145 Description Of Various Laserbased Rapid Manufacturing Techniques

All of the laser-based rapid manufacturing processes have the following three common machine elements:

1. A laser beam delivery system

2. A powder or wire feedstock delivery system

3. A means to manipulate the laser beam or the part or both in at least three linear dimensions

Other enhancements often included in laser-based solid free-form fabrication processes are some type of environmental control apparatus so that processing can be accomplished under an inert or reactive gas environment and a vision system. In some cases, it seems feasible to operate such systems under vacuum. In addition control features are often necessary to control or monitor the process. Each implementation of laser-based solid free-form fabrication claims a unique combination of the three common elements and enhancements that provides certain advantages. There are approximately seven laser-based solid free-form fabrication processes being actively developed and commercialized. Each one of these processes will be described briefly, and then a detailed description of one of these processes (the Penn State process) will be provided.

Los Alamos National Laboratory is developing a process called Directed Light Fabrication (DLF) [12]. In this process, a part is moved in X and Y and the laser-focusing head is moved in the Z direction as the structure is built. A fiber optically delivered Nd: YAG laser is used in this process. The process is conducted in an inert atmosphere. Layer thickness is between 0.075 and 0.25 mm. Materials reported to be processed include 410 stainless steel, P20 tool steel, Ni-Al, molybdenum disilicide, Ag-Cu alloys, and Fe-28%Ni. Deposition rates and other process particulars were not been reported, nor have basic mechanical properties of the deposits for many of these materials. More recently, DLF has been used to process rhenium and iridium with some success [13]. Porosity was reported in the deposits, but this was thought to be due to impure feedstock.

DTM Inc. has commercialized the SLS process using polymer-coated metal powder [14, 15]. In this process a laser is used to cure the polymer-coated metal powder such that the resulting structure has sufficient green strength to be handled subsequent to sintering in a furnace. The results show that dimensional control can be achieved by careful coating of the powder and selection of processing conditions. As expected sintered parts were porous as it is exceedingly difficult to get 100% dense material from a sintering operation. Nevertheless, the SLS process shows great promise in reducing the cost of press and sintered powder metal components by replacing traditional powder metal sintering processes. Materials reported to have been deposited by SLS are Ti-6Al-4V, 1018 steel, BNi-6 and a cobalt braze material [16], and intermetallics [17]. Unfortunately, mechanical properties for the material were not reported. Back infiltration of Cu in ferrous-based SLS structures has also been accomplished [18]. Infiltration of the structure with copper is reported to provide additional structural strength. Cu infiltration of ferrous powder metallurgy products is a common practice. Further, SLS has been used to form integral cans for subsequent hot isostatic pressing (HIP) of the SLS green part [19]. This is the so-called SLS/HIP process. Can removal of conventionally prepared hot isostatically pressed parts is expensive. If the can could be formed from the same material as the component then a near-net-shape part could be produced with little subsequent machining. In yet another adaptation of the process, SLS has been used to fabricate in situ cermets for turbine blade tip manufacture and restoration [20]. In this process a titanium-coated oxide particle, apparently based on a particle described by Cooper et al. [21], is used as the ceramic particle in a metal matrix. Titanium is chosen since it will promote wetting of the oxide particle by the metal matrix. Wear testing has shown this combination of metal matrix and titanium-coated oxide provides improved performance of the turbine.

The University of Liverpool has perfected a process called laser direct casting (LDC) [22]. In LDC a six-axis computer-controlled machine is capable of building three-dimensional objects without the limitation of the build angle or without the need for integral supports. In the LDC a 1.5-kW CO2 laser was used to deposit stainless steel up to 9 mm3/s. Travel speeds between 500 and 1000 mm/min were used, and it was found that fully dense, porosity-free deposits were readily obtained. The efficiency of powder usage was reported to be between 8 and 24% using a powder nozzle that dispersed powder over a wide area, much wider than the melt puddle. When an improved nozzle was used, powder utilization rose to as high as 85%. Microstructure of LDC deposited 316L stainless steel was that of a very fine grained casting with epitaxial growth between layers. Small pores were observed in the deposits of Ni, but its presence was discounted as insignificant, and no mechanical properties were reported.

The University of Michigan has developed a process called Direct Metal Deposition (DMD) [23]. In DMD a 5-kW CO2 laser is used to completely melt and fuse powder feedstock. The DMD process is similar in nature to the previously described DLF processes. DMD has been successively used to deposit H13 tool steels in complex die patterns. It was found that the microstructure of the DMD deposited H13 was finer than that of conventionally processed material and that this may have beneficial effects in terms of reducing wear. Post-DMD heat treatments showed that the H13 material responds to conventional temper operations and that post-DMD tempering is desirable. Hardness, microstructure, and residual stress measurements were reported for DMD H13; however, no mechanical properties have thus far been published.

Sandia National Laboratory has pioneered the development of a process called laser engineered net shapes (LENS) [24]. In the LENS process a fiber optically delivered Nd:YAG laser is used to completely melt and fuse powder feedstock. The process is performed under an inert argon atmosphere where the oxygen content is reported to be less than 10 ppm. Currently, the LENS system utilizes a three-axis motion system where the part translates in X and Y and the laser beam is adjusted in the Z direction as the part builds. Many materials have been processed using LENS, including 316L stainless steel, alloy 625, Ti-6Al-4V, and H13 tool steels [25]. As will be shown later, tensile data has been reported for 316L stainless steel, alloy 625, and Ti-6Al-4V. A schematic diagram of the process, as implemented by Optomec Design Corporation, is shown in Fig. 14.1.

Figure 14.2 represents a complex, hollow shape that was fabricated using the LENS process. The shape encompasses features such as internal passages, angles, corners, and variation in deposit thickness.

Huffman Corporation (Clover, SC) has developed a unique laser cladding machine for the repair of turbine blade tips. It is a five-axis computer-controlled machine usually equipped with a 1- or 2-kW CO2 laser. Although not explicitly used for laser-based free-form fabrication, it has been used to make some interesting shapes. Figure 14.3 shows golf tees fabricated from alloy 625. No metallurgical or mechanical tests were performed on the deposits.

Penn State University has developed a high-power laser-based solid free-form fabrication process under a DARPA/ONR contract (N00014-95-C0029), which was directed by the Applied Physics Laboratory, Johns Hopkins University, and for which MTS Inc. provided a preliminary design of a commercial unit. Material produced under the DARPA/ONR program was equivalent in tensile strength,

X-Y Positioning Stages

FIGURE 14.1 Schematic of LENS process.

FIGURE 14.2 Intricate shape made by the LENS process (part is approximately 7 cm long).
FIGURE 14.4 Schematic of Penn State laser-based rapid manufacturing process.

tensile ductility, charpy impact, and fatigue to conventionally cast and hot iso-statically pressed material of similar composition [26, 27]. A schematic of the process is shown in Fig. 14.4.

In the Penn State process, a 14-kW CO2 laser is introduced into a specially designed processing chamber via an opening in the lid. In this system the laser

FIGURE 14.5 Large laser-based rapidly manufactured Ti-6Al-4V shapes.

beam is translated in the X, Y, and Z directions; the part remains stationary or can be rotated in the X-Y plane. The laser beam manipulation device is attached to the center lid of the processing chamber so that as the laser beam is translated the lids move in unison, keeping the lid opening and the laser beam coincident. Argon or nitrogen gas enters the processing chamber near the bottom and is distributed throughout the chamber through a diffuser plate. The powder feeder is situated above the target and is fed into the processing chamber via a water-cooled copper feed tube. A nozzle directs the powder into the molten metal. After each layer the laser beam and powder nozzle are indexed vertically so that the relationship between laser beam, powder entry point, and target remains unchanged over time. Various sensors are used to monitor the process, including an oxygen sensor and vision system. Other sensors periodically measure laser beam attributes. The information is provided to an operator who can then make adjustments as the process progresses. In some cases brief interruptions in deposition can be made in order to make adjustments to machine and laser conditions that cannot be made during deposition.

Figure 14.5 shows a representative sample of shapes made under the DARPA/ ONR contract. The shapes were made using Ti-6Al-4V powder as the feedstock. Corners, curves, and cylinders have been demonstrated.

Aeromet Corp. has commercialized the DARPA process under the name Lasform [28]. The process currently practiced by Aeromet is limited to titanium alloys and is capable of building complex components with a footprint of approximately 3.6 x 1.2 x 1.2 m.

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