CUSTOM TITANIUM ALLOY FOR 3-D PRINTING AND METHOD OF MAKING SAME

[0001] The present application claims the priority of Provisional Application No. 62/338,018 filed on May 18, 2016 and entitled “CUSTOM TITANIUM ALLOY FOR 3-D PRINTING”.

[0002] 3-D printing technology has advanced into mainstream manufacturing for polymer based material systems and has caused a revolution in computer based manufacturing. Polymers based 3-D manufacturing maturation started with basic printing technology and existing polymer formulations. As it matured, the technology and polymer formulations evolved synergistically to deliver desired performance. Metals based 3-D printing is less mature but is beginning to follow a rapid growth curve. The metals printing technologies have narrowed primarily to powder-bed printing systems based on electron-beam, and laser direct melt and binder-jet technologies. Due to being in the early stages of maturation, little has been done to customize alloy composition to optimize overall 3-D manufactured part performance. Of the alloys being applied, refractory alloys such as titanium are among the least mature in this respect.

[0003] The primary cost driver for all three primary 3-D manufacturing methods for titanium parts is the cost of titanium powder. As a result, the efficient use of the titanium powder is essential to successful market expansion of that product. The powder bed printing methods utilize a build box in which the component is built up layer by layer from powder. At completion the build box is full of powder and the component produced is within the box filled with the powder. After printing, the loose powder is removed from around the part and finishing operations are performed on the part. Since only a small fraction of the powder in the build box is incorporated into the part, there is a significant incentive to reuse the excess high cost powder.

[0004] Of the three primary 3-D printing methods applied to titanium alloys, the direct melt technologies based on electron-beam and laser melting represent the majority of titanium part manufacture but the excess titanium powder suffers from oxygen pickup each cycle through the process. The most common alloy for titanium parts is Ti-6Al-4V, grade 5 with a maximum allowable oxygen content of 0.2 wt %. Consequently the manufacturers want to start with as low an oxygen content in the powder as possible to enable the maximum number of re-use cycles for the powder before the oxygen content exceeds the specification limit.

[0005] At the same time, the customers for the 3-D printed Ti-6Al-4V parts want maximum mechanical tensile strength. The typical approach to achieve high strength Ti-6Al-4V parts is to increase oxygen content close to the upper limit of the Ti-6Al-4V grade 5 specification. This of course results in the minimum number of re-use cycles since the oxygen content would quickly exceed that allowed in the specification. This creates a need for a custom Ti-6Al-4V powder alloy composition to compete with the Ti-6Al-4V grade 5 composition and achieve high strength while having an initial low oxygen content to allow the maximum number of re-use cycles.

[0006] Reviewing the ASTM B348 Grade 5 specification for Ti-6Al-4V grade 5 alloy reveals other strength enhancing elements in the alloy specification that can be used to enhance strength independently of oxygen.

[0007] Table 1 illustrates the standard composition specification for Ti-6Al-4V Grade 5 alloy. Oxygen is typically used to enhance strength because it is easy and as a single element it typically has the most effect on strength. Other elements which affect strength include: aluminum, iron, nitrogen, and carbon, each with a positive effect on strength. These elements are not significantly affected by the 3-D printing process, and a combination of these elements can achieve the same strength enhancing results as oxygen enhancement.

[0008] Table 2 illustrates the specification for Ti-6Al-4V titanium powder alloy with aluminum, iron, nitrogen and carbon composition ranges that, when combined, provide the desired strength enhancement in the alloy without high initial oxygen content. Therefore the baseline strength of 3-D printed Ti-6Al-4V parts produced with this Ti-6Al-4V composition would be similar to higher oxygen Ti-6Al-4V and the Grade 5 parts but would have the low oxygen desired for maximum re-use of the powder. The strength would further increase as the powder picked up oxygen as a result of re-use resulting in an overall higher strength curve and a significantly lower cost of production.

TABLE 1
Composition of Ti—6Al—4V alloy as defined in the ASTM B348
Grade 5 specification
Ti—6Al—4V ASTM B348 Grade 5
		Min	Max
	Element	wt %	wt %
	Aluminum	5.5	6.75
	Vanadium	3.5	4.5
	Iron	—	0.4
	Oxygen	—	0.2
	Nitrogen	—	0.05
	Carbon	—	0.08
	Hydrogen	—	0.015
	Other Elements, each	—	0.1
	Other Elements, total	—	0.4
	Titanium	Balance

TABLE 2
Composition of Ti—6Al—4V enhanced strength titanium alloy.
Enhanced Strength Ti—6Al—4V
		Min	Max
	Element	wt %	wt %
	Aluminum	6.3	6.7
	Vanadium	4.2	4.5
	Iron	0.25	0.4
	Oxygen	0.1	0.13
	Nitrogen	0.02	0.05
	Carbon	0.04	0.08
	Hydrogen	—	0.0125
	Other Elements, each	—	0.1
	Other Elements, total	—	0.4
	Titanium	Balance

[0009] The following table lists the chemical analysis of starting bar stock formulated to produce enhanced strength Ti-6Al-4V powder.

TABLE 3
Composition of Ti—6Al—4V enhanced strength titanium alloy
starting bar stock.
	Element	wt %
	Aluminum	6.44
	Vanadium	4.28
	Iron	0.20
	Oxygen	0.09
	Nitrogen	0.04
	Carbon	0.05
	Hydrogen	0.002
	Yttrium	<0.001
	Titanium	Balance

[0010] The experimentally determined room temperature tensile properties of this starting stock are given in the following table with the required minimum properties for ASTM B348 Grade 5.

TABLE 4
Room temperature properties of enhanced strength titanium alloy
starting bar stock.
	Tensile	0.2% Yield		Reduction
	Strength	Strength	Elongation	of
	ksi (MPa)	ksi (MPa)	%	Area %
Enhanced	145 (1000)	131 (905)	16	44
Ti—6Al—4V
ASTM B348	130 (896) min	120 (827) min	10 min	25 min
Grade 5

[0011] As indicated in Table 4, the room temperature tensile properties of the enhanced Ti-6Al-4V meets the property requirements of the ASTM B348 Grade 5 specification although the oxygen content is well below the typical oxygen content of Grade 5 product. Conversion of this starting stock to powder will result in a small increase in oxygen content which will increase strength further with essentially no detriment to ductility.

[0012] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.