High strength and ductility of titanium matrix composites by nanoscale design in selective laser melting

doi:10.1016/j.jmst.2021.12.020

Abstract

Abstract:

The use of selective laser melting (SLM) to produce titanium matrix composites (TMCs) with high strength while retaining sufficient tensile ductility suitable for structural applications is emerging as an attractive opportunity in the field of advanced manufacturing. However, the presence of coarse ceramic reinforcements as well as difficulties in optimizing the SLM process is a barrier to the application of TMCs. In this study, we demonstrated the production of TMCs reinforced with in situ high aspect ratio TiB nanowhiskers by selective laser melting using nanosized BN powder additions. Pure Ti with 2.5 vol.% nanosized BN powder showed promise for producing high performance TMCs with retained ductility. BN acted to produce TiB nanowhiskers with diameter < 50 nm. Further, by controlling post process furnace annealing TiB retained a low diameter but exhibited a high aspect ratio, up to 400. In addition to TiB refinement, nanosized BN addition promoted grain refinement during SLM, both acting as a solute to induce nucleation events and, as TiB is formed, providing nucleation sites leading to an ultrafine grain structure in as printed samples and after annealing. The produced TMCs exhibit high tensile yield strength, up to 1392 MPa, while retaining tensile ductility up to 10%. This study has shown how nanoscale design in powder bed fusion additive manufacturing techniques can be used to produce high performance TMCs through a combination of refined grain structure and high aspect ratio TiB leading to TMCs with significant improvement in strength, isotropic properties and retained tensile ductility.

Key words: Additive manufacturing, Selective laser melting, Metal matrix composite, Mechanical properties, Nanostructure, Nanowires

Joseph A. Otte, Jin Zou, Matthew S. Dargusch. High strength and ductility of titanium matrix composites by nanoscale design in selective laser melting[J]. J. Mater. Sci. Technol., 2022, 118: 114-127.

Figures/Tables 12

Fig. 1. Schematic of SLM build setup, showing longitudinal and transverse orientation and tensile sample geometry.

Fig. 2. Particle size and distribution. (a) Particle size distribution of CP-Ti as supplied by AP&C. (b) SEM secondary electron image of CP-Ti. (c) TEM image of BN nanoplates as received from Nanografi. (d) SEM secondary electron image of CP-Ti after mixing with 2.5 vol.% BN. (e) SEM back-scattered electron image of CP-Ti mixed with 2.5 vol.% BN.

Table 1. List of processing parameters for SLM prepared samples with relative density measured by the Archimedes method. The values of density are given as a percentage shown as the mean ± SD.

Empty Cell	Name	Laser power (W)	Scan speed (mm/s)	Hatch spacing (mm)	Heat treatment	Relative density (%)
CP-Ti	T1	70	230	0.085	-	99.4 ± 0.2
	T2	70	195	0.1	-	97.2 ± 0.4
	T3	100	325	0.085	-	96.8 ± 0.6
	T4	100	280	0.1	-	96.1 ± 0.3
	T5	120	445	0.075	-	99.6 ± 0.1
	T6	120	335	0.1	-	98.0 ± 0.2
	T7	120	255	0.13	-	97.5 ± 0.4
	T8	140	520	0.075	-	99.5 ± 0.1
	T9	140	390	0.1	-	97.5 ± 0.5
CP-Ti + 2.5 vol.% BN	TC1	120	445	0.075	-	99.6 ± 0.2
	TC1H	120	445	0.075	Yes	99.7 ± 0.3
	TC2	140	520	0.075	-	99.4 ± 0.2
	TC2H	140	520	0.075	Yes	99.7 ± 0.1
	TC3	180	665	0.075	-	99.6 ± 0.2
	TC3H	180	665	0.075	Yes	99.6 ± 0.1
	TC4	180	385	0.13	-	98.8 ± 0.3
	TC5	120	255	0.13	-	98.4 ± 0.2

Fig. 3. SEM images of SLM produced CP-Ti samples. (a) Sample produced with laser power 70 W, scan speed 195 mm/s and hatch spacing 0.1 mm. (b) Sample produced with laser power 70 W, scan speed 230 mm/s and hatch spacing 0.085 mm.

Fig. 4. SEM images of as printed TMCs (TC3 and TC4) after etching. (a) Secondary electron (SE) image of TC3 sample after light etching for 2 min. (b, c) SE and backscattered electron image of TC4 sample after deep etching for 6 min. (d, e) SE image and associated EDS map of TiB nanowhiskers in TC4 sample.

Fig. 5. Characterization of heat treatment effect on the microstructure of pure Ti and Ti + 2.5 vol.% BN. (A1-A4) SEM images taken of deeply etched samples; TC2, TC2H, TC3, and TC3H samples respectively. (A3) and (A4) inset TEM images of extracted TiB nanowhiskers. (B1-B4) Optical micrographs and SEM images of lightly etched samples; T5, T5H TC3 and TC3H respectively. (C) XRD patterns of Ti + 2.5 vol.% BN powder, TC3 and TC3H samples.

Table 2. Sample compositions measured by ICP-AES and infrared combustion analysis in wt.%.

Samples	Ti	O	B	N	Fe	C	H
TC3	Bal	0.29	0.25	0.06	0.02	0.01	<0.002
TC3H	Bal	0.25	0.22	0.06	0.04	0.03	<0.002

Table 3. Tensile test and hardness results. Average results for each sample tested are presented as mean with ± SD. Literature results are presented for comparison.

Sample		Tensile Test				Hardness, H (GPa)
Empty Cell	Empty Cell	E_y (GPa)	σ_y (MPa)	σ_UTS (MPa)	ε_f (%)	Empty Cell
T5	Transverse	112 ± 2	849 ± 23	1021 ± 30	27 ± 5	2.7 ± 0.4
T5	Longitudinal	113 ± 1	799 ± 44	861 ± 41	28 ± 5	2.7 ± 0.4
T5H	Transverse	105 ± 1	755 ± 6	892 ± 10	30 ± 3	2.5 ± 0.4
T5H	Longitudinal	105 ± 2	789 ± 20	875 ± 15	30 ± 2	2.5 ± 0.4
TC3	Transverse	133 ± 2	1322 ± 14	1384 ± 22	6 ± 1	5.1 ± 0.2
TC3	Longitudinal	132 ± 3	1332 ± 17	1386 ± 30	8 ± 1	5.1 ± 0.2
TC3H	Transverse	133 ± 1	1353 ± 42	1407 ± 29	10 ± 1	5.5 ± 0.1
TC3H	Longitudinal	127 ± 3	1392 ± 17	1453 ± 16	9 ± 1	5.5 ± 0.1
Composition & Method		E_y (GPa)	σ_y (MPa)	σ_UTS (MPa)	ε_f (%)	H_nano (GPa)
Grade 1/2 CP-Ti, SLM[17,33,34]		-	555/630	757/732	19.5/20.3	2.39
CP-Ti + 5 vol.% TiB₂, SLM[33]		-	-	-	-	3.33
Ti6Al4V + 1.5 wt.% B₄C, DMD[35]		122	1150	1190	2.9	-
CP-Ti + 0.5 wt.% TiB₂, SLM[19]		-	∼836	∼923	∼8.3	-
CP-Ti + 1 wt.% TiB₂, SLM[19]		-	∼956	∼1021	∼1.8	-
CP-Ti + 5 vol.% TiB, SPS[29]		-	1163	1285	5.8	-

Fig. 6. Tensile test results. (a) Engineering stress-strain curves for T5, T5H, TC3 and TC3H samples with horizontal bars at the point of failure representing transverse samples and the remainder longitudinal. (b-d) Photographs of tensile samples showing print quality.

Fig. 7. Schematic showing the impact of wide hatch spacing and the effect of BN nanopowder addition.

Fig. 8. TEM analysis of TiB as printed (A) and after furnace annealing (B). (A1) TEM image showing cluster of TiB with inset high mag image of TiB interface. (A2) High magnification image of TiB showing stacking faults parallel to (100). (A3) Indexed selected area diffraction pattern (SAED) at TiB-Ti interface showing orientation relationship with [010]TiB//[11$\bar{2}$0]α-Ti. (B1) TEM image of isolated TiB with inset high magnification image. (B2) High magnification image of TiB showing atomic planes parallel to (100). (B3) SAED of TiB along [010] direction.

Fig. 9. Schematic showing the microstructural evolution that occurs during post process heat treatment. All residual BN is reacted, TiB grows with clusters and defects removed and on cooling α-Ti growth is restricted by TiB nanowhiskers.

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