Tensile and Fracture Properties of Aluminium Alloy AA2219-T87 Friction Stir Weld Joints for Aerospace Applications

Metallurgical and Materials Transactions A - The purpose of the present work is to study the mechanical behaviour of AA2219-T87 friction stir weld (FSW) joints at different temperatures (Room...     

Failure Analysis of AISI 440C Steel Ball Screws Used in the Actuator System of a Satellite Launch Vehicle

In view of its excellent wear and corrosion resistance, AISI 440C steel is the material of choice for the fabrication of ball screws used in actuator systems of satellite launch vehicles. During the routine acceptance test of a ball screw, longitudinal cracks were observed at the shaft location of the ball screw. The optical microstructure of the ball screw material (AISI 440C) revealed the presence of aligned carbides (carbide banding). Fractographic observations revealed the cracking to be along the carbide bands. Based on detailed optical and scanning electron microscopic observations, the cracking of the ball screws was attributed to the carbide bands.     

Metallurgical Failure Analysis of Cracked Forward Cylinder Head of Advanced Linear Actuator for Aircraft

Precipitation-hardened stainless steel of 15-5PH grade was used for the fabrication of forward cylinder used in advanced linear actuator of aircraft. The component had undergone a series of qualification tests. During one of the accelerated fatigue tests, an external leak was observed after experiencing more than 1,50,000 cycles of fluctuating stress. Subsequent to disassembly, the leak was found to be because of the hair line cracks in the forward cylinder. The cracked component was subjected to a detailed metallurgical investigation to understand the cause of failure. The failure analysis revealed that the fracture initiated at the hard chrome layer and propagated under cyclic loading. This premature failure of the component was attributed to the surface roughness and the presence of the hard chrome layer at the corner contour of the component. This article brings out the details of the investigation carried out and the results achieved.     

Hydrogen-induced intergranular stress corrosion cracking (HI-IGSCC) of 0.35C–3.5Ni–1.5Cr–0.5Mo steel fastener

This paper brings a failure case study of high strength 0.35C–3.5Ni–1.5Cr–0.5Mo steel fastener, which failed due to hydrogen-induced intergranular stress corrosion cracking (HI-IGSCC). 0.35C–3.5Ni–1.5Cr–0.5Mo steel in hardened and tempered condition, meeting the specified axial tensile stress rating of 1250 MPa is widely used as fasteners in space programmes.In the course of assembly of the structural parts of a satellite launch vehicle, 10 nos of fasteners developed cracks on tightening using a torque wrench set to 6 N m torque surprisingly.Also some fasteners, which were under assembly load of more than 6 months in the same vehicle assembly, were found to be cracked.The failure was attributed to hydrogen-induced intergranular stress corrosion cracking (HI-IGSCC). The details of the analysis and mechanism involved in the HI-IGSCC are presented in detail.Detailed metallurgical analyses of the cracked fasteners support the successive steps of the corrosion enhanced plasticity model, which is based on a local softening in the SCC crack region. The mechanism of a dislocation pileup ahead of a crack under corrosion and stress due to diffusing hydrogen promotes stress concentration against micro-obstacle and caused failure.     

Effect of Prior and Post-Weld Heat Treatment on Electron Beam Weldments of (α + β) Titanium alloy Ti-5Al-3Mo-1.5V

Titanium alloy Ti5Al3Mo1.5V is used in the fabrication of critical engine components for space applications. Double vacuum arc re-melted and (α + β) forged blocks were sliced into 10-mm-thick plates and subjected to electron beam welding (EBW) with five different variants of prior and post-weld heat treatment conditions. Effects of various heat treatment conditions on the mechanical properties of the weldments have been studied. The welded coupons were characterized for microstructure, mechanical properties, and fracture analysis. An optimized heat treatment and welding sequence has been suggested. Weld efficiency of 90% could be achieved. Weldment has shown optimum properties in solution treated and aged condition. Heat-affected zone adjacent to weld fusion line is found to have lowest hardness in all conditions.     

Metallurgical Analysis of Cracked AA7075 Aluminum Alloy Component Used in Control System of a Satellite Launch Vehicle

Journal of Failure Analysis and Prevention - High-strength aluminum alloy AA7075 forgings are widely used in various control system components of liquid engines of satellite launch vehicles. In one...     

Failure Analysis of AISI 304 Stainless Steel Sleeve Used in High-Pressure Hose Assembly of Actuation System of a Satellite Launch Vehicle

Hydraulic control actuation systems in launch vehicles use flexible hoses for transmission of forces to actuators at different locations. The hose ends are connected to the interface adapters by AISI 304 stainless steel sleeves. These sleeves are assembled to nipple end fittings of the high-pressure hose by a swaging operation to obtain leak-proof hose assemblies. The swaging operation is carried out at room temperature by a hydraulically operated swaging machine. Performance of the system as a whole requires strict adherence to the quality standards maintaining the critical interface dimensions within specified tolerances and ensuring the compression given to the swaged joint through the sleeve. If the joining forces are less, this can result in loose joints and gross leakage, whereas excessive compression can cause damage to the parts of the system namely the sleeve, stainless steel braid, PTFE or the adapter. The swaged joint is a single-point failure mode, indicating that a failure in the joint can lead to mission failure. It has to withstand harsh environments such as high pressure, low and high temperature extremes, fatigue loading due to pressure cycling as well as vibration. Therefore, as part of design verification, a detailed qualification plan has been prepared and the hoses were tested as per the plan. As per the test plan, these hoses undergo acceptance tests which include pneumatic and hydraulic pressure testing. During acceptance testing of hoses, one of the hose sleeves was found to be cracked. The cracking was observed after carrying out the pneumatic test when the assembly was being prepared for hydraulic pressure set-up. This paper describes the detailed metallurgical investigation carried out to understand the cause of failure and actions taken to avoid similar failures.