and why) for a casting failure can only be determined if the pertinent

background information is collected, thorough examinations are conducted, representative castings are inspected, the proper material assessments are accomplished, and the service conditions to which the cast components are subjected to are undoubtedly understood [23]. The brake rotor under investigation was newly manufactured and installed in a brand-new car. The rotor had been in service for about one year and three months and covered 10,670 miles before its failure. However, under normal circumstances, a quality set of automotive brake rotors should have an average life expectancy ranging from 30,000 to 70,000 miles traveled, which depends primarily on the size of the vehicle, the way it is driven, and the quality of the brakes (including pads and discs) [11]. The above information suggests the rotor had failed prematurely in service. There are several reasons for undertaking a comprehensive understanding of why and how a cast product failed prematurely. The reasons may include – i) evaluating the effectiveness of an in-process quality-control system, ii) assessing the efficiency of the installation or assembly plant where the part was assembled, iii) improving product design and manufacturing processes; iv) preventing similar problems with identical components, v) absolving a company of liability, and vi) determining and understanding the cause(s) of a failure to resolve financial warranty claims where applicable [23–25].

A failure analysis investigation generally involves collecting and analyzing failure information, identifying the root causes, improving product design, ensuring product compliance and assessing the liability of product failure. Besides, the data collected from the failure analysis can improve component design, regulate maintenance plans, improve maintenance processes where necessary, and improve asset reliability. However, this chapter describes the practical application of root cause analysis in automotive brake rotor failure, details the step-by-step approach to unraveling failure root cause, documents the inspection and characterization of the failed brake rotor, and provides recommendations to avert the recurrence of such a failure in the future.
