0.3C-CrMoV(ESR) grade steel is considered as a potential candidate for realizing solid booster cases in aerospace applications due to its high strength coupled with toughness. To obtain the maximum weld efficiency, these steels are welded using filler wires of the same chemical composition. However, due to inherent corrosive nature, bare filler wires could not be stored for long time. To overcome this problem, a suitable coating technology using copper as coating material has been developed which provides uniform, adherent and thin coating on the wire and do not impair the required mechanical properties of weldment. In order to alleviate the gas pick-up issues associated with electro-plating technique and considering the product configuration, immersion coating technique was adopted. Process optimization to obtain a uniform and adherent coating on wire spools was carried out. Wires were also analysed for coating thickness, adhesion and gases (O, H, N). Coating has been observed to be adherent and gas content were found within the aerospace specification. Further, weldment realized using coated filler wire shows similar quality of weldments and properties as obtained through uncoated filler wire, confirming this process and coating can be adaptable for storage of filler wire.

Mechanical properties at room temperature as well as at cryogenic temperatures (77K, 20K) were evaluated on parent metal as well as electron beam weldments of α–Ti (commercially pure grade I) in this work. The strength of the weldment as well as parent metal increased with decrease in temperature. Weld efficiency >95% at all the temperatures (ambient, 77K, 20K) was observed. Microscopy revealed single phase α in parent metal, which transformed to widmanstatten martensitic structure with feathery morphology in the weld.Increase in dislocation density near the failure tip of tensile test specimen and presence of dimples were observed indicating ductile failure/ sufficient amount of ductility even at 20K.

18%Ni maraging steel (M250) is used in the form of welded hardware in launch vehicles. GTAW process using W2 filler wire is being followed to realise this hardware. As compared to M250, W2 filler has a lower amount of Ti and Mo. Al content is increased to 0.4 wt.% to impart high fracture toughness to the weldment. In recent past, manufacturing industries are experiencing a severe problem of slag formation in M250 weldments. Slag can be removed only by extensive inter-pass grinding. This type of slag formation in TIG welding is an unusual phenomenon and has not been reported for maraging steels. This work attempts to understand the root cause of slag formation. Extensive welding trials have been carried out on M250 steel using W2 filler wire to simulate the slag formation. The slag is hard and adherent to the weldment and it cannot be removed by simple wire brushing. Detailed analyses of slag, M250 base metal, and filler wire were carried out using SEM-EDS and XRD. Slag is found to be a variant of calcium aluminate. The main constituents are alumina (Al2O3) and calcium oxide (CaO). Based on the above observation, different contributing factors for the formation of slag and their role in the severity of slag formation are discussed. Various possible solutions to overcome this problem are also addressed.

0.3C-CrMoV(ESR) steel is an ultrahigh-strength low-alloy steel developed primarily for use as a cost-effective material for aerospace pressure vessel applications. These large-sized (empty set 2,800 mm) pressure vessels are manufactured by auto Tungsten Inert Gas (TIG) welding of plates/end domes. During the welding process, defects are likely to be generated, which will reduce the strength of the material. Therefore, residual strength in the presence of a possible missed defect has to be analyzed to qualify the pressure vessel for aerospace applications. In order to evaluate the residual strength and fracture toughness of welded specimens using surface crack tension specimens, mechanical tests were conducted as per the ASTM E740/E740M-03(2010) standard, Standard Practice for Fracture Testing with Surface-Crack Tension Specimens. Plates of 7.6 mm were TIG welded with filler wire matching the chemical composition of the base metal. The welded plates were hardened at 920 +/- 10 degrees C and tempered at 505 +/- 5 degrees C. The material exhibited a 0.2 % proof strength of 1,417 MPa, ultimate strength of 1,466 MPa, residual strength (sigma(r)) of 1,301 MPa, and apparent fracture toughness (K-Ie) of 86.5 MPa root m in the presence of a surface crack, sized 6 by 2 mm.

0.3%C–CrMoV steel were processed through electric arc furnace melting followed by electro slag refining. 2800 mm diameter class of rolled rings and 8 mm thick plates required for fabrication of solid rocket booster motorcase were realized. Tensile and fracture toughness properties were evaluated as a part of characterization of the steel. Low fracture toughness in initial melts was investigated using optical and scanning electron microscopy. Modifications in methods of alloying additions/processing were suggested and incorporated to achieve the desired mechanical properties in industrial scale melts. Process was also fine-tuned by incorporating additional thermomechanical working and heat treatment cycles to achieve the required mechanical properties. Hardening cycle of 925 °C for 1 h followed by oil quenching and tempering cycle of 505 °C for 2 h followed by oil quenching was found to result in optimum combination of mechanical properties. Repeatability in processing and consistency in achieving the mechanical properties of the steel at industrial scale was demonstrated by processing 24 melts of 6 tons each into large size rings and plates.

Ti6Al4V owing to its high specific strength and Inconel-718 due to its high strength and oxidation resistance at elevated temperatures, have been used in Reusable Launch Vehicle-Technology Demonstrator (RLV-TD). Achieving required properties in large cross-section aerospace quality forgings of these alloys is challenging and has not been reported earlier. Hence, thermo-mechanical processing cycles have been devised to realize forgings of similar to 1400 mm length in these alloys and successfully used in RLV-TD. Forgings of these alloys having desired microstructures and ultrasonic quality of Class-A1 as per AMS 2630 B standard have been realized. This article discusses the processing challenges and solutions thereof.

0.3 % C–CrMoV (ESR) steel has been proposed as an alternate cost effective material for fabrication of solid boosters for satellite launch vehicles. PAW studies on 7.8 mm thick 0.3 % C–CrMoV (ESR) steel have been conducted with filler wires of two different compositions. Welding parameters have been established for the steel. Welding has been carried out with stainless steel and copper back-up bars and effect of the same has been evaluated. Weldment has been characterized through optical microscopy, microhardness and mechanical properties evaluation. It is found that filler wire chemistry (low carbon content) does not significantly affect the properties of weldments subjected to post weld hardening and tempering treatment. It is attributed to low dilution in thicker plate welding and diffusion of carbon from base metal side during hardening treatment of the weldment. Use of different back- up bars also show similar results, except for process difficulties noted while using stainless steel. Weld efficiency >85 % has been observed with weldments having microhardness in the range of 480–520 VHN.

Gas tungsten arc (GTA) welding of Ti alloy Ti6Al4V is carried out in vertical-up direction. Weld parameters for the Ti6Al4V alloy were developed using Ti6Al4V (ELI) alloy filler wire and following two pass welding process. X-ray radiography was carried out to ensure the soundness of the weld. Tensile strength of the weldment was evaluated and microstructure characterization was carried out. It is observed that specimens mostly failed in heat affected zone (HAZ) area toward parent material with occasional failure at the weld. Microhardness mapping and microstructural analysis revealed HAZ as the weaker zone, where dissolution of α and formation of β have initiated. Due to moderate cooling rate at this zone, microstructure remained α–β, whereas weld microstructure is found to have martensitic α′ resulting in an increase in the microhardness. Yield strength (YS) of weldment is found to be more than 90% of parent metal (PM) and also reduction in elongation is noted. Fractography observations of failed specimen away from the weld show mainly ductile failure. Weldment failure fractography shows the presence of dendrite indicating failure near the fusion line.

The effect of nitrogen on the electrochemical corrosion and nanomechanical behaviors of martensitic stainless steel was examined using potentiodynamic polarization and nanoindentation test methods. The results indicate that partial replacement of carbon with nitrogen effectively improved the passivation and pitting corrosion resistance of conventional high-carbon and high-chromium martensitic steels. Post-test observation of the samples after a potentiodynamic test revealed a severe pitting attacks in conventional martensitic steel compared with nitrogen-containing martensitic stainless steel. This was shown to be due to (i) microstructural refinement results in retaining a high-chromium content in the matrix, and (ii) the presence of reversed austenite formed during the tempering process. Since nitrogen addition also resulted in the formation of a Cr2N phase as a process of secondary hardening, the hardness of the nitrogen-containing steel is slightly higher than the conventional martensitic stainless steel under tempered conditions, even though the carbon content is lowered. The added nitrogen also improved the wear resistance of the steel as the critical load (Lc2) is less, along with a lower scratch friction coefficient (SFC) when compared to conventional martensitic stainless steel such as AISI 440C.

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.

Titanium alloy Ti6Al4V is used for manufacturing pressure vessels of launch vehicle in view of its excellent corrosion resistance and fabrication characteristics. This material also offers advantage of good strength to weight ratio necessary for space application. This paper describes the experience gained in the technology development of hemispherical forgings of titanium alloy Ti6Al4V. To achieve α-β microstructure and mechanical properties in the final product, hot working of the alloy from the stage of forging of cast ingots to finish working was carried out in the (α-β) region. The manufacturing parameters are established through state-of-the-art technology and capabilities. Process technology for processing of the hemispherical dome has been established and the same has been presented here.

Nickel-based superalloy Inconel 625 is widely used in aeronautical, aerospace, chemical, petrochemical and marine applications due to its good mechanical properties, weldability and resistance to high temperature corrosion on prolonged exposure to aggressive environments. It is a solid solution strengthened medium strength superalloy, which contains chromium, molybdenum and niobium as alloying additions. Considering the chemistry and specification requirements of the alloy, it was processed through vacuum induction melting (VIM) process followed by electro slag remelting (ESR) route to obtain alloy with controlled gas and inclusion contents. Homogenisation cycle was selected and was carried out at 1170°C temperature to obtain uniformity in chemistry and microstructure. Chemical homogeneity was confirmed through analysis of samples from top, middle and bottom of the secondary ESR ingot. Hot working range was decided considering the flowability of superalloy and the same was carried out under close monitoring of temperature and with specified amount of reduction per stroke. Intermediate reheating and reduction during forging was noted to be an important aspect so to avoid cracking during forging. Processing parameters were established to obtain forgings of different thicknesses with sound ultrasonic quality. Microstructure analysis revealed single phase austenitic grain structure with ASTM grain size no. 4-7, confirming that material has undergone sufficient amount of mechanical working. Mechanical testing was carried out and the mechanical properties were found to be meeting the requirement. Present paper provides details of melting process selection, thermomechanical processing and characterization of the superalloy to achieve the targeted mechanical properties.

Strain hardening is one of the important strengthening mechanisms, which plays significant role in processing and application of metals and alloys. For non-heat treatable alloys, it becomes more important. Its effect is different in different metals and alloys and accordingly specific process and application regime are selected. A large variety of metals and alloys from the family of light alloys (Al, Ti based), high strength steels and high temperature alloys (Co, Ni and Nb based) are used in aerospace systems. This paper analyses importance of strain hardening phenomenon in these alloys. Attempts are made to explain the differential behaviour of various alloys in governing the tensile to yield strength ratio along with % elongation. Role of temperature in this behaviour is also included.

Austenitic stainless steels are extensively used as structural materials for various aerospace systems. Nitrogen containing stainless steels have special role due to their austenite stabilization tendency down to subzero temperatures, improved strength and resistance to sensitization. Primary processing of nitrogen containing cryogenic grade stainless steel 202 has been carried out through two different melting routes viz. (1). conventional melt route of electric arc furnace (EAF) melting followed by vacuum oxygen decarburization (VOD) & vacuum degassing (VD) and other one through (2). vacuum induction melting (VIM) followed by ESR. Chemical analysis and macrostructure analysis was carried out on the samples drawn from these billets. Homogenization and thermomechanical processing parameters were selected and the same were followed for the ingots made through both the melt routes. Mechanical properties evaluation (including tensile properties at subzero temperature of 77K) and micro structure characterization of the products realised from all the two melt routes were carried out. It is observed that, both the melt routes could result in achieving the required aerospace quality of alloy with respect to the chemical composition, metallurgical and mechanical properties. This paper confirms that any of the melt routes studied herein can be adopted according to availability of the facilities. The process development and characterization of the steels processed by conventional EAF+ VD & VOD and VIM+ESR melt routes is presented in this paper.

Material with ultra-low coefficient of thermal expansion (CTE) is required for mounting camera and other optical elements in satellite systems. Invar (64 Fe 36Ni) has been the work-horse material for this purpose. In recent years, modified version of conventional invar i.e. Superinvar with 5% cobalt (replacing 5% nickel) is being used to further bring down the errors in camera mountings due to thermal expansion. Processing of this alloy poses many challenges due to its requirement of ultra-low CTE. In the present work, melting and thermomechanical processing parameters were selected to meet the specified requirement of the alloy. The alloy was melted through vacuum induction melting process to obtain uniform and homogeneous chemistry and properties. Virgin raw material was used to achieve lowest carbon and manganese contents. Chemical composition thus obtained is found to be within the specification. Material was hot worked to refine the microstructure. Three different sizes of forged blocks were produced. Hot worked material was heat treated to obtain desirable and stable microstructure. Heat treatment cycle for stabilization was selected and used to retain carbon in the solution and minimize temporal growth. Mechanical properties (tensile strength and modulus of elasticity) and physical properties (CTE, thermal conductivity) were evaluated. Properties were found to be meeting the specification. It is observed that the material shows uniform single phase austenitic microstructure. The paper presents details of the process selection and challenges in processing of this alloy to obtain the targeted CTE < 0.6x10 -6 per °C in the temperature range of 25°C to 150°C along with other desired mechanical properties.

50CrV4 spring steel is a tough, shock resisting, shallow hardening chromium vanadium steel having high fatigue and impact resistance in the heat treated condition. It is used extensively in gears, pinions, springs, shafts, axles, pins, bolts, etc., which require high modulus of resilience. The alloy was realised through conventional melt route of electric arc furnace (EAF) followed by ESR. The application of the alloy is limited to a section thickness of 15mm [1]. Hence obtaining optimum mechanical properties becomes a challenging task. In this study, the hardening as well as tempering operations were limited to 15mm thickness. The samples from the alloy were subjected to hardening at 860°C for 1.25 h. and oil quenching to room temperature followed by tempering at four different temperatures of 250, 300, 370 & 450°C for 3 h. each with oil quenching to room temperature. It was found that the alloy exhibited good combination of strength and ductility when tempered at 450°C. Microstructural study revealed the presence of fine tempered lath martensite along with the presence of a very small amount of delta ferrite along prior austenitic grain boundaries.

Development of microstructure in the weld pool as well as in the partially molten zone is influenced by weld geometry and cooling of the weld metal besides heat input. Evolution of refined microstructure has definite advantage in AA2219 weldment to restrict the crack path. In the present paper, mechanisms of microstructure development in single pass and multipass gas tungsten arc welding of Al-alloy AA2219 alloy have been studied. Role of backing bar material and weld configuration in development of morphology and growth of microstructure have been discussed.

0.3C-CrMoV(ESR) steel is an ultra-high strength low alloy steel indigenously developed by ISRO for space applications. The steel is used in the form of rings of 2.8 m diameter also. In this paper, the effect of tempering temperature on ring rolled steel for the best combination of fracture toughness and strength properties is studied. The tensile properties and fracture toughness of the steel were evaluated in the as quenched and tempered conditions through the specimens drawn in radial direction of the ring segment. Five tempering temperatures were used in the study: 200, 450, 475, 500 and 510°C. Tensile strength of the steel showed continuous decrease with increasing tempering temperature, but yield strength increased reaching maximum when tempered at 450°C and further decreased with increasing tempering temperature. The elongation was higher for higher tempering temperature. The strain hardening exponent decreased with increasing tempering temperature. The fracture toughness test results showed that tempering between 475 and 510°C exhibited better combination of fracture toughness and strength.

The present investigation is on synthesis of in situ Al-alumina composite and to evaluate the effect of equal channel angular pressing on the refinement of the grain structure and enhancement in the hardness and the strength. The billets pressed in as cast condition has shown cracks during first pass. The billets pressed immediately after solution treatment for one pass and followed by ageing treatment immediately after pressing exhibited very high hardness of 125BHN against 95 BHN to that of the T6 condition of 6061 aluminium alloy. The microstructural refinement from 35 µm to 11 µm is obtained in annealed and ECAP 2 pass condition.

Medium carbon low alloy Ni-Cr-Mo steel is used in the fabrication of aerospace fasteners. It finds application in different heat treated conditions to meet the desired strength level. The alloy was realized through double melting route. Heat Treatment studies have been carried out by following different tempering temperatures to obtain varying strength levels ranging from 1200MPa to 1400MPa. Microstructural analysis has been carried out to find out reasons for variation in mechanical properties. Tempering cycle has been suggested to obtain fully tempered martensitic structure. This paper presents the different hardening and tempering cycles studied to obtain the desired strength level for the intended application.