Chemical Milling Research Articles

Mechanochemical synthesis of sustainable energy materials

The synthesis of new materials for energy storage and conversion has received increasing attention over the last decade. This growing importance reflects the burgeoning energy demand of an escalating world population, the finite supply of fossil fuels and the urgent need to reduce carbon dioxide emissions. Synthetic methods in which reactions are driven mechanically (effectively by the transfer and conversion of kinetic energy) can be considered as a branch of green chemistry. Materials are impacted by pressures that are created between the grinding media and with the container walls through multiple collisions during the milling process. These events induce mechanical deformation and define the nature of the milled products. This article focuses on the application of physical and – principally – chemical (reactive) milling in the preparation of energy-related materials. For hydrogen storage, complex magnesium hydrides prepared by reactive milling exhibit lower desorption temperatures and faster kinetics than those of magnesium hydride itself. The complete conversion of lithium nitride to lithium amide, the thermodynamically unfavorable hydrogenation of magnesium nitride and the synthesis of metal amides from metal hydrides can all be conducted in the mill. In the context of energy storage materials, nanostructuring is also one method by which the volume expansion associated with lithiation in high-capacity alloy anodes in lithium ion batteries can be mitigated. Mechanochemical synthesis provides one method for achieving such nanostructured materials and offers, for example, a unique approach in the synthesis of tin nanowires for use as negative electrodes in secondary lithium ion batteries.

Investigation of typical properties of nanocrystalline iron powders prepared by ball milling techniques

Metallic powder has applications in many fields. In applications for preservation and anti-oxidation, iron powder has been used as an air oxygen reducer which is capable of decreasing microclimatic oxygen concentrations in a hermetic mini-environment. In this role, if we increase the specific surface area by reducing the particle size of the iron powder, the rate and performance of oxygen reduction will be improved significantly. In addition, the porosity of iron powder also contributes considerably. The iron powder can be fabricated using many methods: chemical deposition, powder metallurgy and mechanical milling. The technique of milling has certain advantages, especially for the formation of technical iron powder. The experimental equipment used was a Fritsch P-6 planetary ball mill. The iron powder was prepared with different milling times, from 1 up to 30 h in acetone as a protective environment. The powder products obtained were analyzed using field emission scanning electron microscope (FE-SEM), energy dispersive x-ray (EDX), x-ray diffraction (XRD), dynamic laser scattering (DLS), Brunauer–Emmett–Teller (BET) techniques and also magnetic characterization by vibrating sample magnetometer (VSM). The results show a correlation between the milling time and the crystallite and particle size, specific surface area, magnetic properties and nanoscale porosity of the iron powder. The iron powders obtained were a kind of mesoporous materials. The properties of the iron powder were examined with respect to their oxygen reducing kinetics.

Techno-economic evaluation of a mechanical pulp mill with gasification

The modern concept of “biorefinery” is dominantly based on chemical pulp mills to create more value than cellulose pulp fibres, and energy from the dissolved lignins and hemicelluloses. This concept is characterized by the conversion of biomass into various bio-based products. It includes thermochemical processes such as gasification and fast pyrolysis. In thermo-mechanical pulp (TMP) mills, the feedstock available to the gasification-based biorefinery is significant, including logging residues, bark, fibre material rejects, bio-sludges and other available fuels such as peat, recycled wood and paper products. On the other hand, mechanical pulping processes consume a great amount of electricity, which may account for up to 40% of the total pulp production cost. The huge amount of purchased electricity can be compensated for by self-production of electricity from gasification, or the involved cost can be compensated for by extra revenue from bio-transport fuel production. This work is to study co-production of bio-automotive fuels, bio-power, and steam via gasification of the waste biomass streams in the context of the mechanical pulp industry. Ethanol and substitute natural gas (SNG) are chosen to be the bio-transport fuels in the study. The production processes of biomass-to-ethanol, SNG, together with heat and power, are simulated with Aspen Plus. Based on the model, the techno-economic analysis is made to evaluate the profitability of bio-transport fuel production when the process is integrated into a TMP mill.The mathematical modelling starts from biomass gasification. Dual fluidized bed gasifier (DFBG) is chosen for syngas production. From the model, the yield and composition of the syngas and the contents of tar and char can be calculated. The model has been evaluated against the experimental results measured on a 150KWth Mid Sweden University (MIUN) DFBG. As a reasonable result, the tar content in the syngas decreases with the gasification temperature and the steam to biomass (S/B) ratio. The biomass moisture content is a key parameter for a DFBG to be operated and maintained at a high gasification temperature. The model suggests that it is difficult to keep the gasification temperature above 850 ℃ when the biomass moisture content is higher than 15.0 wt.%. Thus, a certain amount of biomass or product gas needs to be added in the combustor to provide sufficient heat for biomass devolatilization and steam reforming.For ethanol production, a stand-alone thermo-chemical process is designed and simulated. The techno-economic assessment is made in terms of ethanol yield, synthesis selectivity, carbon and CO conversion efficiencies, and ethanol production cost. The calculated results show that major contributions to the production cost are from biomass feedstock and syngas cleaning. A biomass-to-ethanol plant should be built over 200 MW.In TMP mills, wood and biomass residues are commonly utilized for electricity and steam production through an associated CHP plant. This CHP plant is here designed to be replaced by a biomass-integrated gasification combined cycle (BIGCC) plant or a biomass-to-SNG (BtSNG) plant including an associated heat & power centre. Implementing BIGCC/BtSNG in a mechanical pulp production line might improve the profitability of a TMP mill and also help to commercialize the BIGCC/BtSNG technologies by taking into account of some key issues such as, biomass availability, heat utilization etc.. In this work, the mathematical models of TMP+BIGCC and TMP+BtSNG are respectively built up to study three cases: 1) scaling of the TMP+BtSNG mill (or adding more forest biomass logging residues in the gasifier for TMP+BIGCC); 2) adding the reject fibres in the gasifier; 3) decreasing the TMP SEC by up to 50%.The profitability from the TMP+BtSNG mill is analyzed in comparison with the TMP+BIGCC mill. As a major conclusion, the scale of the TMP+BIGCC/BtSNG mill, the prices of electricity and SNG are three strong factors for the implementation of BIGCC/BtSNG in a TMP mill. A BtSNG plant associated to a TMP mill should be built in a scale above 100 MW in biomass thermal input. Comparing to the case of TMP+BIGCC, the NR and IRR of TMP+BtSNG are much lower. Political instruments to support commercialization of bio-transport fuel are necessary.

Large Area Metal Nanowire Arrays with Tunable Sub-20 nm Nanogaps

We report a new top-down nanofabrication technology to realize large area metal nanowire (m-NW) arrays with tunable sub-20 nm separation nanogaps without the use of chemical etching or milling of the metal layer. The m-NW array nanofabrication technology is based on a self-regulating metal deposition process that is facilitated by closely spaced and isolated heterogeneous template surfaces that confine the metal deposition into two dimensions, and therefore, electrically isolated parallel arrays of m-NW can be realized with uniform and controllable nanogaps. Au-NW and Ag-NW arrays are presented with high-density ~10(5) NWs cm(-1), variable NW diameters down to ~50 nm, variable nanogaps down to ~5 nm, and very large nanogap length density ~1 km cm(-2). The m-NW arrays are designed and implemented as interdigitated nanoelectrodes for electrochemical applications and as plasmonic substrates where the coupled-mode localized surface plasmon resonance (LSPR) wavelength in the nanogaps between adjacent m-NW dimers can be precisely tuned to match any excitation source in the range from 500 to 1000 nm, thus providing optimal local electromagnetic field enhancement. A spatially averaged (n = 2500) surface-enhanced Raman scattering (SERS) analytical enhancement factor of (1.2 ± 0.1) × 10(7) is demonstrated from a benzenethiol monolayer chemisorbed on a Au-NW array substrate with LSPR wavelength matched to a He-Ne laser source.

Development of oxide dispersion strengthened ferritic steel prepared by chemical reduction and mechanical milling

The oxide dispersion strengthened ferritic steel with a nominal composition of Fe–14Cr–2W–0.5Ti–0.06Si–0.2V–0.1Mn–0.05Ta–0.03C–0.3Y2O3 (14Cr–ODS) was fabricated by sol–gel method in combination with hydrogen reduction, mechanical alloying (MA) and hot isostatic pressing (HIP) techniques. Pure Fe–1.5Y2O3 precursor was obtained by a sol–gel process and a reduction process at 650°C for 3h and pure 14Cr–ODS alloy powders were obtained from this precursor and the alloying metallic powders by mechanical alloying. The microstructure analysis investigated by transmission electron microscopy (TEM) and energy dispersive spectrometry (EDS) reveal that Y–Ti–O complexes and V–Ti–O complexes with a main particle size of 8nm are formed in the 14Cr–ODS steel matrix. After HIP sintering the weight and the relative density of the compacted ingots are about 0.8kg and 99.7%. The uniform elongation and ultimate tensile strength of the ODS steel obtained by HIP after annealing at 1100°C for 5h are about 13% and 840MPa, respectively.

System analysis of dry black liquor gasification based synthetic gas production comparing oxygen and air blown gasification systems

The black liquor gasification based bio-fuel production at chemical pulp mill is an attractive option to replace conventional recovery boilers increasing system energy efficiency. The present paper studies circulating fluidized bed system with direct causticization using TiO2 for the gasification of the black liquor to the synthesis gas. The advantage of using direct causticization is the elimination of energy-intensive lime kiln which is an integral part of the conventional black liquor recovery system. The study evaluates the effects of gasifying medium i.e. oxygen or air, on the fluidized bed gasification system, the synthesis gas composition, and the downstream processes for the synthesis gas conversion to the synthetic natural gas (SNG). The results showed higher synthetic natural gas production potential with about 10% higher energy efficiency using oxygen blown gasification system than the air blown system. From the pulp mill integration perspective, the material and energy balance results in better integration of air blown system than the oxygen blown system, e.g. less steam required to be generated in the power boiler, less electricity import, and less additional biomass requirement. However, the air blown system still requires a significant amount of energy in terms of the synthesis gas handling and gas upgrading using the nitrogen rejection system.

Effect of chemical milling on low-cycle fatigue behavior of an Al–Mg–Si alloy

The effect of alkaline chemical milling used for dimensionally reducing aluminum-alloy structures is assessed in terms of total fatigue life and crack-initiation mechanisms. Chemically milled Al–Mg–Si specimens exhibited a 50% reduction in average fatigue lives compared to electropolished Al–Mg–Si specimens at comparable peak-applied loads above macroscopic yield. The fatigue-life reduction of the chemically milled specimens is likely associated with early onset of crack initiation due to pit-induced-stress concentrations. Fractographic analyses suggest a transition in the crack-initiation mechanisms from predominantly {111}-slip plane cracking to partly or predominantly pit-induced-stress driven depending on the depth of surface pits.

Ethanol from Nordic wood raw material by simplified alkaline soda cooking pre-treatment

Ethanol production from lignocellulosic raw materials will generate multiple streams, since only a certain fraction of the material can be converted into sugars and then fermented to ethanol. This requires a ‘poly-generation’ approach, where by-products also must have high value (e.g. lignin, sugars from hemicellulose). To reach the large scale required for profitability, it is proposed that the best way is to integrate the new processes with existing industries, preferably those that already operate biomass-to-materials or biomass-to-fuels plants. One of the largest industry branches in this respect is the pulp and paper industry. Production of second generation ethanol (or other products) via sugars from lignocellulosic materials includes a relatively costly pre-treatment of the raw material in order to separate the lignin from the cellulose. This separation of the wood components already takes place in the chemical pulp mill, and the long proven technology in pulp production known as soda cooking (pre-treatment under alkaline conditions) is further evaluated in this study. It can be directly integrated into the recovery of chemicals and energy in the pulp mill. The pre-treatment of the lignocellulosic material studied in this work is alkaline and sulphur-free, and results in a technically pure cellulose to be fed to the hydrolysis stage, which makes it different compared to most of the other processes that aim to produce ethanol from lignocelluloses. The process chain from enzymatic hydrolysis to ethanol is very similar to that being used today for grain ethanol.The aim of this study was to define the conditions in alkaline pre-treatment stage for the separation of wood to a carbohydrate fraction for hydrolysis and ethanol production, and to a lignin fraction for the production of lignin products. Aspen (Populus tremula) and pine (Pinus sylvestris) wood from Nordic mills were studied. The reference case was alkaline pre-treatment according to the well-known soda pulping technique. The pulps of alkaline pre-treated aspen could be enzymatically hydrolysed very efficiently and fermented to ethanol with high yields (82–88% ethanol yield from theoretical maximum). It should be possible to use raw material of lower quality and cost than wood from the pulp industry. However, it can then be important to be able to take out non-process elements (NPEs) that otherwise accumulate in the process. This can be done by introducing an acidic prehydolysis stage prior to the alkaline fractionation. The content of Mg and Mn ions in the wood was possible to reduce by 85–90% and Ba and Ca ions by 75–80%. Potassium was virtually completely removed during the acidic pre-treatment stage.

An activity-based cost calculation for a kraft pulp mill

This study introduces an activity-based costing (ABC) method for a kraft pulp mill. Our ABC model defines the production resources and costs for each process in a chemical pulp mill and allocates the costs to pulp, energy, bark, turpentine, and crude tall oil. The production processes include receiving, unloading and debarking of pulpwood, chipping, chip screening, chip storing, cooking and in-digester washing, pulp washing and screening, oxygen delignification, bleaching, drying, and chemical recovery. We also tested the effect of Scots pine pulpwood properties on the profitability of a virtual greenfield pulp mill located in Finland, where it produced 600000 air-dried (a.d.) metric tons of bleached market pulp annually. Total annual production costs were approximately EUR 216 million (USD 285 million), of which chemical recovery comprised the biggest share (almost 39%). According to the results, the price of market pulp had the most significant effect on the profitability of the mill. The pulpwood properties did not clearly affect pulp production costs; the wood procurement costs had more influence on the profitability of the value chain. Our results also indicate that the profitability of pulp making is strongly dependent on the prices of electricity and heat. This is because the mill is customer and seller in energy markets. ABC proved to be a useful tool and accurate method for cost calculation in this highly competitive branch of the forest industry.

Effects of Chemical Dispersant and Wet Mechanical Milling Methods on Conductive Carbon Dispersion and Rate Capabilities of LiFePO4 Batteries

AbstractThe dispersion state of conductive carbons in the cathodes of lithium ion batteries fabricated with a dispersant and with/without mechanical milling are considered with regard to the batteries' charge–discharge properties. Suspensions of Ketjen black (KB, EC 300J and EC 600JD) are dispersed in an organic solvent. Ball milling is then performed, modifying the particle sizes and structures. Results show improved dispersion but with collateral reduction of both maximum and average particle sizes. The more fragile EC 600JD, after ballmilling, is broken into more small segments than EC 300J, but with segment density high enough for loose coupling to form networks. Therefore, the electron conduction of EC600JD after ball milling is better than EC 300J. The high‐rate discharge capacity of EC600JD after ball milling is also better than that of EC 300J. However, rate capabilities decrease with increasing milling time.

Elastic Modulus of Magnetic Fluids Evaluated by Ultrasonic Test

Magnetic nanoparticles for ferromagnetic fluids and magnetorheological fluids were prepared by chemical coprecipitation and mechanical milling, respectively. The surface-treated particles were dispersed at various weight ratios into a medium of polyethylene glycol. In order to evaluate the elastic modulus of the fluids, ultrasonic pulse velocities were measured with an ultrasonic test using transducers of 5MHz and 2.25MHz. The ultrasonic signals were only available with a transducer of 2.25 MHz at fluid concentrations of 5 mg/ml and lower. In the case of applying transducers over 2.25 MHz and concentrations over 5 mg/ml to the fluids, it was impossible to observe effective ultrasonic signals due to an excessive scattering of the pulses by the dispersed particles. Elastic moduli of the magnetorheological fluids were 5.44 GPa and 6.13 GPa with concentrations of 25 mg/ml and 50 mg/ml, respectively; these values were higher by 40% than the values of 4.04 GPa and 4.28 GPa of ferromagnetic fluids at the same concentrations. As for the effect of an external magnetic field on these dilute fluids, the ultrasonic signals were positioned in a very similar way, which was probably due to insufficient arrangement of the particles even though the reflection energy of the ultrasonic waves apparently increased.

Reductions in greenhouse gas emissions and oil use by DME (di-methyl ether) and FT (Fischer-Tropsch) diesel production in chemical pulp mills

Using energy systems analysis, we examine the potential to reduce CO2 emissions and oil use by integrating motor biofuel production with pulp mills. BLG-DME (black liquor gasification with di-methyl ether production) is compared with solid biomass gasification with BIG-FT (solid biomass gasification with Fischer-Tropsch fuel production). The studied systems are expanded with stand-alone production of biomass-based electricity and motor fuel so that they yield the same functional unit in terms of motor fuel and electricity as well as pulp or paper product, in order to facilitate comparison. More motor biofuel can be produced in integration with the studied mills with BLG-DME than with BIG-FT because the black liquor flow is large compared with other fuel streams in the mill and the integration potential for BIG-FT is limited by the mill’s heat demand. When both systems are required to produce the same functional unit, the BLG-DME system achieves higher system efficiency and larger reductions in CO2 emissions and oil use per unit of biomass consumed. In general, integration of motor biofuel production with a pulp mill is more efficient than stand-alone motor biofuel production. Larger reductions in CO2 emissions or oil use can, however, be achieved if biomass replaces coal or oil in stationary applications.

Composition of intake sugars and emission of gases from paper sludges by Coptotermes formosanus Shiraki

Paper sludge is a by-product of the pulping process and is landfilled or incinerated for disposal. In this study, we evaluated ingestion and digestibility of carbohydrates, by the termite Coptotermes formosanus, in two kinds of sludges: sludge C from the chemical pulp mill and sludge M from the mechanical pulp mill. The no-choice tests using the termite for three weeks showed that the mass loss of sludge C was significantly higher than that of the control samples: a bleached pulp and red pine wood. It is considered that the higher inorganic content of sludge C resulted in the higher mass loss when the same amount of carbohydrates was taken by the pulp- or wood-fed termite. Although the inorganic content of sludge M was almost the same as that of sludge C, the higher lignin content in sludge M is thought to have resulted in the lower mass loss. Analysis of sugar composition in the faecal materials of the termite showed that about 73% of glucose and 81% of xylose in sludge C were digested. It was concluded that the digestibility of these sugars in sludge C was the same as that of the control samples despite containing high amounts of inorganic compounds. However, the hydrogen conversion rate by the termites that were fed sludge was lower than that of the termites that were fed pulp in the no-choice test for three days: one mole of glucose from the sludge and pulp was converted to 0.51 and 0.80 moles of hydrogen, respectively.

Research on Tooth Trace Modification of Spur Bevel Gear

Based on the gear geometry theory and the normal meshing motion equation of gear pairs， changes of meshing points and angles are analyzed and the effect of tooth trace modification on gear pair meshing movement is discussed. It is concluded that chemical milling can effect tooth trace involute modification of fine-forged spur bevel gear, improve its surface quality and dimensional precision and therefore prolong its service life.

High Pressure Waterjets – An Innovative Means of Alpha Case Removal for Superplastically Formed Titanium Alloys

Ultra high-pressure waterjets (WJ) have gained consideration as a viable alternative to conventional material removal and cleaning methods such as chemical milling, grit blasting, and grinding. Waterjets have often been considered for the stripping of unwanted coatings, but they also exhibit an inherent ability for the controlled milling of metallic materials. The waterjet material removal process poses a secondary benefit – the material erosion occurs due to the repetitive impact of droplets within the waterjet stream, which also serve to induce compressive residual stresses in the shallow sub-surface layers of the work piece. An experimental study was conducted to explore the removal of the alpha case layer on Superplastically Formed conventional and fine grain Ti-6Al-4V specimens. The resulting surface roughness parameters and completeness of alpha layer removal were characterized. Based on the experimental results, waterjet removal processes may serve as an alternate method for the post-processing of Superplastically Formed titanium alloys.

Superplastic Forming – Cost Effective

When Superplastic Forming (SPF) was offered as a production process in the mid 70’s, it became the panacea of all processes for sheet metal products designed to be made from Titanium and Aluminium materials. The claims were (1) reduced part count (2) reduced assembly time (3) weight reduction (4) monolithic parts and (5) stronger structures. Following Pearson’s work in the mid 30’s with Lead-Tin and Bismuth-Tin alloys [1], showing higher than 1000% elongation without failure, the Aluminium industry developed SPF alloys and launched into numerous commercial applications. Other research facilities focused on the potential of achieving superplasticity in Titanium alloys. This was demonstrated in the late 60’s using the now well established Ti6Al4V alloy. Considerable funding was allocated, both in the USA & UK, specifically for the development of the process. The USA focused on the military programmes and the UK on the civil (Concord) and some military aircraft. Success in these programmes and the claims made, resulted with a production process. Companies invested in suitable plant and equipment, and designers grasped the process potential and applied SPF to their sheet metal designs expecting to reap the claimed benefits. The claims are valid if applied to correctly chosen components. All too often, the SPF manufacturing choice did not deliver its claims. In many cases cost of material, need to chemical mill and higher energy costs, were either not envisaged or taken into account. Today all processes, material cost and alternative material types have to be assessed before the manufacturing method is chosen. The aerospace industry is attacking the Buy-Fly ratio. Energy and labour cost are at a premium and these have caused the SPF and Hot Forming community to examine ways of producing products (a) from less material (b) by Hot Forming (eliminating the need to apply chemical milling to remove the alpha case) (c) questioning the material choice (CP instead of Ti6Al4V) and (d) by applying modern fabrication methods. The paper will illustrate this change in philosophy; shows today’s choices and demonstrate how the SPF process can be cost effective, and in fact still does have a major role to play in producing airframe and engine structures.

SU-DD-A4-05: Development of a Tetrahedron Beam Computed Tomography Benchtop System with a 75 Pixel Field Emission X-Ray Tube: Collimator Design and Image Reconstruction

Purpose: We are developing a novel Tetrahedron Beam Computed Tomography (TBCT) benchtop system using a multiple pixel x‐ray tube with 75 carbon nanotubefield emissioncathodes and an in‐house curved CTdetector array. In this paper, we describe the development of multi‐slot collimators and image reconstruction algorithms.Material and Methods: The 75 x‐ray sources are arranged in a linear array with 4 mm cathode spacing. The 5‐row detector array is curved with a 145 cm radius. The linear scanning x‐ray source array and a curved detector array are aligned perpendicular to and within the rotation plane, respectively, with the source array positioned precisely on the central axis of the detector array. A group of multi‐slot collimators were designed to collimate the beam from each source exactly to the detector array. An FDK TBCT reconstruction algorithm was developed. Image reconstruction was performed using numerical and actual projection data. Results: The shapes of each slot opening are partial ellipses. The slot geometries were derived and drawn using CAD software. Actual collimators were made using a chemical milling technique. The FDK TBCT image reconstruction algorithm was adapted from the cylindrical, equiangular detector cone beam reconstruction algorithms. The TBCT reconstruction was evaluated using numerical Shepp‐Logan phantom as well as real phantom scans.Conclusions: Multi‐slot collimators were designed and built. The TBCT image reconstruction algorithm was able to accurately reconstruct 3D volume with both numerical and real projection data. This work is partially supported by NIH grant 1R21CA130330‐01A1 and DOD Prostate Cancer Research Program W81XWH‐07‐0083.

High Pressure Waterjets – An Innovative Means of Alpha Case Removal for Superplastically Formed Titanium Alloys

Ultra high-pressure waterjets (WJ) have gained consideration as a viable alternative to conventional material removal and cleaning methods such as chemical milling, grit blasting, and grinding. Waterjets have often been considered for the stripping of unwanted coatings, but they also exhibit an inherent ability for the controlled milling of metallic materials. The waterjet material removal process poses a secondary benefit – the material erosion occurs due to the repetitive impact of droplets within the waterjet stream, which also serve to induce compressive residual stresses in the shallow sub-surface layers of the work piece. An experimental study was conducted to explore the removal of the alpha case layer on Superplastically Formed conventional and fine grain Ti-6Al-4V specimens. The resulting surface roughness parameters and completeness of alpha layer removal were characterized. Based on the experimental results, waterjet removal processes may serve as an alternate method for the post-processing of Superplastically Formed titanium alloys.

Superplastic Forming – Cost Effective

When Superplastic Forming (SPF) was offered as a production process in the mid 70’s, it became the panacea of all processes for sheet metal products designed to be made from Titanium and Aluminium materials. The claims were (1) reduced part count (2) reduced assembly time (3) weight reduction (4) monolithic parts and (5) stronger structures. Following Pearson’s work in the mid 30’s with Lead-Tin and Bismuth-Tin alloys [1], showing higher than 1000% elongation without failure, the Aluminium industry developed SPF alloys and launched into numerous commercial applications. Other research facilities focused on the potential of achieving superplasticity in Titanium alloys. This was demonstrated in the late 60’s using the now well established Ti6Al4V alloy. Considerable funding was allocated, both in the USA & UK, specifically for the development of the process. The USA focused on the military programmes and the UK on the civil (Concord) and some military aircraft. Success in these programmes and the claims made, resulted with a production process. Companies invested in suitable plant and equipment, and designers grasped the process potential and applied SPF to their sheet metal designs expecting to reap the claimed benefits. The claims are valid if applied to correctly chosen components. All too often, the SPF manufacturing choice did not deliver its claims. In many cases cost of material, need to chemical mill and higher energy costs, were either not envisaged or taken into account. Today all processes, material cost and alternative material types have to be assessed before the manufacturing method is chosen. The aerospace industry is attacking the Buy-Fly ratio. Energy and labour cost are at a premium and these have caused the SPF and Hot Forming community to examine ways of producing products (a) from less material (b) by Hot Forming (eliminating the need to apply chemical milling to remove the alpha case) (c) questioning the material choice (CP instead of Ti6Al4V) and (d) by applying modern fabrication methods. The paper will illustrate this change in philosophy; shows today’s choices and demonstrate how the SPF process can be cost effective, and in fact still does have a major role to play in producing airframe and engine structures.

Characterization of residual stresses in heat treated Ti-6Al-4V forgings by machining induced distortion

To provide a solid base for improved material exploitation in dimensioning calculations it is necessary to determine the stress state in the part prior to service loading. In order to achieve higher material strength at elevated temperatures, the surface temperature gradient with respect to time has to be sufficiently high during heat treatment. This results in non-negligable residual stresses that can reduce the allowable load level upon which yielding occurs. For titanium alloys there are two common heat treatments, namely solution treatment and mill annealing. The latter one is the method of choice within the presented project. Mill annealing is utilized in order to significantly reduce the residual stresses in the parts without loosing much of the improved strength at elevated temperatures. Quantification of residual stresses is done by solving an inverse problem. From the measurement of distortion, induced by dividing the investigated part, the residual stress state can be calculated via analytical modeling or correlation with finite element models. To assure a minimum perturbation of the residual stress state during specimen production, dividing of the part is accomplished by electric discharge machining.The parts of interest are v-shaped prisms with a length of approximatly 450 mm and a thickness in the cross sectional area from about 20 mm to 45 mm. Figure 1(a) shows the forged part and 1(b) the dimensions of the cross section in millimeters as well as the material properties considered in the finite element model. The heat exchange between the part and the environment is modelled as heat transfer by convection superimposed with heat radiation. Since the parts are exposed to air during forging and heat treatment, the surface develops a strongly adhesive oxide layer, the so called alpha-case. After forging the parts are cooled in air and heat treated at a temperature of 720° C for a duration of 120 min. Subsequent air cooling and removing the alpha-case by shot peening and chemical treatment in a mixture of nitric and hydroflouric acid finishes the processing. The residual stress state in the parts is quantified through correlation of results obtained by finite element simulations and high precision deflection experiments. Experimental measuring errors are minimized by using the capability of the wire cutting machine to measure locations on the specimens with respect to the fixed machine coordinate system. A sophisticated design of cutting operations allows an analysis of the part without removing the fixture and hence makes it possible to achieve a very high accuracy of the displacement measurements of about 4 µm. The quality of the obtained results also depends on the finite element models describing the mechanics of the experimental investigations as precise as possible. Since numerical modeling of shot peening and chemical milling is rather complicated, the whole processing route of the shapes is simplified assuming that the individual contribution of all process steps to the residual stress state can be described by virtual heat transfer coefficients. The overall virtual heat transfer coefficient for the whole processing route is calculated from the linear superposition of the individual heat transfer coeffcients for each processing step. Detailed analysis of the obtained overall virtual heat transfer coefficients in comparison with experimentally obtained ones shows, that other processing steps besides the heat treatment considerably influence the residual stress state. In order to make the conducted simulation scheme applicable in practise, advanced finite element modeling techniques are developed. The experimentally derived deflection curves are correlated to the finite element results via a least square fit.