Welding of Ti-6Al-4V: Influence of welding process and alloy composition on microstructure and properties

Titanium alloys are widely used for components in the fan and compressor sections of aeroengines mainly because of their superior strength-to-weight ratio. Large static compressor components can be manufactured by welding together smaller subcomponents, which has potential to provide benefits such as higher buy-to-fly ratio and improved performance of the components. This is the background for why welding and the mechanical properties of welds have been investigated in this project. The aim of the work was to study what kind of microstructures and defects are formed in welding of Ti-6Al-4V with different welding processes and how these aspects affect the mechanical properties of the welds. Furthermore, the influence of chemical composition of the alloy on the formation of microstructures and defects was studied. The welding processes compared were electron beam welding, laser beam welding, plasma arc welding and TIG welding. High energy beam welding processes rendered a finer weld microstructure in comparison to the coarser microstructure produced by arc welding processes. The finer weld microstructure was found to be beneficial for tensile ductility and low cycle fatigue performance. Porosity was observed in welds produced by all the processes. Large pores and pores located close to the specimen surface the most detrimental to the fatigue strength.\ua0 Fatigue life in the welds produced by arc weld processes was more sensitive to porosity than in the high energy beam welds. The finer microstructure has a higher resistance to micro crack initiation and growth which contributed to the better fatigue performance of welds produced by electron beam welding and laser beam welding. The alloy composition had a significant influence on the microstructure of the welds and the formation of defects. A small boron addition induced significant grain refinement in weld in boron alloyed material. Narrow columnar prior-β grains were formed in the fusion zones of the boron alloyed welds. The α colonies and α plates were also refined, as compared to the standard Ti-6Al-4V welds. In the cast base material, the TiB particles were located along the prior-β grain boundaries restricting the grain growth in the heat affected zone. In the fusion zone of welds, TiB particles had decreased in size and formed networks of stripes along the interdendritic regions. EBSD combined with prior-β grain reconstruction was an effective method to reveal the prior-β grain structure in the different weld zones. A significant batch to batch variation in amount of porosity was observed in laser welding of Ti-6Al-4V. The most significant factors affecting formation of porosity were the material batch, pulse length and welding speed. The material batches that were most susceptible to formation of porosity had increased amount of carbon and oxygen. The formation of porosity could be minimized in all material batches by optimizing the welding parameters

Applying a Multi-Criteria Project Portfolio Tool in Selecting Energy Peat Production Areas

This study demonstrates the characteristics of the new generic project portfolio selection tool YODA (“Your Own Decision Aid”). YODA does not include a mathematical aggregation model. Instead, the decision maker’s preferences are defined by the interactive articulation of acceptance thresholds of project-level decision criteria. Transparency and ease of adopting the method in participatory planning are sought using the method’s simple preference input. The characteristics of the YODA tool are introduced by presenting how it has been applied in participatory land use planning in northern Finland in selecting a combination of peat production sites to attain the goals defined at municipal level. In this process, each stakeholder first constructed a project portfolio that best met his or her preferences. In doing this, acceptance thresholds for project-level decision criteria were defined. In total, eight decision criteria were related to economic value, biodiversity, social impacts, and ecosystem services. Subsequently, the portfolios of different stakeholders were combined in line with the principles of robust portfolio modelling. Core projects were accepted by all stakeholders, while exterior projects were not accepted, and borderline projects by some of the stakeholders. Although the land use planning situation at hand was highly sensitive, because it was related to various aspects of sustainability, the use of YODA provided useful results. The first meeting with stakeholders identified 52 out of 99 sites that none of the stakeholders would use for energy peat production, due to their characteristics, whereas, in the second meeting, a smaller stakeholder group found 18 core projects and 26 borderline projects which could be potential areas for energy peat production. We conclude that YODA—as a generic project portfolio tool—can be used in various planning situations

Applying a Multi-Criteria Project Portfolio Tool in Selecting Energy Peat Production Areas

This study demonstrates the characteristics of the new generic project portfolio selection tool YODA (“Your Own Decision Aid”). YODA does not include a mathematical aggregation model. Instead, the decision maker’s preferences are defined by the interactive articulation of acceptance thresholds of project-level decision criteria. Transparency and ease of adopting the method in participatory planning are sought using the method’s simple preference input. The characteristics of the YODA tool are introduced by presenting how it has been applied in participatory land use planning in northern Finland in selecting a combination of peat production sites to attain the goals defined at municipal level. In this process, each stakeholder first constructed a project portfolio that best met his or her preferences. In doing this, acceptance thresholds for project-level decision criteria were defined. In total, eight decision criteria were related to economic value, biodiversity, social impacts, and ecosystem services. Subsequently, the portfolios of different stakeholders were combined in line with the principles of robust portfolio modelling. Core projects were accepted by all stakeholders, while exterior projects were not accepted, and borderline projects by some of the stakeholders. Although the land use planning situation at hand was highly sensitive, because it was related to various aspects of sustainability, the use of YODA provided useful results. The first meeting with stakeholders identified 52 out of 99 sites that none of the stakeholders would use for energy peat production, due to their characteristics, whereas, in the second meeting, a smaller stakeholder group found 18 core projects and 26 borderline projects which could be potential areas for energy peat production. We conclude that YODA—as a generic project portfolio tool—can be used in various planning situations

Indoor air problems and the perceived social climate in schools : A multilevel structural equation analysis

Peer reviewe

Tehokkaat ohjauskeinot jatkuvapeitteisen metsänkäsittelyn edistämiseksi runsasravinteisilla turvemailla

Raportissa tarkastellaan, onko luonnontieteellisiä perusteita ohjata metsänkäsittelyä runsasravinteisilla paksuturpeisilla turvemailla nykyistä enemmän kohti jatkuvapeitteistä kasvatusta – ja jos tällaisia perusteita on, miten sitä koskeva ohjaus olisi mahdollista ja tarkoituksenmukaista toteuttaa ja mitkä sen yhteiskunnalliset vaikutukset voisivat olla. Olemassa olevan luonnontieteellisen tutkimustiedon perusteella näyttäisi olevan perusteita ohjata ja kannustaa metsänomistajia siirtymään runsasravinteisilla turvemailla jatkuvapeitteiseen metsänkäsittelyyn. Avohakkuita välttämällä ja metsänkäsittelyn intensiteettiä muutenkin vähentämällä pystytään pienentämään ravinne- ja kiintoainekuormitusta vesiin. Vesistökuormitusta koskevia tutkimustuloksia jatkuvapeitteisestä metsänkasvatuksesta on erityisesti rämeiltä, mutta samat riippuvuudet pätevät myös runsasravinteisemmilla turvemailla. Kasvihuonekaasupäästöistä tutkimustietoa on runsaasti puustoisilta soilta, ja päästöjen riippuvuus vedenpinnan tasosta sekä puuston vaikutus vedenpinnan tasoon tunnetaan. Avohakkuun jälkeen päästöt kasvavat voimakkaasti. Siirtymällä runsasravinteisten turvemaiden päätehakkuuikäisissä kuusikoissa jatkuvapeitteiseen metsänkasvatukseen voidaan välttää avohakkuiden jälkeiset suuret kasvihuonekaasupäästöt. Tällaisen siirtymän avulla voitaisiin vähentää päästöjä ja kasvattaa Suomen vuosittaista hiilinielua jopa miljoonalla hiilidioksiditonnilla. Lukessa tehtyjen laskelmien (Lehtonen ym. 2023a) perusteella siirtymä jatkuvapeitteiseen kasvatukseen runsasravinteisilla turvemailla ei juuri vaikuttaisi puuston kasvuun koko maan tasolla. Raportissa tuodaan esiin 14 keinoa, joilla jatkuvapeitteistä metsänkasvatusta voitaisiin edistää runsasravinteisilla turvemailla. Niistä kolmea tarkastellaan raportissa lähemmin. Muutamaa poikkeusta lukuun ottamatta keinot eivät ole toisiaan poissulkevia vaan pikemminkin toisiaan täydentäviä. Osa niistä olisi verraten helppoja toteuttaa, mutta uudentyyppiset taloudelliset tuet avohakkuiden aiheuttamien ympäristöhaittojen välttämiseen runsasravinteisilla turvemailla vaatisivat tukiehtojen tarkempaa määrittelyä, yhteiskehittelyä ja kokeiluhankkeita erilaisissa olosuhteissa. Metsätalouden tukijärjestelmä (kestävän metsätalouden rahoituslaki, kemera) oli voimassa jokseenkin muuttumattomana yli 25 vuotta (1997‒2023). Puuntuotannon tukemisen järjestelmä on ollut voimassa vielä paljon pitempään, lähes sata vuotta. Aivan uudentyyppisten, markkinattomien ympäristöhyötyjen tuottamiseen tai ympäristöhaittojen vähentämiseen tähtäävien metsätalouden tukien suunnittelu ja toteutus vaatii paitsi avarakatseisuutta ja ennakkoluulottomuutta, myös lisää tutkimusnäyttöä eri metsänkasvatustapojen vaikutuksista runsasravinteisilla turvemailla. Euroopan unionin valtiontukisääntöjen näkökulmasta uudentyyppisten tukien soveltamiselle ei näyttäisi olevan estettä. Päinvastoin, Euroopan unionin uudet 1.1.2023 voimaan tulleet uudet valtiontukisäännöt avaavat kokonaan uusia mahdollisuuksia ottaa käyttöön ympäristöhyötyjen tuottamiseen kannustavia tukia metsätaloudessa. Näitä mahdollisuuksia olisi tarpeen ottaa käyttöön huomattavasti laajemmin kuin 1.1.2024 voimaan tulevassa metsätalouden uudessa tukijärjestelmässä (Metka) tehdään. Uudentyyppiset taloudelliset tuet markkinattomien ympäristöhyötyjen tuottamiseen tai ympäristöhaittojen vähentämiseen myös lisäisivät metsänomistajien valinnanmahdollisuuksia suometsiensä käsittelyssä ja käytössä. Kyselyiden perusteella metsänomistajat suhtautuvat tällaisiin tukiin myönteisesti, ja haastatteluiden perusteella myös valtaosa sidosryhmistä näkee tarvetta uudentyyppisille ohjauskeinoille turvemaametsissä

Quantification and valuation of ecosystem services to optimize sustainable re-use for low-productive drained peatlands

LIFEPeatLandUse (LIFE12 ENV/FI/000150) 2013 – 2018, LAYMAN’S REPORT201

Microstructure and Porosity of Laser Welds in Cast Ti-6Al-4V with Addition of Boron

Addition of small amounts of boron to cast Ti-6Al-4V alloy has shown to render a finer microstructure and improved mechanical properties. For such an improved alloy to be widely applicable for large aerospace structural components, successful welding of such castings is essential. In the present work, the microstructure and porosity of laser welds in a standard grade cast Ti-6Al-4V alloy as well as two modified alloy versions with different boron concentrations have been investigated. Prior-β grain reconstruction revealed the prior-β grain structure in the weld zones. In fusion zones of the welds, boron was found to refine the grain size significantly and rendered narrow elongated grains. TiB particles in the prior-β grain boundaries in the cast base material restricted grain growth in the heat-affected zone. The TiB particles that existed in the as cast alloys decreased in size in the fusion zones of welds. The hardness in the weld zones was higher than in the base material and boron did not have a significant effect on hardness of the weld zones. The fusion zones were smaller in the boron-modified alloys as compared with Ti-6Al-4V without boron. Computed tomography X-ray investigations of the laser welds showed that pores in the FZ of the boron modified alloys were confined to the lower part of the welds, suggesting that boron addition influences melt pool flow

TIG welding and laser welding of boron alloyed Ti-6Al-4V

TIG welds and laser welds have been performed on cast i) Ti-6Al-4V, ii) Ti-6Al-4V+0.06B, and iii)Ti-6Al-4V+0.11B. The effect of boron on the weld microstructure and microhardness is here presented. In theconventional Ti-64 alloy without boron addition large prior β grains exist in the fusion zone and in the heataffected zone, whereas in the alloys with 0.06 wt% and 0.11 wt% boron the prior β grains are found to beapproximately 5 times smaller. In the heat affected zones of the boron doped alloys incoherent TiB precipitates aredistributed mainly along the prior β grain boundaries, inhibiting grain growth at elevated temperature and beingthe main reason for the grain refinement. In the fusion zone TiB precipitates are found in stripes at interdendriticregions. Prior β grains are elongated and their size is refined by boron addition. The microstructure in the fusionzones and heat affected zones consist of fine acicular α. TiB precipitates in the fusion zone restricts the length ofindividual α laths and the size of α colonies. Microhardness measurements across welds reveal increasinghardness with fine acicular microstructure. Boron addition is not found to have significant effect on microhardnessin the welds

Microstructure and Mechanical Properties of Ti-6Al-4V Welds Produced with Different Processes

The effect of defects and microstructure on the mechanical properties of Ti-6Al-4V welds produced by tungsten inert gas welding; plasma arc welding; electron beam welding; and laser beam welding was studied in the present work. The mechanical properties of different weld types were evaluated with respect to micro hardness; yield strength; ultimate tensile strength; ductility; and fatigue at room temperature and at elevated temperatures (200 °C and 250 °C). Metallographic investigation was carried out to characterize the microstructures of different weld types, and fractographic investigation was conducted to relate the effect of defects on fatigue performance. Electron and laser beam welding produced welds with finer microstructure, higher tensile ductility, and better fatigue performance than tungsten inert gas welding and plasma arc welding. Large pores, and pores located close to the specimen surface, were found to be most detrimental to fatigue life