Laser Based Direct Fabrication

Eric J. Whitney, Applied Research Laboratory, The Pennsylvania State University; John E. Smugeresky, Sandia National Laboratories; David M. Keicher, Optomec Design Company

Many rapid prototyping methods are under development, and some use metals or ceramics separately, while others are exploring using metals and ceramics together. In one process, metal powder can be deposited at a rate of about 0.1 to 10 g/min in precise amounts and with positional accuracy such as a net-shape part results. In another process, metal powder is deposited at a higher rate, about 10 to 100 g/min, resulting in a near-net-shape part. Both of the processes are completely computer controlled such that there are no molds used in the fabrication of a shape. Generally, the processing atmosphere is controlled to limit interstitial contamination from oxygen and nitrogen. The processes completely melt and fuse the metal powder, often achieving full theoretical density. These approaches eliminate or reduce the need for nonfunctional prototyping, such as the traditional trial-and-error prototyping approach using wood, clay, or plastic and the new rapid-prototyping (RP) processes that integrate computer-aided engineering, CAD, and CAM techniques with polymer deposition processes. These RP processes are also known as stereolithography or selective laser sintering (SLS).

During the period that RP was being developed, several laser laboratories experimented with using CNC-controlled laser-cladding systems to build structural parts (Ref 30, 31, 32). These laboratories concentrated on obtaining structurally sound material and addressed making only rudimentary shapes. The importance of this early work, however, was the demonstration that structurally useful material could be deposited using laser technology. More recently, several laboratories have taken advantage of the RP software concepts to push laser-based direct-fabrication technology to a much higher level of shape definition (Ref 33, 34, 35, 36, 37, 38). While there are a number of terms used to describe this technological evolution, for example, rapid manufacturing (RM), flexible fabrication (FF), and solid free-form fabrication (SFF), they are laser-based direct-fabrication techniques for structural rather than model or surrogate materials.

The material deposited by these processes must have structurally useful properties, such as high tensile strength, adequate ductility, good fracture toughness, and good fatigue life. At a minimum, the properties must be equivalent to conventionally cast material of the same composition. The potential of the process, however, is the ability to make materials that have unique characteristics, in shapes that are not constrained to have a single composition throughout the part.

For example, turbine engine parts often operate with severe temperature gradients. The limiting mechanical property in the high-temperature region may be creep while at the lower-temperature region the limiting property may be tensile or fatigue strength. In designing such a chemically homogeneous component manufactured using current processes, a compromise between creep resistance and high fatigue must be made. However, with the laser-based direct-fabrication techniques, parts are formed by depositing a small volume of material and building the part a layer at a time. This allows for composition changes on a scale equivalent to the width and height of the deposit such that gradients in the composition can be intentionally built into the part. Appropriate selection of the composition gradients provides the opportunity to adjust the properties as a function of position, creating so-called functionally graded materials. Thus, a design engineer may be able to increase the life of a component by metallurgically optimizing each region of the part for the anticipated thermomechanical environment. Both thermal profiling and composition profiling, in principle, can be engineered into a part to optimize its design.

Lasers are an ideally suited heat source for laser-based direct-fabrication processes. The laser provides an intense heat source that is easily configured into highly automated systems. Laser-based direct fabrication has much in common with laser cladding and surfacing techniques. Laser power level (energy density), powder deposition rate, and travel speed are parameters that can be scaled to suit the requirements for thermal profiling, size and composition of the deposit to control microstructure, mechanical properties, dimensional tolerances, and surface finish. Figure 11 (Ref 5) shows a schematic of a simple laser-based direct-fabrication setup.

Fig. 11 Basic approach to laser-depositing metallic materials. Source: Ref 5

Many types of lasers can be used, depending on the size and sophistication of the desired part and the type of material being processed (Ref 39). Selection of laser type depends on the light absorption, thermal transport, and thermodynamic properties of the material being deposited. For small intricate parts, a laser power of 1 kW or less can be used (Ref 40). For larger parts, such as those shown in Fig. 12, with thick-wall sections multikilowatt lasers must be used to achieve economical deposition rates. Adjustments to processing parameters are also required to meet dimensional and surface finish specifications.

Fig. 12 Thick-walled Ti-6Al-4V laser-based direct fabrication. Source: Ref 41

Laser-Based Direct Fabricated Microstructures. A wide variety of microstructures, some of which are unique, can be achieved with laser-based direct fabrication, depending primarily on the power level of the laser utilized. At power levels less than about 5 kW, a small molten pool of material is created on a substrate, the added feedstock increases the size of the pool momentarily, and solidification occurs when the laser beam is traversed. Solidification rates are generally high enough, due to the heat-sink effect of the substrate, that metastable microstructures characteristic of rapid-solidification technology are readily generated. This corresponds to ultrafine, extended solid-solution microstructures capable of producing enhanced mechanical properties. In welding terms, cellular dendritic microstructures are produced where the secondary dendrite arm spacing can be used to estimate the solidification rate, with rates of approximately 104 s-1 being realized. Figure 13 (Ref 42) shows 316 stainless steel produced by two different laser-based direct-fabrication processes.

Fig. 13 Fine-grained ductile 316 stainless steel produced under nonoptimized conditions. Left: optical macrostructure, showing layer geometry. Source: Ref 5. Right: transmission electron micrograph showing ultrafine grain size. Source: Ref 42

In making thick-section parts, high-power lasers in the 10 to 45 kW range are generally used. Here the solidification rates are slower, possibly approaching those of thin-walled castings. Figure 14 shows the microstructure for a laser-based direct-fabrication-processed Ti-6Al-4V using a 14 kW CO2 laser and a thin-walled conventional casting.

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Fig. 14 Microstructure of laser-based direct-fabricated Ti-6Al-4V

The chemical and mechanical properties of laser-based direct-fabricated materials are excellent. The following alloys were deposited and tested: Ti-6Al-4V, Ti-5Al-2.5Sn, 316 stainless steel, alloy 625, alloy 690, alloy 718, H13 tool steel, and nickel-aluminum-bronze. Some representative data are presented here.

Ti-6Al-4V. It is well known that titanium alloys are subject to embrittlement by oxygen and nitrogen, impurities commonly introduced during conventional processing. Table 5 shows a comparison of the composition of laser-based direct-fabricated deposited Ti-6Al-4V and the starting powder composition. The data show that, under properly controlled conditions, the increase in oxygen and nitrogen can be held very low. Tensile and hardness properties of laser-based direct-fabricated Ti-6Al-4V material are also within ASTM B 367 Grade C5.

Table 5 Chemical composition of laser-based direct-fabricated Ti-6Al-4V and starting powder

Sample(a)

Composition,

wt%

O

N

H

Fe

Al

V

Starting powder

0.23

0.032

0.012

0.199

6.01

3.95

Laser-based direct-fabricated material(b)

0.23

0.048

0.0072

NC

5.83

3.75

(a) Powder sample average of two tests.

(b) Vacuum mill annealed

The data shown in Tables 5 and 6 are from laser-based direct-fabricated Ti-6Al-4V material made from prealloyed hydride-dehydride powder and using a 14 kW CO2 laser. After forming, the material was heat treated in vacuum at 788 °C (1450 °F) for 2 h. Fatigue testing has shown that laser-based direct-fabricated Ti-6Al-4V is equivalent to cast and hot isostatically pressed material.

Table 6 Tensile properties of laser-based direct-fabricated Ti-6Al-4V

Test

ASTM B 367 C5

Laser-based direct-fabricated Ti-Al-4V

Hardness, HRC

39 max

36

Ultimate tensile strength, MPa (ksi)

896 (130)

1007 (146)

Yield tensile strength, MPa (ksi)

827 (120)

876 (127)

Elongation, (in 25.4 mm, or 1 in.)

6 min

8.5

Alloy 718 is a widely used 7"-strengthened superalloy that is available in cast and wrought forms. The alloy generally exhibits good weldability. Table 7 shows the 538 °C (1000 °F) tensile properties of laser-based direct-fabricated alloy 718 after postform aging.

Table 7 Tensile properties of laser-based direct-fabricated Alloy 718

Test

Typical cast value

Laser-based direct-fabricated alloy 718

Ultimate tensile strength, MPa (ksi)

860 (125)

931 (135)

Yield tensile strength, MPa (ksi)

725 (105)

793 (115)

Elongation, %

9

15

Alloy 625 and 316 stainless steel are austenitic materials that are based on nickel and iron, respectively. The alloys are available in wrought and cast forms. Alloy 625 is a high-temperature material that is corrosion and oxidation resistant. It is used in the chemical, petroleum, paper, nuclear, and aerospace industries and is generally regarded as the most widely used nickel-base alloy in the world. At lower temperature, 316 stainless steel is used in many of the same industries as alloy 625. Both alloys represent a significant portion of the premium metals market.

Excellent properties for 316 stainless steel, where yield strengths double that of wrought annealed bar with no sacrifice in ductility, are routinely achieved by laser-based direct-fabrication processing (Table 8) (Ref 42). The properties of similarly processed alloy 625 exceed those of annealed bar. Properties are dependent on processing parameters, and work on 316 stainless steel demonstrated that mechanisms as simple as grain refinement over conventionally processed materials lead to Hall-Petch strengthening. Figure 14 shows a grain size of approximately 10 /' m compared to 50 ^m for wrought 316 stainless steel. Hardness values have been reported (Ref 42) from 180 to 232 on the Knoop scale, equivalent to Rockwell B values of 85 to 96. Rockwell B values in the 80 to 85 range for stainless steels correspond to tensile yield strengths in annealed bar of about 240 MPa. Tensile yield strengths greater than 480 MPa have been reported (Ref 43) for similarly processed material, for samples with Knoop hardness in the 230 to 240 range (96 HRB). In that study, the grain size within the fabricated structures was reported to be on the order of 5 to 10 /'m, whereas the grain size for the annealed 316 stainless steel is typically around 100 tm. This difference in grain size is believed to be the primary cause of the improved strengths for the laser-based direct-fabricated structures and is consistent with the Hall-Petch relationship between yield strength and grain size. Consequently, full-size parts can be made with a wide range of strengths.

Table 8 Mechanical tensile test data for laser-based direct-fabricated 316 stainless steel and alloy

Plane orientation with respect to

Ultimate

strength

Yield

strength

Elongation

tensile direction

MPa

ksi

MPa

ksi

(in 25 mm, or 1 in.)

316 stainless steel perpendicular

790

115

450

65

66

316 stainless steel parallel

805

117

590

86

33

316 stainless steel anneal bar

585

85

240

35

50

Inconel 625 parallel

930

135

635

92

38

Inconel 625 perpendicular

930

135

515

75

37

H13 Tool Steel. The fabrication and maintenance of tools is a huge cost element for many manufacturing operations. Laser-based direct-fabrication of tools is of immense economical importance. Utilizing the unique metallurgical qualities of laser-based direct fabrication to increase the useful life of tools may also reduce costs. H13 tool steels processed by laser-based direct fabrication can have both a coarse and fine structure (Ref 44), depending on processing conditions. Figure 15 shows the tempering response of laser-based direct-fabricated H13 as compared to a conventionally prepared alloy of the same composition.

700 600 500 400 300

Q

Q

nu o

_ o Subst □ Clad

o

Tempering temperature, °C

200 400 600

Tempering temperature, °C

Fig. 15 Tempering response of H13 tool steel

0 0

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