1、_ SAE Technical Standards Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising there
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3、ublication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: +1 724-776-497
4、0 (outside USA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.org SAE values your input. To provide feedback on this Technical Report, please visit http:/www.sae.org/technical/standards/ARP1331B AEROSPACE RECOMMENDED PRACTICE ARP1331 REV. B Issued 1974-03 Revised 199
5、7-03 Reaffirmed 2015-03 Superseding ARP1331A The Chemical Milling Process RATIONALE ARP1331B has been reaffirmed to comply with the SAE five-year review policy. FOREWORDChanges in the revision are format/editorial only.1. SCOPE:1.1 This recommended practice provides recommendations concerning the ch
6、emical milling ofaluminum, magnesium, and titanium alloys, and specialty alloys.1.2 The detailed recommendations are based on practical engineering and production experiencethat will enable industry to make proper use of the chemical milling process.1.3 Safety - Hazardous Materials:While the materia
7、ls, methods, applications, and processes described or referenced in thisspecification may involve the use of hazardous materials, this specification does not address thehazards which may be involved in such use. It is the sole responsibility of the user to ensurefamiliarity with the safe and proper
8、use of any hazardous materials and to take necessaryprecautionary measures to ensure the health and safety of all personnel involved.2. REFERENCES:There are no referenced publications specified herein.3. GENERAL:Chemical milling is a process used for controlled dissolution of metals in order to obta
9、in weightreduction and specific design configurations. The amount of metal removed is a function of thematerial being etched, the etchant composition and temperature, and the time of immersion in theetchant solution.3.1 Application of Chemical Milling Process:This process can be used in the followin
10、g ways:3.1.1 Remove metal from the surface of formed or irregular shaped parts, such as forgings, castings,extrusions, or formed wrought stock. Metal may be removed from the entire surface of the partor from selected areas.3.1.2 Reduce web thicknesses below practical machining, forging, casting, or
11、forming limits.3.1.3 Taper sheets and preformed shapes.3.1.4 Produce stepped webs, resulting in the consolidation of several details into one integral piece.3.1.5 Remove metal from the surface of tough, hard-to-machine alloys.3.2 Advantages of Chemical Milling Process:The advantages of this process
12、are as follows:3.2.1 A part may be chemically milled on both sides simultaneously. In so doing, the part isprocessed twice as fast and warpage, which might result from the release of “locked in“stresses, is minimized.3.2.2 Production capacity is increased because many parts can be chemically milled
13、at one time. This can be done by chemically milling a large piece before cutting out the parts or by millingmany separate pieces in the tank at one time.3.2.3 Close tolerances may be held when chemically milling.3.2.4 There are many ways to save weight by using this process. Extrusions, forgings, ca
14、stings,formed sections, and deep drawn parts may be lightened considerably. Raw stock, such assheet or bar, which would normally be heavier because of the limitations due to standard sizesor minimum thickness restrictions required for forming, forging, or casting may also belightened considerably. U
15、sing this process, parts may be produced with very thin web sectionswithout fear of excessive warpage or distortion by observing the proper relationship betweenpocket size and web thickness.3.2.5 Tapering of sheets, extrusions, or formed sections may be readily accomplished by using thisprocess. Var
16、ious tapers may be made on one or both sides of a part.3.2.6 The design of sandwich construction parts can be improved by leaving heavy bands orstiffeners (integral with one or both skins) at attachment points.3.2.7 Parts may be formed and heat treated prior to chemical milling. Since forming is eas
17、ier prior tothe machining operation, less expensive forming dies are required and costly “check andstraighten“ work is largely eliminated. Warpage resulting from heat treating is also minimized.SAE INTERNATIONAL ARP1331B 2 OF 123.2.8 The process permits the design of lighter-weight, integrally-stiff
18、ened parts which are simplifiedby the elimination of riveting, welding (seam, spot, and fusion), or metal bonding.3.2.9 Parts manufactured by this process normally require no subsequent sanding or polishing of themilled surface.3.2.10 The surface finish of many castings can be improved by chemical m
19、illing.3.2.11 Thin parts may be blanked out using special techniques incorporating the use ofphotosensitive masks and spray etching equipment.3.2.12 Machine practicality or conventional machining methods need not limit the designer ormanufacturer of chemical milled parts.3.3 Limitations of Chemical
20、Milling Process:The limitations of this process are as follows:3.3.1 Fillet radii are determined by factors such as depth of cut, alloy, etchant, and maskant and areapproximately equal to the depth of cut. Inside corners take a spherical shape; outside cornersremain sharp.3.3.2 Aluminum alloy castin
21、gs are normally difficult to chemical mill due to the porosity andinhomogeneity of the cast material. Such castings may be chemical milled where neithersmooth surfaces nor high strengths are required.3.3.3 Welded parts must be considered individually because chemical milling over a welded areaoften
22、results in pits and uneven etching. Many welded materials can be satisfactorily chemicalmilled; however, individual tests should be performed to determine the advisability of chemicallymilling a particular part. Parts may be chemical milled by masking the welded area.3.3.4 Surface irregularities, su
23、ch as dents and scratches, are reproduced in the chemical milledsurface of aluminum alloys. Surface dents and scratches in magnesium alloys tend to wash outor disappear as a result of chemical milling. Selective masking of scratches and dents can beused to produce good parts.3.3.5 Surface waviness a
24、nd thickness variations are reproduced but not enlarged.3.3.6 Normally, cuts in excess of 0.500 inch (12.70 mm) deep are not recommended.3.3.7 Holes, deep and narrow cuts, narrow lands, and sharp, steep tapers should not be attempted.3.3.8 No one etchant can be used on all alloys. Caution should be
25、exercised in selection of theetchant for the alloy being chemical milled.SAE INTERNATIONAL ARP1331B 3 OF 124. RECOMMENDATIONS:4.1 Design Considerations:4.1.1 Depth of Cut: Although cuts up to 2.0 inches (51 mm) deep have been made in plate, thefollowing limitations may be used as a guide:TABLE 1Maxi
26、mum Depth Per Surface Maximum Depth Per SurfaceInch MillimetersSheet and Plate 0.500 12.70Extrusion 0.150 3.81Forging 0.250 6.354.1.2 Depth of Cut Tolerance:4.1.2.1 Because the etchant solutions used in the chemical milling process reproduce the thicknessvariations of the original raw stock, the que
27、stion of expected process tolerances is indeedcomplex. When forging or machining operations precede chemical milling, either the finaltolerances must be enlarged to allow for the thickness variations introduced by theseoperations or these variations must be removed.4.1.2.2 The process tolerances can
28、 be improved by using premium or close tolerance stock, bypregrinding or chemical sizing stock to a close tolerance, by segregating incoming stockaccording to actual thickness by individually handling during the etching cycle or, wherepossible, by using narrower widths of standard sheet which are co
29、ntrolled to a closertolerance by the producing mill.4.1.2.3 A reasonable production tolerance for chemical milling is 0.002 inch (0.05 mm). To thismust be added the actual raw stock tolerance prior to chemical milling. With the abovecomments in mind, the tolerances shown in Figure 1 are recommended
30、as a guide forproduction chemical milling.4.1.3 Lateral Tolerances: For cuts up to 0.250 inch (6.35 mm) deep, a lateral tolerance of0.030 inch (0.76 mm) is normal as shown in Figure 2. This tolerance can be reducedconsiderably, as also shown, with maximum effort.4.1.4 Minimum Land Width: The minimum
31、 land width should be twice the depth of cut but not lessthan 0.125 inch (3.18 mm). It need not be greater than 1.0 inch (25 mm). Narrower lands arepossible but more expensive to achieve.SAE INTERNATIONAL ARP1331B 4 OF 12FIGURE 1SAE INTERNATIONAL ARP1331B 5 OF 12FIGURE 2 - Lateral Tolerances Versus
32、Depth of CutSAE INTERNATIONAL ARP1331B 6 OF 124.1.5 Minimum Width of Cut: The minimum width of cut should be twice the depth of cut plus0.060 inch (1.52 mm) for cuts up to 0.125 inch (3.18 mm) deep and twice the depth of cut plus0.125 inch (3.18 mm) for cuts over 0.125 inch (3.18 mm) deep.4.1.6 Cuts
33、 on Opposite Sides: Cuts on opposite sides of the metals should not be directly oppositeone another because of the heat transfer problem. The recommended minimum distancebetween cuts on opposite sides for normal production work is 0.250 inch (6.35 mm).4.1.7 Tapers: Tapers greater than 0.100 inch per
34、 lineal foot (8.33 mm/m) are not recommended foraluminum. Larger gradual tapers are recommended for other metals.4.1.8 Grain Direction: Aluminum parts should be chemical milled in such a way that the length of thecut will be parallel to the grain.4.1.9 Surface Finish: The surface finish of chemical
35、milled parts is determined by the initial surfacefinish, alloy, heat treat condition, depth of cut, and etchant used. In general, good quality stockfree from scratches, pits, and other damage should be specified for chemical milled parts. Thefollowing surface finishes can be expected:Aluminum - 53 t
36、o 142 RHR (1.35 to 3.62 m)Magnesium - 27 to 63 RHR (0.69 to 1.60 m)Steel - 27 to 223 RHR (0.69 to 5.68 m)Titanium - 13 to 45 RHR (0.33 to 1.15 m)4.1.10 Forming and Heat Treating: Parts should be formed and, when necessary, heat treated priorto chemical milling.4.1.11 Trim Area: Whenever possible, ch
37、emical milled parts should be designed with trim area. Trim area is excess material surrounding the actual part. The excess material is trimmed offafter chemical milling. When no trim area is provided, much time and money is spent inprotecting the edge of the part.4.2 Tooling Considerations:4.2.1 Te
38、mplate Materials: The two most popular template materials are aluminum sheet and glasslaminates. Aluminum templates are recommended for flat or slightly curved surfaces. Glasslaminates are recommended when contoured or irregular shapes are to be scribed.4.2.2 Template Design: Whenever possible, temp
39、lates should be designed with a minimum ofstiffeners across scribe lines so that a knife slip, during the scribing operation, would affect onlythe area of strippable mask.4.2.3 Undercut Ratio: Inasmuch as the process etchant solution dissolves metal not only in adirection perpendicular to the expose
40、d surface but also undercuts the mask, this undercut ratioor etch factor must be known for each alloy and designed into the template.SAE INTERNATIONAL ARP1331B 7 OF 124.2.4 Color Coding: On chemical milling templates that have more than one level of cut, a color codedesignating the sequence of mask
41、removal is recommended. In addition, the depth of cut andfinal thickness for each level should be permanently marked on the tool.4.3 Production Considerations:4.3.1 Cleaning:4.3.1.1 An absolutely clean surface, free of oil, grease, electrochemical or chemical conversioncoatings, primer coatings, mar
42、king inks, scale, soil, and other foreign matter, is the primerequirement to ensure proper adhesion of the mask and uniform etching of the material. Because parts or stock received for chemical milling normally have surfaces contaminatedwith one or more of the above types of soil, it is recommended
43、that standard cleaningprocedures be established for each of the primary alloys and that the correct procedure beused on all parts prior to masking.4.3.1.2 One or more of the following steps is required to satisfactorily clean the alloys of aluminum,magnesium, titanium, and steel.4.3.1.2.1 Solvent wi
44、ping and/or vapor degreasing are used to remove the visible oil or grease, primercoatings, and marking inks. Parts may be hand wiped using a clean rag and a freshsuitable solvent until the soil is removed (normally, solvent wiping is used to remove primeror marking inks) or the parts may be suspende
45、d in a standard vapor degreasing tank untildegreased. Titanium alloys parts shall not be vapor degreased using halogenatedhydrocarbon solvents. In either case, this step greatly prolongs the life of the alkalinecleaner solutions.4.3.1.2.2 Alkaline cleaners remove invisible oil, surface dust, traces
46、of primer, marking inks, andminor inclusions which have been pressed into the surface in the forming or handlingoperations. High pH alkaline cleaners may cause excessive attack on aluminum alloysurfaces.4.3.1.2.3 Scale, oxides, and conversion coatings must also be removed to ensure uniform maskadhes
47、ion. This is done by immersing the parts or stock in solutions which have beenspecially compounded for the specific task, such as the deoxidizers for aluminum alloys orthe descalers for steel and titanium alloys.4.3.1.3 The success of most chemical milling operations depends upon the efficiency of t
48、he rinsingsteps. It is, therefore, very important that all parts be thoroughly rinsed immediately followingeach chemical treatment.4.3.1.4 After cleaning, parts should be handled only with clean, cotton gloves to avoidrecontamination with the natural oils present on the hands.SAE INTERNATIONAL ARP13
49、31B 8 OF 124.3.2 Masking:4.3.2.1 Except where all-over metal removal is desired, all chemical milled parts should be maskedas soon as possible after the parts have been cleaned, dried, and cooled to roomtemperature.4.3.2.2 The mask is a coating of any kind which is used to control the location of the unetched ornonmilled areas of a part by protecting these areas from the action of the etchant solution.4.3.2.3 A mask must have suffici
