Directed Energy Deposition of AA7075
Cobian Gonzalez, Lucia (2020)
Cobian Gonzalez, Lucia
2020
Materiaalitekniikan DI-tutkinto-ohjelma - Degree Programme in Materials Engineering, MSc (Tech)
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Hyväksymispäivämäärä
2020-06-08
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202006055940
https://urn.fi/URN:NBN:fi:tuni-202006055940
Tiivistelmä
Directed energy deposition (DED) is an additive manufacturing technique which is commonly used to produce high dimension components, made from steels, titanium, aluminum and nickel alloys, in a short period of time. In addition, there is an increasing demand for producing high strength components using DED. AA7075 aluminum alloy is a high strength alloy, whose properties are enhanced by heat treatments. However, like most high strength aluminum alloys, AA7075 is non-weldable, as it suffers from hot cracking when it is welded or additively manufactured, which risks the structural integrity of the component. Nevertheless, there are studies where AA7075 components have been produced by additive manufacturing while trying to suppress hot cracking. A proposed way to reduce the hot cracking tendency is by refining the microstructure with the addition of inoculants.
The objective of this master thesis is to develop a procedure to 3D print the aluminum alloy AA7075 using DED and to solve this problem.
The printability of the alloy is studied analyzing and comparing the results obtained with 3D printed samples, of pure 7075 powders and functionalized with 1.7% and 3.4% vol. TiC nano-particles, subjected to T6 heat treatments and characterized with optical and electron microscopy, EBSD and hardness tests to determine: 1) Will nanoparticles enhance the printability of AA7075 in the laser DED process?, 2) How different is the microstructure in AA7075 printed with and without nanoparticles? 3) Is it overall possible to print AA7075 using laser DED?
Hot cracking is produced in non-functionalized samples by the segregation of Al2CuMg on the grain boundaries. Evaporation and entrapment of Zn on the melt pool cause large single pores on the base of the samples and compromises the mechanical properties of the alloy. T6 heat-treated samples show a minor hardness increase which could have been produced due to Zn evaporation and possible overageing.
Although TiC is not homogeneously distributed in the Al matrix, but in clusters, the addition of TiC successfully suppresses hot cracking by inhibiting dendritic growth produced by an in-creased and more uniform nucleation which results in fine equiaxed grains, and thus enhancing the printability of the material.
The objective of this master thesis is to develop a procedure to 3D print the aluminum alloy AA7075 using DED and to solve this problem.
The printability of the alloy is studied analyzing and comparing the results obtained with 3D printed samples, of pure 7075 powders and functionalized with 1.7% and 3.4% vol. TiC nano-particles, subjected to T6 heat treatments and characterized with optical and electron microscopy, EBSD and hardness tests to determine: 1) Will nanoparticles enhance the printability of AA7075 in the laser DED process?, 2) How different is the microstructure in AA7075 printed with and without nanoparticles? 3) Is it overall possible to print AA7075 using laser DED?
Hot cracking is produced in non-functionalized samples by the segregation of Al2CuMg on the grain boundaries. Evaporation and entrapment of Zn on the melt pool cause large single pores on the base of the samples and compromises the mechanical properties of the alloy. T6 heat-treated samples show a minor hardness increase which could have been produced due to Zn evaporation and possible overageing.
Although TiC is not homogeneously distributed in the Al matrix, but in clusters, the addition of TiC successfully suppresses hot cracking by inhibiting dendritic growth produced by an in-creased and more uniform nucleation which results in fine equiaxed grains, and thus enhancing the printability of the material.