Influence of Hot Isostatic Pressing On

Home , Hot isostatic pressing, Spray forming

JMEPEG ASM International DOI: 10.1007/s11665-014-0869-z 1059-9495/$19.00 Inﬂuence of Hot Isostatic Pressing on the Microstructure and Mechanical Properties of a Spray-Formed Al-4.5 wt.% Cu Alloy S. Devaraj, S. Sankaran, R. Kumar, and G. Appa Rao

(Submitted September 6, 2013; in revised form December 4, 2013)

Al-4.5 wt.% Cu alloy was spray atomized and deposited at varied spray heights ranging from 300 to 390 mm. The average grain sizes decreased from 29 to 18 lm and a concomitant increase in the hardness and the 0.2% yield strength (YS) with increase in the spray height. The respective hardness values of SF-300, SF-340, and SF-390 are 451 ± 59, 530 ± 39, and 726 ± 39 MPa and the YS are 108 ± 7, 115 ± 8, and 159 ± 10 MPa. The transmission electron micrographs revealed the morphological changes of the Al2Cu phase from irregular shaped to small plate-shaped and then subsequently to spheroidal shape due to high undercooling encountered during spray atomization with increase in spray height from 300 to 390 mm. The porosity of the spray formed deposits varied between 5 to 12%. Hot isostatic pressing of spray deposits reduced the porosity to less than 0.5% without any appreciable increase in grain size. A dislocation creep mechanism seems to be operative during the secondary processing. A comparison between as-spray formed and hot isostatically pressed deposits exempliﬁes improvement in mechanical properties as a result of elimination of porosity without affecting the ﬁne grain sizes achieved during the spray-forming process.

In a typical spray forming process, the molten metal from Keywords hardness, hot isostatic pressing, microstructure, porosity, spray forming, yield strength the tundish nozzle gets atomized by the high velocity inert gas and deposited over the copper substrate. The microstructure of the spray formed deposit strongly depends on the atomizer design and process parameters such as spray height, melt super- heat, gas-metal ratio (Ref 15-19). During atomization, small 1. Introduction droplets at the periphery of the spray cone solidify faster than the big droplets residing at the centre of the cone due to high surface to volume ratio. While, the higher cooling rates of the Al-4.5 wt.% Cu alloy is a high strength, light weight, atomized droplets results in finer grain sizes of the deposit precipitation hardening alloy widely used for aircraft and (Ref 20-22), the spray typically consists of liquid, semi-liquid automobile applications (Ref 1-4). This alloy processed through droplets and solid particles. However, the solid particles present conventional route exhibits coarse dendritic microstructure on the deposition surface can be critical in controlling the because of the slow cooling rates encountered and hence the microstructure and porosity in the spray deposit (Ref 23, 24). desired mechanical properties cannot be achieved for critical Porosity is inevitable and act as sites for initiation and applications (Ref 5-8). Furthermore, alloys produced via propagation of cracks and deteriorates the mechanical proper- powder metallurgy (P/M) route involves various processing ties such as ductility, yield and ultimate tensile strength. steps such as powder production, compaction, and sintering Therefore, it is most important to reduce the porosity to thereby resulting in larger time consumption and making the enhance the properties of an alloy. The elimination of porosity process very expensive (Ref 9). However, spray forming can however can be achieved by secondary processes such as hot prove to be an alternate technique possessing immense isostatic pressing (HIPing), rolling, or extrusion (Ref 25, 26). potential for the fabrication of dense near-net complex shaped HIPing is a well established technique widely used for defect components through rapid solidification (Ref 10-13). This is an healing/elimination of porosity as well as to improve the attractive manufacturing process since it uniquely combines the microstructural homogeneity in the cast components of any advantages of both ingot as well as powder metallurgy routes. complex geometry. HIP has a greater potential to produce the The process results in refined microstructures, extended solid near-net-shaped components with homogeneous microstructure solubility, elimination of macro-segregation, reduction in and near theoretical density from particulate materials in a micro-segregation and uniform distribution of secondary phases single step (Ref 27). It is well known that HIP involves by careful optimization of process parameters (Ref 14). simultaneous application of high gas pressure and high temperature and the densification occurs during HIPing mainly S. Devaraj, S. Sankaran, and R. Kumar, Department of Metallurgical due to dislocation and diffusion creep mechanisms (Ref 28). and Materials Engineering, Indian Institute of Technology Madras, In the earlier studies, the microstructures of spray-formed Chennai 600036, India; and G. Appa Rao, Defence Metallurgical Al-4.5 wt.% Cu alloy deposits were investigated and the Research Laboratory, Hyderabad 500058, India. Contact e-mail: [email protected]. average grain size varied between 30 and 75 lm depending on

Journal of Materials Engineering and Performance the location within the deposit (Ref 29). The porosity was found to be maximum adjacent to the substrate due to insufficient liquid fraction at the onset of spray forming. It was seen that as spray-forming proceeds, the liquid fraction increases and the porosity reduces at the top of the substrate. The porosity distribution in the deposit was reported to be V-shaped by a mathematical model considering higher solid fraction in the spray at the top of the deposit (Ref 25). The influence of copper content on the spray formed microstructures was reported earlier and the results showed that with increase in Cu content from 4.5-40 wt.%, the grain morphology changed from equiaxed at 4.5 wt.%, to irregular non-dendritic at 15 wt.%, to particulate eutectic at 33 wt.% and to non- faceted growth morphology at 40 wt.% respectively exhibiting a strong dependence on the amount of copper influencing the grain morphology (Ref 30). However, the influence of the grain size on the mechanical behavior of spray formed Al-Cu alloys was not reported in the past, albeit limited work on the microstructural evolution. Also, the influence of secondary processing such as HIPing on spray formed Al-4.5 wt.% Cu Fig. 1 Spray atomization experimental setup alloy on hardness and the tensile properties were not reported in the literature to the best of our knowledge. Hence, in this work, level of 5 9 10À5 Pa. The vacuum sealed capsules were loaded structural characterization in conjunction with mechanical into ASEA-Quitus-32 HIP and the HIP was carried out at characterization were carried out on spray formed and HIPed 490C under a pressure of 83 MPa for 3 h. The heating and Al-4.5 wt.% Cu alloys for varied spray heights. The influence cooling rates of the HIP cycle adopted for these samples were of grain refinement and porosity on the mechanical properties 10C/min. of as-spray formed and HIPed alloys were investigated by carrying out quasi-static uni-axial tensile tests. 2.3 Metallurgical Characterization The polished samples of both as-spray-formed and spray- formed HIPed alloy were etched with KellerÕs reagent to reveal 2. Experimental the microstructures and the specimens were examined by optical microscopy (OM) using a Leitz, Laborlux 12 ME 2.1 Spray forming (Germany) and scanning electron microscopy (SEM) using FEI The as-cast Al-4.5% Cu alloy was melted and superheated Quanta 200 (USA) fitted with lithium-doped silicon energy (200C) in an electrical resistance furnace (schematic shown in dispersive x-ray spectrometer (EDS) of AMETEK Process and Fig. 1) and for each experiment around 1.5 kg was considered. Analytical Instrument. Also, spray-deposited HIPed samples The superheated melt was atomized into very fine droplets of were examined by transmission electron microscopy (TEM) different sizes by using high velocity nitrogen gas and using Philips CM 12 (The Netherlands) operating at 120 kV. consolidated on a copper substrate. The mass flow rate of the The grain sizes of the spray deposits and the HIPed samples melt was 1 kg/min. The spray forming was conducted at three were measured by line intercept method (ASTM E 112-96). different spray heights (standoff distance between nozzle tip The porosity measurements were done using image analysis and the substrate) of 300, 340, and 390 mm. The deposits according to ASTM B-276 standards across the entire section produced at different spray heights mentioned above are of the deposits. designated as SF-300, SF-340, and SF-390 hereafter in the article. Spray deposits of 300 mm in diameter and 16 mm in 2.4 Physical and Mechanical Characterization height at the centre and approximately 2 mm at the peripheral The density of the spray deposits and HIPed samples were region were produced. To prepare rectangular billets of uniform measured by displacement method (ArchimedesÕ principle) thickness, 2 mm from the bottom surface and 4 mm from the using CP124S Sartorius electronic digital-weighing balance. top surface of the deposit and 50 mm from each side of the The hardness of the samples from SF-300, SF-340, and SF-390 circular deposit were machined. For microstructural investiga- and their corresponding HIPed counterparts were measured tion samples were taken from different regions (central to using WOLPERT 402 MVD Vickers hardness tester (USA) periphery) of SF-300, SF-340, and SF-390. using a load of 49 N and held for 10 s. The hardness was measured at different deposition distances across the deposit 2.2 HIP and the average value of six indentations was taken. For tensile For HIP, samples of 150-mm length and 60-mm width were testing, flat samples of 25-mm gauge length and 3-mm cut from the spray-formed material. These samples were placed thickness were prepared according to specifications stated in in a stainless steel sheet metal capsule with a packing of ASTM E8/E8M-04 (Ref 31). Sub-sized tensile specimens were alumina powder to facilitate uniform shrinkage of capsule machined in the longitudinal direction with computerized during HIP. The container of the spray-formed Al-alloy samples numerical control wire cutting electronica sprint cut machine and alumina packing was evacuated and vacuum degassed at (India). Three samples from each of the spray-deposited and 450C for 8 h and vacuum sealed under a dynamic vacuum spray-HIPed samples were tensile tested using MTS 810

Journal of Materials Engineering and Performance material test system machine (USA), at a strain rate of 3.3 9 10À4 sÀ1 at room temperature until fracture.

3. Results

3.1 Physical Properties and Microstructural Characterization The average relative density of the spray-formed samples across the deposit (central and peripheral) for SF-300, SF-340, and SF-390 were 2.41 ± 0.01, 2.35 ± 0.02, and 2.28 ± 0.01 g/cc, respectively. The relative density of the spray-formed samples increased with decreasing spray height. After HIP, the respective densities of the spray deposits were 2.80 ± 0.01, 2.80 ± 0.01, and 2.79 ± 0.01 g/cc, and the increase in relative density after HIP was 13.9, 16.0, and 19.3%, respectively. The average porosity present in SF-300, SF-340, and SF-390 was 5, 8, and 12%, respectively. Subsequent to HIP, the porosity of all the spray-formed samples decreased signiﬁcantly and was less than 1%. The variation in microstructures (representative micrographs) of SF-300, SF-340, and SF-390 approximately taken 7 mm from the substrate are shown in Fig. 2(a), 3(a), and 4(a), respectively. The grain sizes of SF-300 were measured from cross section adjacent to the substrate, middle section, and top section ( 4, 7, and 10 mm from the substrate) and the values were around 28, 31, and 27 lm, respectively. With increase in spray height to 340 (for SF-340), the grain sizes adjacent to the substrate, middle section, and top section ( at 4, 7, and 10 mm) are 23, 24, and 22 lm, respectively. For SF-390, the grain sizes are 18, 20, and 16 lm for the respective deposition distances adjacent to the substrate, middle section, and top section of the deposit. The difference in grain size was rather minimum from the adjacent portion of the substrate to the top Fig. 2 Micrographs of spray-formed Al-4.5 wt.% Cu alloy of section of the deposit. The average grain sizes of SF-300, SF-300, (a) 7 mm from the substrate. Respective micrographs of the SF-340, and SF-390 were 29, 23, and 18 lm, respectively. HIPed samples are shown in (b) The microstructures shown in Fig. 2(a) (SF-300) and 3(a) (SF-

340) consist of smaller and larger grains of different sizes due of as-spray-formed deposit (Fig. 7). The distribution of Al2Cu to different cooling rates experienced by the droplets. The phase of as-spray-formed and HIPed samples are shown in microstructure of SF-390 showed a homogeneous equiaxed Fig. 6(a) and 7(a) respectively. As it is noticed from the microstructure as the spray height was increased from 300 to Fig. 6(a), the fraction of Al2Cu phase in the as-spray formed 390 mm. The typical optical microstructures (representative alloy is higher than that of the HIPed sample (Fig. 7a). micrographs) of the HIPed SF-300, SF-340, and SF-390 Figure 8(a-d), 9(a-d), and 10(a-d) represent the TEM images of samples are shown in Fig. 2(b), 3(b), and 4(b), respectively. HIPed SF-300, HIPed SF-340, and HIPed SF-390 samples, The respective average grain sizes were 29, 23, and 18 lm. respectively. The size, shape, and the distribution of Al2Cu In comparison with as-spray-formed deposits, the variation in phase of the HIPed SF-300, HIPed SF-340, and HIPed SF-390 grain size for the HIPed counterpart is small and the grains are shown in Fig. 8(a) and (b), 9(a) and 10(a), respectively. The were equiaxed. The distribution remains unaffected after HIP. Fig. 9(d) and 10(d) are the corresponding dark-field images of A comparison of grain-size distribution of spray formed Fig. 9(c) and 10(c) confirming the presence of plate-shaped and alloy for three different spray deposits of SF-300, SF-340, and spheroidal precipitates, respectively. The TEM images of SF-390 and respective HIPed deposits are shown in Fig. 5. The HIPed SF-300, HIPed SF-340, and HIPed SF-390 are shown variation of grain size in SF-390 was between 5 to 50 lm and in respective Fig. 8(d), 9(b), and 10(c) exemplifying the 90% of the grains were less than 25 lm, in contrast to SF-340 interaction of dislocations with the precipitates. Figure 8(c) and SF-300 in which the variation was between 5 to 70 lm, exemplifies the presence of dislocation in the HIPed sample. respectively. The spray deposits also revealed characteristic Figure 9(d) and 10(d) exemplify the dark-field images of fine-equiaxed a-Al and Al2Cu phase. The EDS report shown in Fig. 9(c) and 10(c). Fine Al2Cu particles are located along the Fig. 6(a) confirm the presence of Al2Cu phase in an aluminum grain boundaries as indicated in Fig. 10(b). Irregular shaped, matrix, and Fig. 6(b) confirms the distribution of the Al2Cu coarse, and plate-shaped Al2Cu precipitates were observed in phase inside the grain and as well as the grain boundaries. HIPed SF-300 and HIPed SF-340, whereas HIPed SF-390 However, in the SEM images of HIPed samples, the exhibited fine spheroidal and plate-shaped Al2Cu precipitates. distribution of Al2Cu phase is significantly different from that It was observed that as the spray height increases the

Journal of Materials Engineering and Performance Fig. 3 Micrographs of spray-formed Al-4.5 wt.% Cu alloy of SF- Fig. 4 Micrographs of spray-formed Al-4.5 wt.% Cu alloy of 340, (a) 7 mm and from the substrate. Respective micrographs of SF-390, (a) 7 mm from the substrate. Respective micrographs of HIPed samples are shown in (b) HIPed samples are shown in (b) morphology of the precipitates changes from irregular shaped to spheroidal and plate shape.

3.2 Mechanical Characterization The hardness was measured at various distances from the surface adjacent to the substrate and also from center to peripheral region of the samples, and the results were uniform. The Vickers hardness of the spray-formed alloys SF-300, SF-340, and SF-390 were 451 ± 59, 530 ± 39, and 726 ± 39 MPa, respectively. There is a signiﬁcant decrease in hardness for SF-300 in contrast to SF-390. The Vickers hardness value of the respective deposits after HIP was 460 ± 55, 538 ± 40, and 732 ± 37 MPa. The results showed marginal improvement in hardness after HIP. Figure 11 shows the stress- strain curves of SF-300, SF-340, and SF-390 before and after HIP. The 0.2% yield strength (YS), the ultimate tensile strength (UTS), and ductility in terms of % of elongation are given in Table 1. As evident from the plot, the stress-strain behavior of Fig. 5 Grain-size distribution in spray-formed Al-4.5 wt.% Cu the spray-formed alloy for the spray height of SF-300 and alloy (SF-390, SF-340, and SF-300) and respective HIPed samples SF-340 exhibit distinctly lower 0.2% YS of 108 and 115 MPa in (HIPed SF-390, SF-340, and SF-300) for different spray heights contrast to SF-390 of 0.2% YS of 159 MPa. The failure in SF-390 occurred without any appreciable plastic deformation SF-340 after HIP increased from 108 to 140 and 115 to after reaching the 0.2% YS. After HIP, there is a signiﬁcant 150, whereas HIPed 390 exhibits 154 ± 20 MPa. The increase increase in 0.2% YS and UTS. The 0.2% of YS of SF-300 and in 0.2% YS of HIPed SF-300 was 22% compared to

Journal of Materials Engineering and Performance Fig. 6 (a) EDS report of spray-formed Al-4.5 wt.% Cu alloy show- Fig. 7 (a) EDS report of HIPed Al-4.5 wt.% Cu alloy showing the ing the presence of Al Cu phase. (b) Spray-formed Al-4.5 wt.% Cu 2 presence of Al2Cu phase. (b) Uniform distribution of Al2Cu precipi- alloy reveal the distribution of Al Cu with in the grains and along 2 tates in Al matrix after HIP. Inset shows distribution of Al2Cu pre- the grain boundaries. Inset in the micrograph shows the distribution cipitates at low magniﬁcation of Al2Cu phase with-in the grains at higher magniﬁcation

completely filled by the liquid droplets. Shukla et al. (Ref 30) as-spray-formed sample of SF-300. A similar increase of 23% explained this kind of solidification dominates at the early was observed in HIPed SF-340 and the increase observed in stages of deposition resulting in low solidification porosity. HIPed SF-390 is not significant due to a large standard deviation However, with increase in the deposition distance from the of 20 MPa. The UTS of spray-formed deposits after HIP substrate surface, the porosity could increase due to unfilled increased from 150 to 214 MPa, 142 to 220 MPa, and 159 to interstices. 216 MPa. The % of elongation of HIPed deposits of SF-300, SF- The microstructural variations described in Fig. 2(a), 3(a), 340, and SF-390 were 12%, 27%, and 73% higher than SF-300, and 4a are rationalized in the light of solidification behavior of SF-340, and SF-390. the droplets during flight-path and deposition. The various sizes of droplets (40-180 lm) were produced during the spray atomization due to the interaction of gas with the melt. These 4. Discussion droplets were subjected to different cooling rates during their travel and deposition on the substrate. Due to the differences in thermal gradient, the spray comprises solid particles, semi- 4.1 Porosity and Microstructure of Spray-Formed Alloy liquid, and liquid droplets before they impact on a deposited With an increase in the nozzle to substrate distance, an surface. Smaller droplets solidify faster due to high heat transfer increase in porosity is observed due to the absence of liquid rate at the droplet gas interface, resulting in a large under- fraction in the spray as it is reaches the surface of the substrate. cooling and become solid. Singh et al. (Ref 32) discussed the At shorter deposition distances close to the substrate surface critical factor in order to achieve reduced segregation and and due to relatively high liquid fraction, the interstices are refinement of grains. Relatively, larger semi-solid droplets are

Journal of Materials Engineering and Performance Fig. 8 Bright-ﬁeld TEM images of SF-300 showing the presence of Al2Cu phase of different morphologies (a, b) irregular and coarse precipitates with different orientations. Inset shows the corresponding diffraction pattern of b; (c, d) the presence of high dislocation and interaction of dislocations with precipitates

Fig. 9 Bright-ﬁeld TEM images of SF-340 showing the presence of Al2Cu phase of different morphologies (a) plate-shaped and spheroidal precipitates. (b) Interaction of dislocations with precipitates. (c) High dislocation density. (d) Corresponding dark-ﬁeld image of (c). Inset shows the diffraction pattern on (d)

Journal of Materials Engineering and Performance Fig. 10 Bright-field TEM images of SF-390 showing the presence of Al2Cu phase of different morphologies (a) uniform distribution of spherical precipitates. (b) Precipitates located along the grain boundaries. (c) Interaction of dislocations with precipitates. (d) Corresponding dark-filed image of (c). Inset is the diffraction pattern of d fragmented into large number of particles due to impingement of high velocity gas on the deposited surface. Such a large number of disintegrated particles increase the nucleation frequency on the deposit surface. Srivastava et al. (Ref 33) presented the steady state condition of heat transfer is achieved on the deposit surface by reducing the heat transfer through the substrate. This is considered to be an unfavorable condition for the formation of the dendritic growth in the matrix due to lack of constitutional supercooling, finally resulting in non-dendritic equiaxed grain microstructure (Ref 34). The SEM and EDS report (Fig. 6a) confirm the presence of Al2Cu phase in spray- formed alloys. However, the distribution of fine precipitates of Al2Cu (Fig. 6b) phase is also seen within the grains in concomitant to its presence along the grain boundaries in the spray-formed alloy and it can be attributed to higher cooling rates. Since the cooling rate of the droplets of SF-390 was higher than SF-340 and SF-300 it is reasonable to state that the amount of solid fraction in the spray is more. This provides more number of nucleation sites resulting in relatively smaller Fig. 11 Stress-strain diagram of spray-formed and HIPed size equiaxed grains in contrast to SF-300 and SF-340. Al-4.5 wt.% Cu alloy 4.1.1 Precipitate Morphology. Figure 8(a) shows irregular- and plate-shaped precipitates of different size and shape with the effect isotropic interfacial and anisotropic interfacial energies, different orientations. All irregular-shaped grown precipitates respectively. The anisotropy interfacial energy is introduced due seem to be oriented in one direction, in contrast to plate-shaped to the difference in interfacial widths along the semi-coherent and precipitates oriented in two different directions perpendicular to coherent interfaces (Ref 35). The length and thickness of the each other (Fig. 8b). The irregular-shaped precipitates were plate-shaped precipitates depend on whether the precipitates are observed in SF-300 due to sufficient time available for growth of growing or shrinking and interaction with diffusion fields of the the precipitates. The irregular-shaped growth was reduced with neighboring precipitates (Ref 35). The coalescence of precipitates increase in spray height which was confirmed from the TEM occurs when the interfacial energy of the precipitates are reduced. images of Fig. 9(a) and 10(a). The plate-shaped and spheroidal This occurs due to diffusion fields of the two neighboring precipitates shown in Fig. 9(a) and 10(a) were produced due to precipitates of the same crystallographic variant. Vaithyanathan

Journal of Materials Engineering and Performance Table 1 Properties of as-spray-formed and spray HIPed Al-4.5 wt.% Cu alloy

Alloy processing details 0.2 % YS, MPa UTS, MPa Elongation, %

SF-300 108 ± 7 150 ± 2 2.82 ± 0.1 SF-340 115 ± 8 142 ± 7 2.34 ± 0.2 SF-390 159 ± 10 159 ± 10 <0.8 HIPed SF-300 140 ± 4 214 ± 4 3.19 ± 0.2 HIPed SF-340 150 ± 5 220 ± 6 3.20 ± 0.1 HIPed SF-390 154 ± 20 216 ± 17 3.01 ± 0.2 et al. (Ref 35) investigated the effect of isotropic interfacial, pressure at high temperatures. Moreover, the pressure applied anisotropic interfacial energy, anisotropic strain (elastic energy), during HIP was adequate enough to disintegrate the brittle and combined anisotropic interfacial and elastic energy on shape, continuous Al2Cu phase located along the grain boundaries of size, and morphology of the precipitates. The TEM micrographs the as-spray-formed alloys. The disintegrated Al2Cu phase also of HIPed SF-390 (Fig. 10c) revealed the presence of spheroidal- distributed uniformly within the grains and at the grain and plate-shaped precipitates. Understandably, the plate-shaped boundaries (Fig. 7(a) and (b)). precipitates observed in TEM micrographs was found to be h¢ The coarsening of grains was insignificant in HIPed phase based on the aspect ratio (Ref 36). The aspect ratio of the deposits. In order to investigate the influence of pressure on Al2Cu precipitates shown in TEM micrographs (Fig. 8(b), 10(c)) the coarsening effect, samples from SF-300, SF-340, and SF- are reduced from 30 ± 7to11± 4 as the spray height increased 390 were heat treated at 490C in an argon atmosphere under from 300 to 390 mm. The decrease in aspect ratio of the atmospheric pressure for 3 h adopting similar temperature and precipitates could be attributed to strain energy involved when holding time used for HIP. The results of the heat-treated the droplets impact on the deposition surface (Ref 36). More deposits show that, there is no significant grain growth which strain energy was involved when the solid particles (SF-390) could be possibly because of the presence of secondary phases impact on liquid droplets than that of the liquid droplets (SF-300) (Al2Cu) pinning the grain boundaries and preventing any grain impacting on the semi-liquid deposition surface. Gao et al. boundary migration. (Ref 36) examined the effect of strain and strain rate on h¢ precipitates by conducting the experiment on Al alloy sample for 4.3 Mechanical Properties different strain rates. It was reported that from the TEM The high hardness value achieved for the spray deposit of micrographs there is a decrease in the aspect ratio of the SF-390 is predominantly attributed to grain refinement. Despite precipitates as the strain rate is increased beyond the certain limit. the porosity, the reduced grain size with equiaxed grains and distribution of Al Cu secondary phase hard particles in Al 4.2 Microstructure Evolution During HIP 2 matrix contribute to high hardness of the spray-formed alloy. The porosity in all the spray deposits is reduced to less than Precipitate morphology could be another reason for enhancing 1% after HIP. Atkinson and Rikinson (Ref 37) reported the the hardness of the spray-formed alloy. Based on the TEM stresses developed during HIP processes promote annihilation micrographs it is evident that the plate shaped and spheroidal of the pores and achieve relative density near to the theoretical precipitates facilitate higher hardness than the irregular-shaped value of the alloy. During HIP, the plastic flow of the materials precipitates. After HIP, not much change in hardness was occurs in all the directions at high temperature under hydro- observed. During the HIP of Al-4.5 wt.% alloy, some amount static pressure. Typically, creep mechanisms such as Nabarro- of Al2Cu phase is likely to get dissolved in the matrix. The Herring and Coble creep based on dislocations and diffusion evidence for decrease in the volume fraction of Al2Cu phase play a dominant role in annihilating the pores. Prabir and can be observed subsequent to HIP (Fig. 7b). However, the Farghalli (Ref 38) reported that the creep characteristics of a major reason for marginal improvement in hardness of the Al-3.0 Cu alloy in the temperature range of 500-580C and HIPed deposits is that the solid solution strengthening is less coupled with stresses varying between 0.3 and 1.5 MPa yielded effective than precipitation hardening (Ref 40, 41). In contrast a stress exponent n 4.5. Kloc et al. (Ref 39) investigated that to SF-390, the spray-formed alloy of SF-300 and SF-340- 2024 alloy over a temperature range of 250-330C and stresses exhibited significant plastic deformation before fracture. The in the range of 25-50 MPa exhibited a stress exponent 5 increase in YS and UTS of spray-formed alloy directly implying a dislocation-based creep mechanism to be dominant. correlates with refined grain microstructure and distribution of It is obvious that under the conditions employed for HIP, the Al2Cu (Fig. 6b) phase particles. The deformation behavior is alloy is likely to plastically deform via dislocation-based creep associated with interaction of dislocations with grain bound- mechanism. The HIPed samples revealed the presence of aries or particles (Fig. 10b). The presence of equiaxed grains in dislocations which further corroborated the proposed afore- spray-formed alloy acts as a critical barrier against dislocation mentioned mechanism (Fig. 8(c) and (d), 9(b) and (c), and movement and results in high YS and UTS. The dispersion of 10(b) and (c)). second phase particles in a matrix is also responsible for The microstructures shown in Fig. 2(b), 3(b), and 4(b) enhancing the yield and ultimate tensile strength of the spray- indicate that, after HIP the grain size remains unaltered. The formed alloy (Fig. 10a). However it is finally the competition flow lines observed in HIPed SF-390 shown in Fig. 4(b) is between grain refinement, entrapment of intermetallic precip- prominent and not clearly seen in HIPed SF-300 and HIPed itates and porosity that determines the final mechanical SF-340 (Fig. 2(b), 3(b)). This is an indication of the fact that a properties of the spray formed alloy. pronounced deformation that has occurred in SF-390. Spherical Subsequent to HIP, the spray-formed alloy-exhibited sig- pores are closed completely by the application of isostatic nificant increase in 0.2% YS and UTS. Some amount of Cu

Journal of Materials Engineering and Performance Fig. 12 Fracture surfaces after the tensile test (a) SF-300 and (b) HIPed SF-340

Fig. 13 Fracture surfaces of HIPed samples after the tensile test (a) SF-300 and (b) SF-340 seems to dissolve in the a-Al matrix because of dissolution of movement (Fig. 8(d), 9(b) and (c), and 10(c)) and hence Al2Cu phase (Fig. 7b). The increase in 0.2% YS might be not increasing the 0.2% YS. In addition to grain boundary and only due to grain reﬁnement but also due to solid solution solid solution strengthening, the Al2Cu precipitates also strengthening (more amount of Cu is dissolved in Al matrix). contribute to an improvement in 0.2% YS. Signiﬁcant plastic Before HIP the Al2Cu phase is located along the grain deformation was observed in HIPed SF-390 since the Al2Cu boundaries as a continuous phase and is very brittle. When an phase was disintegrated into a number of individual particles alloy is HIPed at high temperature and pressure, the Al2Cu in contrast to its spray-formed counterpart. After HIP, the phase is disintegrated in to number of individual particles and increase in % of elongation could be due to closure of is no longer interconnected to each other (Fig. 7a). After HIP, porosity, distribution, size, and shape of the Al2Cu (Fig. 10(a) the disintegrated Al2Cu phase was uniformly distributed and (c)) particles. The enhancement of 0.2% yield strength within the grains and at the grain boundaries and act like and ductility of the HIPed samples could be directly attributed individual precipitates (Fig. 8(a) and (b), 9(a), and 10(a)). The to the distribution secondary phase particle within the grains precipitates effectively acts as an obstacle to dislocation and along the grain boundaries.

Journal of Materials Engineering and Performance 4.4 Fractography 3. G. Hongli, X. Bin, N. Yuchao, W. Weimin, and S. Bailai, Effects of Different Pouring Conditions on the Microstructure of Al-Cu alloy, The room temperature fractured surfaces of SF-300, SF-340, Spec. Cast. Non-ferrous Alloys., 2006, 26, p 10–13 and SF-390 are shown in Fig. 12. The formation of dimples and 4. E. Chen, Economic and Environmental Evaluation of End-of-Life ridges were the indication of ductile fracture by coalescence of Aerospace Aluminum Options using Optimization Methods, Bachelor of Science Dissertation, Massachusetts Institute of Technology, USA, voids. Few voids were also observed on the fractured surface of 2008, p 1–59 the spray-formed alloy (Fig. 12a). The equiaxed dimples are 5. H.D. Brody and M.C. Flemings, Solute Distribution in Dendritic observed on the fractured surface of SF-390. The size of the Solidification, Trans. Met. Soc. AIME, 1966, 236, p 615–623 dimples and the tearing ridges of SF-390 were smaller than 6. T.Z. Kattamis, J.C. Coughlin, and M.C. Flemings, Influence of SF-300 and SF-340 since the grains were smaller in size and the Coarsening on Dendritic Arm Spacing, Trans. Met. Soc. AIME., 1967, 236, p 1504–1511 Al2Cu phase was located along the grain boundaries. The 7. T.Z. Kattamis, Y.V. Mirty, and J.A. Reffner, Microstructure and fractured surfaces of SF-300 and SF-340 exhibited relatively Segregation in Splat-Cooled Aluminum-Copper Alloy, J. Cryst. large dimples (Fig. 12a). The porosity and secondary phase act as Growth, 1973, 19, p 237–241 favorable sites to initiate the cracks and propagate along the 8. G. Baumeister, B. Okolo, and J. Ro¨gner, Microcasting of Al bronze: interface. Influence of Casting Parameters on the Microstructure and the Fractographs of HIPed samples of SF-300, SF-340, and SF- Mechanical Properties, Microsyst. Technol., 2008, 14, p 1647–1655 390 are shown in Fig. 13(a) and (b). These samples also exhibit 9. S.M. Lee, J.H. Jung, E. Fleury, W.T. Kim, and D.H. Kim, Metal Matrix Composites Reinforced by Gas Atomized Al-Cu-Fe Powders, Mater. ductile fracture (dimples) similar to that of spray formed Sci. Eng., A, 2000, 294–296, p 99–103 samples. Figure 12(b) exemplifies the decoration of the sec- 10. A.R.E. Singer, Principles of Spray Rolling of Metals, Met. Mater., ondary phase particles inside the grains indicating possible role 1970, 4, p 246–257 in the fracture process. 11. E.J. Lavernia, E.M. Gutierrewz, J. Szekely, and N.J. Grant, A Mathematical Model of Liquid Dynamic Compaction Process, Part 1: Heat Flow in Gas Atomization, Int. J. Rapid Solidif., 1988, 4, p 89–124 12. S.N. Ojha and S.N. Singh, On Spray Deposition of High-Speed Steel, 5. Conclusions J. Mater. Sci. Lett., 1991, 10, p 893–895 13. A.K. Srivatsava, S.N. Ojha, and S. Ranganathan, Microstructural Features and Heat Flow Analysis of Atomized and Spray Formed Spray-formed Al-4.5 wt.% Cu alloys were produced at three Al-Fe-V-Si alloy, Metall. Mater. Trans., 1998, 29a, p 2205–2219 different spray heights of 300, 340, and 390 mm and hot 14. F.W. Kang, G.Q. Zhang, J.F. Sun, Z. Li, and J. Shen, Hot Deformation isostatic pressed to reduce the porosity. The grain size of the Behavior of a Spray Formed Super Alloy, J. Mater. Process. Technol., spray-formed samples reduced from 29 to 18 lm as the spray 2008, 204, p 147–151 15. P. Mathur, S. Annavarapu, D. Appelian, and A. Lawley, Process height was increased due to high cooling rates. The porosity of Control, Modeling, and Applications of Spray Casting, J. Met., 1989, the spray-formed billet is increased from 5 to 12% as the spray 41, p 23–28 height is increased from 300 to 390 mm due to a reduced liquid 16. G.M. Janowski, F.S. Biancaniello, G.E. Hicho, R.J. Fields, S.D. fraction present or lack of liquid fraction present in the spray. Ridder, and F.H. Froes, P/M in Aerospace, Defense and Demanding The hardness values and 0.2% YS of the spray-formed alloy Applications, MPIF, Princeton, NJ, 1993, p 363–370 increased by 38 and 32 %, respectively, with increase in spray 17. S. Hariprasad, S.M.L. Sastry, K.L. Jerina, and R.J. Lederich, Micro- structure and Mechanical Properties of Dispersion-Strengthened High- height. Fine equiaxed and homogeneous grains were obtained Temperature Al-8.5 Fe 1.2 V 1.7 Si Alloys Produced by Atomized Melt and the morphology of the Al2Cu phase changed from irregular- Deposition Process, Metall. Trans. A, 1993, 24, p 865–873 to plate-shaped and spherical shape with increase in spray 18. H. Liu, E.J. Levernia, and R.H. Rangel, Modeling of Molten Droplet height. After HIP, relative density of 99% is achieved by the Impingement on Non-flat Surface, Acta Metall. Mater., 1995, 43, elimination of porosity. During HIP, the plastic flow on material p 2053–2072 19. H. Liu, R.H. Rangel, and E.J. Lavernia, Modeling of Droplet-Gas seems to occur by dislocation creep mechanism. The HIPed Interactions in Spray Atomization of Ta-2.5 W alloy, Mater. Sci. Eng., deposits exhibited similar grain size compared to spray-formed A, 1995, 191, p 171–184 deposits. After HIP, the 0.2% YS of SF-300 and SF-340 20. E.J. Lavernia, Evolution of Microstructure During Spray Atomization improved by 23 and 24%. HIPed SF-300, HIPed SF-340, and and Deposition, Int. J. Rapid Solidif., 1989, 5, p 47–85 HIPed SF-390 exhibited increase in % elongation by 12, 27, and 21. S.N. Ojha, Spray Forming: Science and Technology, Bull. Mater. Sci., 73%, respectively, in contrast to SF-300, SF-340, and SF-390. 1992, 15, p 527–542 22. P.S. Grant, Spray Forming, Prog. Mater Sci., 1995, 39, p 497–545 23. P.S. Grant, B. Cantor, and L. Katgerman, Modeling of Droplet Dynamic and Thermal Histories During Spray Forming. II. Effect of Acknowledgements Process Parameters, Acta Metall. Mater., 1993, 41, p 3109–3118 24. A. Upadhyaya, N.S. Mishra, and S.N. Ojha, Microstructural Control by The authors would like to thank Mr. Raghukiran N. and Mr. Spray Forming and Wear Characteristics of a Babbit Alloy, J. Mater. Sujith R. for their technical support. Mr. Subramanyam S. V. and Sci., 1997, 32, p 3227–3235 Mr. Dhanapalan P. are thankfully acknowledged for their technical 25. W.D. Cai and E.J. Lavernia, Modeling of Porosity During Spray support. Also, sincere thanks are extended to Ms. D. Kanchanamala Forming, Mater. Sci. Eng., A, 1997, 226–228, p 8–12 26. M. Eddahbi, F. Carrenõ, and O.A. Ruano, Deformation Behavior of an and Mrs. S. Veerabathiran for their assistance in conducting TEM. Al-Cu-Mg-Ti Alloy Obtained by Spray Forming and Extrusion, Mater. Lett., 2006, 60, p 3232–3237 27. H.Y. Bor, C. Hsu, and C.N. Wei, Influence of Hot Isostatic Pressing on References the Fracture Transitions in the Fine Grain MAR-M247 Superalloys, Mater. Chem. Phys., 2004, 84, p 284–290 1. E.J. Lavernia and N.J. Grant, Spray Deposition of Metals a Review, 28. H.V. Atkinson and S. Devies, Fundamental Aspects of Hot Isostatic Mater. Sci. Eng., 1988, 98, p 381–394 Pressing: An Overview, Metall. Mater. Trans. A, 2000, 31A, p 2981– 2. Y. Guilin and Z. Xin, The Casting Process of Large Closed Annular 3000 Castings Made of High Strength Al-Cu Alloy, J. Mater. Eng., 2000, 9, 29. P.S. Grant, W.T. Kim, and B. Cantor, Spray Forming of Aluminum- p 42–45 Copper Alloys, Mater. Sci. Eng., A, 1991, 134, p 1111–1114

Journal of Materials Engineering and Performance 30. P. Shukla, R.K. Mondal, and S.N. Ojha, Microstructural Modifications 36. Z. Gao, X. Zhang, and M. Chen, Influence of Strain Rate on the Induced by Spray Forming of Al-Cu Alloys, Mater. Sci. Eng., A, 2001, Precipitate Microstructure in Impacted Aluminum Alloy, Scr. Mater., 304–306, p 583–586 2008, 59, p 983–986 31. ASTM. E8-04. Standard Test Methods for Tension Testing of Metallic 37. H.V. Atkinson and B.A. Rikinson, Hot Isostatic Processing (Transac- Materials. 2004 Annual Book of ASTM Standards, 03-01-3.06 tions of the Royal Microscopical Society), IOP Publishing Ltd, 32. S.N. Singh and S.N. Ojha, Microstructural Investigation of Rap- England, 1991, p 21–49 idly Solidified Cast Iron Powders, Int. J. Rapid Solidif., 1992, 7, 38. K.C. Prabir and A.M. Farghalli, Creep and Ductility in an Al-Cu Solid- p 201–217 Solution Alloy, Metall. Trans. A, 1987, 26, p 2105–2114 33. V.C. Srivastava, A. Upadhyay, and S.N. Ojha, Microstructural Features 39. L. Kloc, S. Spigarelli, E. Cerri, E. Evangelista, and T.G. Longdon, Induced by Spray Forming of a Ternary Pb-Sn-Sb Alloy, Bull. Mater. Creep Behavior of an Aluminum 2024 Alloy Produced by Powder Sci., 2000, 23, p 73–78 Metallurgy, Acta Mater., 1997, 45, p 529–540 34. M.C. Flemings, Solidification Processing, McGraw-Hill, New York, 40. E.A. Starke, Jr., T.H. Sanders, Jr., and I.G. Palmer, New Approaches to NY, 1974, p 160 Alloy Development in the Al-Li System, J. Metal., 1981, 8, p 24–33 35. V. Vaithyanathan, C. Wolverton, and L.Q. Chen, Multiscale Modelling 41. H.M. Flower and P. Gregson, Solid State Phase Transformations in of h¢ Precipitation in Al-Cu Binary Alloys, Acta Mater., 2004, 52, Aluminum Alloys Containing Lithium, J. Mater. Sci. Technol., 1987, p 2973–2987 3, p 81–90

Journal of Materials Engineering and Performance