MIL offers numerous metal finishing processes. These methods improve the surface finish of a material. After finishing, parts may have improved performance in specific environments, abrasion resistance, color dyes, etc.
MIL offers numerous metal finishing processes. These methods improve the surface finish of a material. After finishing, parts may have improved performance in specific environments, abrasion resistance, color dyes, etc.
Aluminum alloys are extremely valuable and low-maintenance materials that, due to their high temperature control and overall versatility, can be used in a variety of applications from cooking to automotive to use in air and space craft. They are not, however, very hard materials. Unlike pure aluminum, they are not self-passivating and are therefore more susceptible to corrosion and wear. In order to better utilize the benefits of aluminum and to achieve enhanced durability, hardness, corrosion and wear resistance, these industries and others have turned to the process of Aluminum Anodizing.
What is Anodizing?
Many people have mistaken anodizing for a plating process. This is inaccurate, as the surface to be anodized does not receive a superficial coating deposited in the same manner as a surface to be plated. Plating is a process where a coating is applied to the surface. Anodizing is a process that modifies the surface. By applying electricity to aluminum, the parts become the anode against the cathode in a completed electrical circuit while immersed in an electrolyte acid bath. The electricity and acid bath work together to open the surface’s texture and crystal structure, and build the thickness of the natural oxide layer. In this same process the surface also has increased hardness which is crucial in the many applications for anodized aluminum.
Depending on the type required, MIL can provide an anodize thickness range as follows:
As the surface texture and crystal structure is modified, the anodized surface becomes ideal for absorption of dyes in color application. From black to gold to red to many other colors, anodized aluminum surfaces offer color stability under ultraviolet rays and do not chip where a painted surface would.
After the parts are immersed in an electrolyte solution made up of chromic, sulfuric, boric sulfuric acid, or are dyed, they are sealed per specification or purchase order requirements. This forces the pores that have been opened to swell shut and prevent unwanted dye or other fluid absorption, and provide the corrosion resistance that bare aluminum does not have.
While an anodized aluminum part can stand up on its own, the process can also be used as a pre-treatment for other coatings processes. Anodized surfaces provide increased adhesion under coatings such as Solid Film Lubricant and Prime and Paint, for optimal performance and endurance.
In order to take advantage of the many inherent qualities that aluminum has to offer, anodizing of the surface can enhance the product for durability that far outlasts bare aluminum. MIL can provide a number of different types of anodize, ranges of thickness and color, and multiple sealing solutions. Magnetic Inspection Laboratory also retains numerous aerospace Prime and Nadcap approvals to support these process requirements.
Black oxide is a conversion coating formed by a chemical reaction produced when parts are immersed in the alkaline aqueous salt oxidizing solution operated at approximately 290°F. The reaction between the iron of the ferrous alloy and the hot oxide bath produces a magnetite (Fe3O4) on the part surface.
Black oxide is commonly applied to carbon and low alloy steels, but different chemistries can be used to form black oxides on stainless steel and copper based alloys. These coatings are particularly suited for moving parts, particularly for sliding or bearing surfaces, by providing a finish coating that will retain an oil film. These coatings are also suited for parts that cannot tolerate dimensional buildup of a more corrosion-resistant finish.
The coatings present a pleasing appearance frequently employed for decorative purposes or decreases in light reflection. Very limited corrosion protection, under mildly corrosive conditions, is obtained with black oxide coating. Usually the parts are oiled or waxed.
Precision gear assemblies benefit from black oxide treatments due to its oil-retaining properties. It also aids in the inspection of wear patterns of mating gears.
As with other plating / conversion processes, hardened steels greater than 40 HRC may require hydrogen embrittlement relief bake dependent on specification requirements.
Electrodeposits of Cadmium are used to protect various steel substrates from corrosion. Because cadmium is anodic to iron, the underlying ferrous metal is protected at the expense of the cadmium plating even if the cadmium becomes breached, exposing the substrate.
Besides having excellent corrosion protective properties, cadmium has many useful engineering properties including natural lubricity.
Cadmium also has excellent electrical conductivity, low contact resistance, good solderability and excellent ductility properties.
The obvious downside of cadmium plating is its toxicity, health, safety and environmental concerns. There have been many attempts to replace cadmium, and some have been successful with certain properties, but none have incorporated all of its unique capabilities.
Many specifications have various classes and types defining thickness and dichromate post treatments. Typical thickness ranges are 0.0001” to 0.0006” per surface. Dichromate post treatments do not add to thickness but must be performed after hydrogen embrittlement baking.
Heat treated steels, particularly those plated and used at 35 HRC or greater are susceptible to hydrogen embrittlement. Pre-plating stress relief and post process embrittlement relief baking may be required for parts with a hardness of 35 HRC or greater. Each cadmium process specification is has different requirements. Be cautious and thorough about defining requirements on a contract or purchase order. Caution: An omitted or time-delayed embrittlement relief bake can have catastrophic results!
Racking of detailed machined parts can be challenging in throwing cadmium into deep bores requiring complex conforming anode fixturing.
Barrel cadmium plating can be utilized on small, non-critical parts when allowed by specification. Barrel rotational speed is limited to 7 RPM leaving the parts free of rack marks, nicks and dings.
Few surface treatments offer the same protection and enhancement as a chemical conversion coating while maintaining as-received dimensional tolerances. Chem Film, also referred to by proprietary names suchs as Irridite and Alodine, is traditionally a chrome based conversion coating that can be applied to aluminum to offer corrosion resistance, increased adhesion properties, and depending on the type required, electrical conductivity. In contrast with anodized and phosphate finishes, these properties can be achieved without adding discernable thickness or altering the part.
Every day, MIL works with countless parts that will be exposed to environmental conditions and subject to deterioration. Just as passivation is performed to prevent corrosion on stainless steel, chem film is ideal for protecting aluminum. A good chem film coating can withstand salt spray testing for as many as 168 hours without showing signs of oxidation or corrosion whereas the same material left bare is far more susceptible to the affects of exposure.
As aluminum is notorious for its poor adhesion properties, aerospace industry specifications nearly always require a pre-treatment prior to application of coatings like prime and paint. Unlike a pre-treatment like anodize, which builds and changes the structure of the part surface, chem film is simply a very thin coating that is applied to the surface. Before it dehydrates, the coating behaves as a gelatinous film that due to its soft nature, acts as an open molecular bonding structure similar to glue. This makes it an ideal base to increase paint adhesion and prolong the life of inorganic and organic coatings alike. When anodizing is required, masking adhesion is also greatly increased in the same way to prevent anodizing leakage onto surfaces with tight tolerances. Parts that are masked without the aid of chem film as a pre-treatment are immediately at greater risk for anodizing leakage which can lead to Disposition: Scrap.
There are two classes widely used in the aerospace industry. As established by MIL-DTL-5541, both Class 1A and Class 3 refer to coatings that offer increased corrosion resistance and adhesive properties. Class 3 however, also maintains electrical conductivity. Even after 168 hours of salt spray testing, Class 3 coatings must exhibit electrical resistance at no greater than 10,000 microhms per square inch. This property supports frequent integration of aluminum into applications such as aircraft controls where electrical conductivity is crucial.
MIL-DTL-5541 also has established two types of chem film coating. Type I is the traditional hexavalent chrome based coating that is most recognizable by its color that can range anywhere from yellow to gold to green to brown, and even clear upon request. Besides the value of the coating itself, the appearance lends itself well for cosmetic or identification purposes. Type II is the environmentally friendlier one and can come in the form of a trivalent-chrome or chrome-free coating. The introduction of this type has resulted in increasing frequency of engineering drawing revisions due to new European standards like RoHS, and an overall push in the aerospace industry for greater environmental responsibility.
To protect aerospace, medical, and aluminum parts from many other industries, MIL has included both types and classes established in MIL-DTL-5541, and many other chem film specifications to its roster of processing capabilities. Just as with nearly all of the processes it performs, MIL maintains a large number of Prime and NADCAP approvals for chem film as evidence of its processing excellence. Aluminum parts coated at MIL are thus uniquely protected and supportive to offer enhanced life span and efficiency.
Chemical Milling is defined as a process for the selective and controlled removal of metal from a surface through the use of chemical etchants. Sounds simple enough. So why is so difficult?
Nearly all metallic material types can be chemically milled. For what MIL Inc. does, it’s limited to decarb / alpha case removal of forgings and stock reduction of titanium aircraft skins.
Decarb removal of ferrous forgings may require up to 0.125” be removed. Verification of material removal is accomplished by use of “witness pads” in defined locations of the part and by hard dimensional reads before and after. Surface finish, usually 32 micro inches or better is required, because only a portion of the forging is machined and the remaining as-forged portion is essentially in its final blueprint condition.
Stock reduction of titanium sheet is required to selectively reduce material thickness in low stress areas of sheet material with the goal of weight reduction. Typically, the entire section of sheet is masked and then a template is used to scribe a pattern in the area to be chemically milled.
Regardless of chemical milling method, there are numerous process variables in a state of flux requiring the utmost in process controls and technician experience. Masking, racking, chemical control and technician experience are key attributes in controlling this one-shot, get-it-right process.
Hydrogen embrittlement is a critical concern in milling ferrous materials as is intergranular attack and end grain pitting. Hydrogen absorption is a key control attribute in chemical milling of titanium alloys in addition to IGA and EGP.
Electropolishing is commonly referred to as a “reverse plating” process. Electrochemical in nature, electropolishing combines rectified DC current and electrolyte to remove defects or a controlled amount of material from a metallic part surface. Material removal can be controlled to within 0.00005” per surface.
While the electropolishing is best known for the bright, polished finish, other benefits include deburring, micro finish enhancement, size control and the concurrent benefit of passivation.
Etching is used for many things. The primary goal of etching is to remove material and prepare the surface for plating, anodizing, conversion, anodize, etc. You must be careful with etch because you can introduce hydrogen which may embrittle the part depending on the alloy and strength.
Passivation is intended to restore corrosion resistance of parts machined from austenitic, ferritic, and martensitic corrosion-resistant steels of the 200, 300, 400 series and precipitation hardened corrosion-resistant steels.
The non-rusting properties of stainless steels are attributable to a very thin, invisible oxide film that inherently covers the surfaces of the part and prevents corrosion from occurring. Theoretically, a just-machined, polished or pickled part will acquire this film quickly from atmosphere. In practice, however, such fabricated parts may be contaminated with small particles of foreign material that must be removed to impart the full properties of stainless steel. As an example, residual cutting tool fragments may be impinged into the machined part during manufacture. The primary purpose of passivation is to remove surface contamination and restore optimum corrosion resistance of stainless steel alloys. Passivation IS NOT a scale removal treatment and will not affect part tolerances.
Stainless parts that have been carburized or nitrided shall not be passivated. Additionally, some stainless steel types such as 303, 416, 420, 430, and 440 are problematic and may flash attack, during passivation. A commom practice of Alkaline – Acid – Alkaline processing is used as a means of resolving such concerns.
There are many Types of passivation solution make-up formulated specifically for the type of material being passivated. Most specifications have a table or chart defining recommending the best method.
Many specifications require post process validation testing to ensure process effectiveness. Testing methods may require one of the following:
This conversion coating is typically used to provide a coating on titanium that is receptive to anti-galling and organic finishes such as paint and dry film lubricants.
Manganese phosphate coatings are a popular phosphate coating due to its hardness and outstanding wear resistance. This coating is often used for both corrosion resistance and lubricity and are applied only by immersion.
Zinc Phosphate coatings are used for corrosion resistance, as a lubricant-holding layer, and as a paint/coating base and can also be applied by immersion or spraying.
The controlled chemical removal of surface oxides (scale) from iron and steels by immersion in an aqueous acid solution. The most common pickling solutions are carbon steels sulfuric and hydrochloric acid solutions and nitric and hydrofluoric acid solutions for stainless steels and titanium alloys.
Pickle and etching is limited to oxide removal and seldom exceeds material removal greater than 0.001” per surface. Removal of 0.0001” to 0.0002” is usually sufficient to remove oxides.
Hydrogen embrittlement relief bakes may be required for high strength steels heat treated to HRC 39 and greater.
A very light etching process utilized to remove traces of smeared metal introduced during previous processes, or light oxide layers that may have formed. The etch assists in revealing flaw conditions that would otherwise have been masked. Maximum metal removal typically does not exceed .0002” and is often times even less than that. Performance of prepenetrant etching is driven by specification or customer requirements.
This cleaning procedure removes scale, tarnish films, and other contaminants that form or are otherwise deposited on the surface of titanium during processing.
Ultrasonic cleaning uses sound frequencies of 15 to 40 KHz in combination with chemicals to clean parts. Sound frequencies sets up cavitation (formation of small air bubbles) that helps scrub contaminants off parts. Various chemicals may be used depending on contaminant to be removed.
Solvent vapors resulting from boiling solvent are utilized to clean heavy oils and greases from parts. Parts are positioned in the vapor zone, and the solvent vapors condense on the cooler parts and flush away surface contaminants. When parts are removed from the vapor zone through the vapor-air interface, the solvent flashes off the surface and leaves the surface clean and dry.
Shot peen, a cold-working metal process, is performed by directing a controlled stream of round particles (shot) against a metallic surface, creating a compressive layer in that surface. While the primary purpose of this layer of compressed material is to provide resistance to fatigue cracking, the consequential work hardening of the surface can provide the added benefit of resistance to fretting and wear. To accurately repeat this process requires a heightened degree of control and precision.
MIL provides the shot peen process to accompany our other capabilities, serving the aerospace, medical, and power generation industries. We currently have two machines online with additional capacity forthcoming.
Fracture Critical Components such as:
Landing Gear Systems such as:
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