IPE 3105 Chemical Milling PDF

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IPE 3105 Modern Machining Process

CHEMICAL MILLING (CHM) Rezaul Karim Nayeem Assistant Professor, MPE, AUST

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CHM • Chemical milling (CHM) is the controlled chemical dissolution (CD) of the work piece material by contact with a strong reagent. • Mostly used on metals, though other materials are increasingly important. • Special coatings called Maskants protect areas from which the metal is not to be removed.

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CHM • The process is used to produce pockets and contours and to remove materials from parts having a high strength-to-weight ratio.

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CHM Process 1. Preparing and pre-cleaning the work piece surface This provides good adhesion of the masking material and assures the absence of contaminants that might interfere with the machining process. 2. Masking Readily strippable mask is used, which is chemically impregnable and adherent enough to stand chemical abrasion during etching.

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CHM Process 3. Scribing Scribing of the mask is guided by templates to expose the areas that receive CHM. The type of mask selected depends on the size of the work piece, the number of parts to be made, and the desired resolution of details.

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CHM Process 4. Etching The work piece is then etched and rinsed During CHM, the depth of the etch is controlled by the time of immersion.

5. De-masking the mask is removed before the part is finished. Etching Process

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CHM Process

Sequence of processing steps in Chemical Milling; (1) Cleaning of Raw part, (2) Apply maskant, (3) scribe, cut & peel the maskant from areas to be etched, (4) etching, (5) remove maskant and clean to yield finished part

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CHM Process

CHM Setup

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CHM Process • Typical reagent temperature range from 37 °C to 85°C. Faster etching rates occur at higher temperatures, but must be controlled within ±5°C of the desired temperature in order to attain uniform machining. • When mask is used, the machining action proceeds both inward from the mask opening, laterally beneath the mask thus creating the etch factor • In order to avoid uneven machining, the chemicals that impinge on the surface being machined should be fresh. • The chemicals used are very corrosive and, therefore, must be handled with adequate safety precautions.

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CHM Process • The etch factor is the ratio of the undercut d to the depth of etch T. 𝑬𝒇 = 𝒅Τ𝑻 Where, 𝐸𝑓 = Etch factor d = Undercut T = Depth of etch

Etch factor after CHM

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CHM Process • Successive steps of mask removal and immersion can achieve stepped cuts. • Tapered cuts, can also be produced without masking the work piece by controlling the depth and rate of immersion. E = 𝒔Τ𝒕 Where, E = Rate of etching s = Depth of cut t = Immersion time Contour cuts by CHM

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CHM Process

Machining tapers and steps by CHM

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Tooling for CHM • Tooling for CHM is relatively inexpensive and simple to modify. • Four different types of tools are required.  Maskants  Etchants  Scribing templates  Accessories

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Tooling for CHM 1. Maskants • Generally used to protect parts of the work piece where CD action is not needed. • Synthetic or rubber base materials are frequently used. • Multiple coats of maskant are frequently used to increase the etchant resistance and avoid the formation of pinholes on the machined surfaces.

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Tooling for CHM • Maskants should possess the following properties:  Be tough enough to withstand handling  Adhere well to the work piece surface  Scribe easily  Be inert to the chemical reagent used  Be able to withstand the heat generated by etching  Be removed easily and inexpensively after etching

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Tooling for CHM 2. Etchants • Etchants are acid or alkaline solutions maintained within a controlled range of chemical composition and temperature. • Their main technical goals are to achieve the following:

   

Good surface finish Uniformity of metal removal Control of selective and inter-granular attack Control of hydrogen absorption in the case of titanium alloys

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Tooling for CHM  Maintenance of personal safety  Best price and reliability for the materials to be used in the construction of the process tank  Maintenance of air quality and avoidance of possible environmental problems  Ability to regenerate the etchant solution and/or readily neutralize and dispose of its waste products

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Table: Maskants and Etchants for Different Work piece Materials

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Tooling for CHM 3. Scribing templates • Scribing templates are used to define the areas for exposure to the chemical machining action. • The most common method is to cut the mask with a sharp knife followed by careful peeling of the mask from the selected areas. Layout lines or simple templates of metal or fiberglass guide the scribing process. • The etch factor allowance must be included in any method used for the scribing operation.

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Tooling for CHM

Numerical Control (NC) laser scribing of masks for CHM of a large surface area

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Tooling for CHM 4. Accessories • Accessories include tanks, hooks, brackets, racks, and fixtures. • These are used for single-or-multiple-piece handling into and out of the etchants and rinses.

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Process Parameters Solution Type

Circulation Mixing Concentration

Maskant Properti

Operating Temperature

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Process Parameters • These parameters will have direct impacts on the work piece regarding the following: 1. Etch factor (d/T ) 2. Etching and machining rate 3. Production tolerance 4. Surface finish • For high-quality and low-cost parts, heat treatment state of the work piece is needed to be considered.

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Material Removal Rate • The material removal or etch rate depends upon the chemical and metallurgical uniformity of the work piece and the uniformity of the solution temperature. • Etching rates were high for hard metals and were low for softer ones (Metals Handbook, 1989). • Generally, the high etch rate is accompanied by a low surface roughness

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Material Removal Rate

CHM average roughness of some alloys after removing 0.25 to 0.4 mm (El-Hofy, 1995 )

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Material Removal Rate

Surface roughness and etch rate of some alloys after removing 0.25 to 0.4 mm (El-Hofy, 1995)

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Accuracy • The undercutting behavior in simple contouring is essentially the same as in chemical blanking. • Each factor in chemical contouring is defined as the undercut divided by the depth of cut. • Allowance for undercut is made in the design itself. With optimum time, temperature and solution control, accuracies of the range of +/- 0.01 mm can be achieved on relatively shallow depths of cut. • Sharp radii cannot be produced in the cutting direction.

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Surface Finish • The surface produced by CHM process are stress free and show no thermal effects. • The quality of finish is lower for extrusions, forgings and castings. • Aluminium alloys show better surface of the order of 1.6 µm. • Hydrogen embrittlement may occur owing to the absorption of hydrogen in chemical machining in some metals. So considerable care should be taken for steel, stainless steel, copper alloys and nickel alloys. • Hydrogen embrittlement can be overcome by heating the work piece 120°C for 1 to 4 hours.

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Advantages • Weight reduction is possible on complex contours . • Simultaneous material removal, from all surfaces, improves productivity and reduces wrapping. • No burrs are formed. • No stress is introduced to the work piece, which minimizes the part distortion and makes machining of delicate parts possible. • A continuous taper on contoured sections is achievable. • Design changes can be implemented quickly.

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Advantages The capital cost of equipment is relatively low. A less skilled operator is needed. Tooling costs are minor. The good surface quality in addition to the absence of burrs eliminates the need for finishing operations. • Decorative finishes and extensive thin-web areas are possible. • There are low scrap rates (3%). • • • •

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Limitations • • • • •

Handling and disposal of chemicals can be troublesome. Sometimes can be time-consuming, repetitive, and tedious. Metallurgical homogeneous surfaces are required for best results. Porous castings yield uneven etched surfaces. Welded areas frequently etch at rates that differ from the base metal.

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Limitations • Material removal from one side of residually stressed material can result in a considerable distortion. • The absence of residual stresses on the chemically machined surfaces can produce unfavorable fatigue strength compared with the processes that induce compressive residual stresses. • Hydrogen pickup and inter-granular attack are a problem with some materials. • Scribing accuracy is limited and complex designs become expensive. • Steep tapers are not practical.

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Applications • Elimination of the decarburized layer from low alloy steel forgings • Elimination of the recast layer from parts machined by EBM and EDM • Removal of sharp burrs from conventionally machined parts of complex shapes • Removal of a thin surface from forgings and castings prior to penetration inspection below the surface (required for the detection of hidden defects)

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Applications

Thinning of parts by CHM (Tlusty, 1999)

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