Titanium alloys and CNC machining
Titanium alloys are extensively used in the aerospace industry besides other industries like medical, marine, jewelry, etc. Most aircraft engines have a significant amount of titanium alloys in their parts with the most commonly used being Ti-6Al-4V alloy. However, the unpredictable machining behavior of the titanium alloy makes it very difficult to machine.
One factor responsible for limiting the cutting parameters during CNC machining of titanium alloys is the cutting temperature. Success in CNC machining of titanium alloys depends largely on overcoming several of the inherent properties of the metal. Titanium alloys have low chip-tool contact area and the thermal conductivity of these metals is considerably low (about 7,3 W/m K). This combination of small contact area and low thermal conductivity results in very high cutting temperatures and impose a constraint on the usage of higher cutting speeds.
In addition, during the cutting of metal, high temperatures are generated in the region of the tool cutting edge, and these temperatures have a controlling influence on the rate of wear of the cutting tool and on the friction between chip and tool.
CNC machining needs a new process paradigm
In today’s CNC machining process paradigm, tool path programming and online control of the machine are separate tasks. The current CNC machining process begins with the CAD model of the part. A plan is then made to determine how to machine the part according to the stock material form, the machining operations and the availability of machines and tools.
Tool paths are generated accordingly and frequently the paths are simulated to detect potential interference problems between the machine tool and the part as well as to check feed rates and tool dimensions. This paradigm has changed little from its inception except perhaps for the addition of the simulation process.
Limited computing power at that time necessitated the division between CNC control and tool path generation, however, the cost and performance of today’s computers no longer warrants this division of labor.
StereoLithography Apparatus
In 1986, inventor Charles W. Hull and entrepreneur Raymond S. Freed founded 3D Systems, Inc., which in 1988 came out with the first commercial Rapid Prototyping (RP) system, the StereoLithography Apparatus (SLA). Today several SLA models are available, including the series of SLA 250/30A, SLA 250/50, SLA-250/50HR, SLA 3500, SLA 5000, SLA 7000 and Viper si2.
StereoLithography (SL), uses the concept of layer additive construction, and builds objects by curing successive thin slices or layers of specific ultraviolet (UV) light-sensitive liquid photopolymer resins, with a low-power UV laser.
The successive layers are built on top of the previous layer, thereby fabricating a 3D part. The self-adhesive characteristic of the photopolymer bonds the layers securely. However, the irradiation of the UV light does not fully cure the photopolymer and most parts have to undergo a post-curing process, which increases their mechanical properties in a direct proportion.
A cleaning solvent is also used to clean up any residual resin after the part has been built on the machine.
3D Systems, Inc., developed the STL format for use with the SLA, which produces a physical 3D model based on an STL format file. The format became an industry standard due to its simplicity of comprising only triangles represented by three vertices and a surface normal vector.
With advancing research however, this format needs updating as it can sometimes lead to incorrect geometry.
SL produces the greatest accuracy and the most acceptable finish among all the RP technologies. Continuing research is going on all around the world, and further improvements in SL, STL, and SLA are anticipated in the near future along with the growth of RP technologies.
Shape Deposit Manufacturing
Shape Deposition Manufacturing (SDM) was developed in Stanford University.
In this approach, when a complicated model is to be made, the cutting and deposition are alternatively used after the model is decomposed into the smallest level. This system has been developed as a hybrid system combining 5-axis CNC equipment and a powder based Rapid Prototype (RP) machine.
SDM is a RP process that systematically combines the flexibility of the additive layer manufacturing process with the precision and accuracy attained with the subtractive CNC machining process.
The basic SDM fabrication methodology is that the material is firstly deposited, and then the part is transferred to the shaping station center where the material is removed by CNC machining to form the desired or net shape of the part, thence sacrificial support material is deposited to support the part.
This method can fabricate a wide variety of materials, including metals, plastics (plastic manufacturing), ceramics, etc. In addition, it has the ability to build heterogeneous structures due to both permitting pre-fabricated components to be embedded within the built shapes and using support materials.
Select Laser Sintering
Beaman and Deckard developed Selective Laser Sintering (SLS) process at the University of Texas at Austin. In the SLS process, a layer of powder is deposited on a support and leveled by a rolling device. A laser beam scans a two-dimensional pattern on the deposited powder layer to sinter the powder layer. After sintering of a layer, a new layer of powder is deposited in the same manner. Successive powder deposition and laser scanning then builds a 3D part.
In the first three years of commercial use, the SLS process was used primarily to produce durable nylon-based prototype and patterns for making silicone rubber molds. Rubber molds are used to vacuum cast durable plastic prototype parts from liquid urethane or epoxy systems.
The direct production of nylon-based parts is generally economical when a limited number of parts is needed. The vacuum casting is attractive when more parts are required, particularly if the parts are larger. While nylon-based and cast prototype parts are useful for some level of functional evaluation, it is necessary to have prototype parts made with the production material and process if more rigorous testing and analysis are required.
As a result, there has always been a great deal of interests in using Rapid Prototyping (RP) processes to produce durable prototype molds for injection molding plastic parts. A plastic pattern is first made by a RP process and then converted to a mold using a secondary process. While these options have some attraction, particularly as the quality of RP patterns has improved, they all suffer from limitations associated with fabrication time, accuracy, durability, stairstepping, or a combination of these factors.
Therefore, the SLS process has since been developed to fabricate metallic, plastic, ceramic, and composite functional parts. SLS now uses two processes, namely, indirect and direct SLS processes, but continuing research promises greater advances in overcoming the limitations and further improving the SLS process.
Rapid Prototyping. Technologies
Rapid Prototyping (RP) employs several technologies to fabricate high-quality parts with the direct utilization of Computer Aided Design (CAD) generated data.
Usually RP is laser (Light Amplification by Simulated Emission of Radiation) based; nevertheless, it also uses other technologies that do not utilize lasers.
Some of the technologies that are utilized in RP are
- CAD
- Selective Laser Sintering (SLS)
- Fused Deposition Modeling (FDM)
- Laminated Object Manufacturing (LOM)
- StereoLithography Apparatus (SLA)
- Laser Engineered Net Shaping (LENS®)
- Electron Beam Melting (EBM)
- Multi Jet Modeling (MJM)
- Patternless Casting Modeling (PCM)
- three dimensional printing (3DP) technologies
- Jetted Photopolymer (J-P) technology
The use of robotics technology is known as Robocasting, which is also used in RP.
RP’s underlying technology is to splice the CAD diagram into cross-sectional triangulated blocks via a stereolithographic file and then build the object block by block at specific points in physical space, using iterative layers of the building materials, which may be in the form of a sheet, liquid, or powder.
The smaller the cross-sectional blocks, the sturdier is the prototype and the better is the finish. As each block is added to the previous block, this method is also known as additive fabrication, although some subtraction may also take place, according to the needs of the design. Due to the creation of smaller blocks, complicated three-dimensional objects can be made easily and quickly with RP.
Any technology that can rapidly produce a part has now come to be known as a RP technology; however, in its truest sense, a RP technology is additive in nature and thus, SLA and SLS are the foremost RP technologies.
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