32
Remanufacturing and Advanced Machining
combines electrochemical with mechanical action (Mohammad and Wang, 2016).
As part of the ECMP, a passive film forms on the anode (workpiece) surface given
certain electrochemical parameters due to anodic dissolution. This film is scratched
at high spot regions of the surface with abrasives, then fresh layers of the substrate
are immediately exposed to the electrolyte. Approximately 90% of the material is
removed through electrochemical action while the mechanical abrasion is reduced
to a minimum. The mechanical action increases the removal rate in the high spot
region of the anode surface and accelerates the rate of anodic smoothing. Because
only a small amount of material is removed mechanically, the abrasive tool life is
ca. ten times longer than that of a conventional mechanical tool (Mohammad and
Wang, 2016).
In terms of remanufaturing technologies, interesting reports are published on
electrochemical honing (ECH) applications to improvement of surface quality and
geometrical accuracy of recovered cylindrical shafts (Singh and Jain, 2016). In this
process, most of the metal is removed on the atomic scale by anodic dissolution,
while the honing process acts as a performance multiplier. Before ECH, the twin
wire arc technique was applied to discarded surfaces of cylindrical shafts, recovering them with SS-316 material. The authors demonstrate that ECH of recovered
surfaces provides a glazed texture and average surface roughness of 0.347 μm after
the processing time of 90 s.
Machining and micromachining can be also performed with purely chemical
methods, such as chemical milling (CM). CM is a subtractive machining process
using baths of temperature-regulated etching chemicals to remove material for producing a required shape and blind features like pockets, channels, etc. (Bhattacharyya
and Doloi, 2020). It also removes material from the entire surface of components for
the purpose of weight reduction. This method is extensively used on metals with
removal depths in excess of 10 mm (Tomlinson and Wichmann, 2014), although
other materials are gradually becoming more essential.
The CM process consists of five steps: cleaning, masking, scribing, etching,
and demasking. Solvent-based maskants can be applied by spraying, brushing,
or direct immersion. Today, several new technologies have emerged regarding
chemical milling maskants, so that recovery rates of 95% are available. Etchant
aluminum regeneration performed today is 100% effective (Tomlinson and
Wichmann, 2014). The efficiency of the chemical milling is rather low, between
0.4 and 1.2 mm/h, but in the case of large surfaces, CM appears to be very effective (Koryagin et al., 2000).
1.7 ULTRASONIC MACHINING METHODS
Another efficient and economical process for precision machining, especially of glass
or ceramic materials, is ultrasonic machining (USM). Unlike other nonconventional
processes, ultrasonic machining does not thermally damage the workpiece, which
is important for brittle materials in service (Thoe et al., 1998). Compared to EDM
and ECM, USM is advantageous because it is applicable to dielectric materials, too.
Remanufacturing and Advanced Machining
combines electrochemical with mechanical action (Mohammad and Wang, 2016).
As part of the ECMP, a passive film forms on the anode (workpiece) surface given
certain electrochemical parameters due to anodic dissolution. This film is scratched
at high spot regions of the surface with abrasives, then fresh layers of the substrate
are immediately exposed to the electrolyte. Approximately 90% of the material is
removed through electrochemical action while the mechanical abrasion is reduced
to a minimum. The mechanical action increases the removal rate in the high spot
region of the anode surface and accelerates the rate of anodic smoothing. Because
only a small amount of material is removed mechanically, the abrasive tool life is
ca. ten times longer than that of a conventional mechanical tool (Mohammad and
Wang, 2016).
In terms of remanufaturing technologies, interesting reports are published on
electrochemical honing (ECH) applications to improvement of surface quality and
geometrical accuracy of recovered cylindrical shafts (Singh and Jain, 2016). In this
process, most of the metal is removed on the atomic scale by anodic dissolution,
while the honing process acts as a performance multiplier. Before ECH, the twin
wire arc technique was applied to discarded surfaces of cylindrical shafts, recovering them with SS-316 material. The authors demonstrate that ECH of recovered
surfaces provides a glazed texture and average surface roughness of 0.347 μm after
the processing time of 90 s.
Machining and micromachining can be also performed with purely chemical
methods, such as chemical milling (CM). CM is a subtractive machining process
using baths of temperature-regulated etching chemicals to remove material for producing a required shape and blind features like pockets, channels, etc. (Bhattacharyya
and Doloi, 2020). It also removes material from the entire surface of components for
the purpose of weight reduction. This method is extensively used on metals with
removal depths in excess of 10 mm (Tomlinson and Wichmann, 2014), although
other materials are gradually becoming more essential.
The CM process consists of five steps: cleaning, masking, scribing, etching,
and demasking. Solvent-based maskants can be applied by spraying, brushing,
or direct immersion. Today, several new technologies have emerged regarding
chemical milling maskants, so that recovery rates of 95% are available. Etchant
aluminum regeneration performed today is 100% effective (Tomlinson and
Wichmann, 2014). The efficiency of the chemical milling is rather low, between
0.4 and 1.2 mm/h, but in the case of large surfaces, CM appears to be very effective (Koryagin et al., 2000).
1.7 ULTRASONIC MACHINING METHODS
Another efficient and economical process for precision machining, especially of glass
or ceramic materials, is ultrasonic machining (USM). Unlike other nonconventional
processes, ultrasonic machining does not thermally damage the workpiece, which
is important for brittle materials in service (Thoe et al., 1998). Compared to EDM
and ECM, USM is advantageous because it is applicable to dielectric materials, too.