In this work, crack-free samples with a relative density of 99.5 ± 0.1% were produced from a gas-atomized Al–Cu–Ag–Mg–Ti–TiB2 powder via laser powder-bed fusion. The homogeneous equiaxed microstructure without preferred grain orientation shows the α-Al grains’ mean size to be 0.64 μm ± 0.26 μm TiB2 particles with sizes of several tens of nm up to 1.5 μm were observed in the as-built component. Small TiB2 particles of up to approx. 200 nm are located within the α-Al grains, which show a semi-coherent interface to the α-Al phase. Larger TiB2 particles of up to 1.5 μm accumulate in the liquid between the growing α-Al grains during solidification and inhibit grain growth. Al2Cu phase is precipitated at the α-Al grain boundaries. Coarse Al2Cu precipitates, which are slightly enriched with silver and magnesium, are also observed within the grains preferentially precipitated on small pores and TiB2 particles. The novel fine-grained microstructure results in the as-built state in a tensile strength of 401 ± 2 MPa and total elongation at fracture of 17.7 ± 0.8%.

Microstructure and mechanical properties of a TiB2-modified Al–Cu alloy processed by laser powder-bed fusion

TiB 2 particulates were produced in-situ in aluminum matrices through chemical reactions of K 2 TiF 6 and KBF 4 salts with aluminum melts. The formation and growth of the in-situ diboride particulates are studied in the presence of different alloying elements, Mg, Zr, Ti and Si respectively, modifying the reaction chemistry. Microstructural analyses revealed alloying elements playing a significant role on the formation and growth of the in-situ particulates. The addition of 5 w.t. pct. in-situ TiB 2 particulates, for example, resulted in a 14% increase in ultimate tensile strength for an as-cast Al-Si alloy. The strengthening effect is attributed to both the direct strengthening of TiB 2 particulates as a reinforcement and their effects on modifying eutectic Si in the castings. The morphology and dispersion of the in-situ reinforcing particulates are discussed in terms of reaction chemistry, the heterogeneous nucleation of alpha aluminum grains and the modification of Si in eutectic Al-Si phase.

Tailoring in-situ TiB2 particulates in aluminum matrix composites

Grain refining can improve the solidification-cracking resistance and mechanical properties of welds. Mg alloys are increasingly used for vehicle weight reduction, AZ91 Mg being the most widely used Mg casting alloy. The present study was conducted to study grain refining in gas–tungsten arc welding of AZ91 Mg, focusing on identifying the mechanism of grain refining. Significant grain refining was obtained by transverse arc oscillation. Dendrite fragmentation was identified as the grain refining mechanism, instead of heterogeneous nucleation or grain detachment. This was done by using an overlap welding procedure in which the weld made with transverse arc oscillation (test weld) overlapped with another weld made previously without arc oscillation (preweld). It was found that repeated prewelding followed by welding with transverse arc oscillation resulted in very fine grains.

Mechanism of grain refining in AZ91 Mg welds by arc oscillation

Effect of friction stir processing on corrosion of Al-TiB2 based composite in 3.5 wt.% sodium chloride solution

Aluminum-matrix nanocomposites offer advantageous properties over conventional aluminum alloys. However, controlling the size and size distribution of ceramic nanoparticles during in situ synthesis at high temperatures has been a long-term challenge due to a lack of effective size-control mechanisms. Here, we successfully synthesized titanium diboride (TiB2) nanoparticles with an unprecedented narrow size distribution in molten aluminum. The average size of TiB2 nanoparticles was tunable from 22.1 to 171.4 nm by solely controlling the reaction temperature under a diluted reactant salt solution. To uncover the mechanism of particle size control, an interface diffusion-controlled model was developed. The dilution of reactant salt was crucial to achieve a steady reaction environment while confining the growth of the particles in a shallow region. The model suggests that the average size of as-synthesized nanoparticles is mostly controlled by reaction temperature and unaffected by the titanium salt concentration in a diluted solution due to a steady diffusion of titanium and boron. Temperature controls the diffusion of reactants and nucleation rate to dictate the average size of the as-synthesized nanoparticles.

Size Control of In Situ Synthesized TiB2 Particles in Molten Aluminum

In the grain refinement of aluminum, Al3Ti and TiB2 particles are introduced to reduce the casting grain size down to 200 micrometer level, which makes cold working possible. The particles are brought in by the addition of Al-Ti-B-type master alloys. It is generally believed that TiB2 particles are stable and nucleate α-Al grains in solidification in the presence of titanium in solution from the dissolution of Al3Ti particles in the master alloys. The titanium in solution either forms Al3Ti layers on the surface of TiB2 particles to promote the nucleation of α-Al grains or remains as solute to restrict the growth of α-Al grains in solidification. However, a consensus on a grain refinement mechanism is still to be reached due to the lack of direct observation of the three phases in castings. This paper presents finding of the TiB2/Al3Ti interfaces in an Al-Ti-B master alloy. It demonstrates a strong epitaxial growth of Al3Ti on the surface of TiB2 particles, a sign of the formation of an Al3Ti layer on the surface of TiB2 particles in grain refinement practice. The Al3Ti layer has a crystal coherency with α-Al and hence offers a substrate for heterogeneous nucleation of α-Al grains. However, the layer must be dynamic to avoid the formation of compounded Al3Ti and TiB2 particles leading to the loss of efficiency in grain refinement.

Haibin Ma

Papers

The Interface of TiB 2 and Al 3 Ti in Molten Aluminum