You can form an arbitrary iron shape by simultaneous electroforming from a large number of parallel needles? In a near-boiling, oxygen-starved solution of hydrochloric acid, you can run current through some needles, but not others, to promote the deposition of iron on a cathode surface a short distance away (millimeters or less) around just those needles. By withdrawing the array of needles as the cathode grows, the inter-electrode distance remains constant. If the needles are themselves iron, they will dissolve anodically and be deposited (in electrolytically purified form) on the cathode, and will need to be fed in through some kind of wire feed mechanism, but if they are graphite or a noble metal, then iron must instead be supplied in ferrous form by pumping in more electrolyte to replace the spent electrolyte; pumping it through the centers of the needles themselves is one possibility.
If the needles are moved laterally as well as being withdrawn, they can produce features of finer detail than the spacing between needles.
And, of course, if the current is reversed, the same method produces local anodic dissolution and becomes selective electrochemical machining, rather than selective electrodeposition.
By dispersing fine graphite or amorphous carbon particles in the electrolyte, so that some of it gets included in the iron deposits, it is possible to deposit iron that can later be converted to steel by heat treatment, causing the carbon to diffuse; this can be localized to just certain layers of the workpiece. Alternatively, the iron can be case-hardened if the softness and ductility of pure iron is undesirable.
Other metals commonly electroplated can be 3-D printed in the same way; an Argentine savant has already demonstrated this process with copper, but zinc, tin, chromium, nickel, gold, silver, cadmium, cobalt, lead, and even some alloys such as bronze and brass can be shaped in this way. Additionally, layers of different metals can be alternated, and fillers such as graphite, aluminum oxide, and clay particles can be included, especially if the cathode voltage is kept moderate enough to prevent bubbling.
Other liquid electrolytes, such as ionic liquids and perhaps even deep eutectic systems, might permit the use of a wider range of metals, more rapid electrodeposition of iron, or lower risks than a near-boiling strong hydrochloric acid solution.
The needles, if iron, need not be pure iron; they can contain metallic impurities as long as their standard electrode potential is significantly more negative than iron's -0.44V. In particular, zinc, magnesium, aluminum, and the rare earths should not be a problem. Most other metals, however, would plate out in preference to the iron, including virtually everything you can electroplate in water (except zinc and maaybe chromium).
In https://www.finishing.com/94/56.shtml there is some discussion of different ways to electroform or electroplate with iron; Colin Braathen writes:
I agree that ferric chloride is not a good basis for plating; most documents on the subject stress the importance of keeping iron 3+ levels in the bath low. Air for agitation is likely to oxidise the ferrous ion to ferric, so I plan to hermetically cover the bath with clingwrap and agitate using argon… Bath heating will be by quartz tube with a Nichrome coil inside, salvaged from a cheap radiant room heater, glanded into the bath walls with silicone. Bath lining will be PVC, heat gun welded at the seams and bonded to a support shell with air-moisture-curing polyurethane glue (bath temperature will be uncomfortably close to the glass transition temperature of PVC).
I, too, have plated iron from a sulphate solution as a tryout. I got about 1 mm before my power supply burned out (15A on about 0.5 sq.ft, i.e. 30 A/ft). The iron was brittle to the point of being crumbly I could almost crush it in my fingers, and I had incipient dendrites. … Chloride baths are run at higher temperature and, apparently, can produce a ductile stress-free deposit if done right.
… The bath will be Fisher-Langbein, FeCL2.4H2O 300-450 g/l, CaCl2 150-190 g/l, 85 °C, pH 1.5, current density 2 9 A/dm2 (20 85 A/ft2). The low pH should help to minimize formation of the ferric salt.
See also:
https://ukdiss.com/examples/electrodeposition-iron-co-deposits-des.php is an academic fraud company ("dissertation writing service") publishing a purported dissertation on iron electrodeposition in ionic electrolytes whose author's name has been removed.
https://encyclopedia2.thefreedictionary.com/Iron+Plating:
an electrolyte whose main constituent is ferrous sulfate or chloride. … electrodeposition proceeds at room temperature with an insignificant concentration of acid in the electrolyte at a rate of the order of 1 micron per hr. For repair work, the temperature and acid concentration are increased. The iron layer is deposited more quickly, the ferrous chloride solution is more concentrated, and the temperature is about 100°C.
https://www.scientificamerican.com/article/electro-plating-with-iron/ https://archive.org/details/scientific-american-1869-11-27
Electro-Plating with Iron
Scientific American volume 21, number 22, p. 346
November 27, 1869
The Hon. Cassius M. Clay†, late U. S. Minister to Russia, has recently returned from St. Petersburg, bringing with him some fine specimens of iron electrotypes, done after the process of Prof. Jacobi and Klein. We have before alluded to this important discovery. By its use, nearly all forms of electro-plating, such as engravings, stereotypes, medallions and ornaments, may be done in iron, with a fineness of texture which is really surprising.
Its importance and value will be appreciated when we reflect that the iron electro-plates are about five times more durable than the ordinary copper electro-plates.
Mr. Clay has presented us with an iron electro-plate copy 'Of a copperplate engraving of the Prince Imperial of Russia. This plate is six inches square,. and beautifully done. It is one thirty-second of an inch in thickness, and has a color closely resembling that of zinc. These iron electrotypes are now used by the Russian Government with complete success for the printing of bank notes.
The process was patented in this country through the Scientific American Patent Agency, Sept. 29, 1868, and further information can be had by addressing C. M. Clay & Co., 45 Liberty St., New York.
The following description of the process we copy from the patent specification :
“Our invention consists in the application of a practical galvano-plastic process as to the deposits of iron on molds, or any other form, for reproducing engravings, stereotypes, and for other useful or ornamental purposes.
“The galvano-plastic bath we use is composed of sulphate of iron, combined with the sulphates of either ammonia, potash, or soda, which form, with sulphate of iron, analagous [sic] double salts.
“The sulphate of iron may also be used, in combination with the chlorides of the said alkalies, but we still prefer the use of sulphates.
“The bath should be kept as neutral as possible, though a small quantity of a weak organic acid may be added, in order to prevent the precipitation of salts of peroxide of iron.
“A small quantity of gelatin will improve the texture of the iron deposit.
“As in all galvano-plastic processes, the elevation of the temperature of the bath contributes to the uniformity of the deposit of iron, and accelerates its formation.
“For keeping up the concentration of the bath, we use, as anodes, large iron plates, or bundles of wire of the same metal.
“Having observed that the spontaneous dissolution of the iron anode is, in some cases, insufficient to restore to the bath all the iron deposited on the cathode, we found it useful to combine the iron anode with a plate of gas-coal, copper, platinum, or any other metal being electro-negative toward iron, and which we place in the bath itself.
“As a matter of course, this negative plate may also be placed in a separate porous cell, filled with an exciting fluid, as diluted nitric or sulphuric acid, or the nitrates or sulphates of potash and soda.
“For producing the current, we usually take no more than one or two cells of Daniels' or Smee's battery, the size of which is proportioned to the surface of the cathode.
“It is indispensable that the current should be regulated, and kept always uniform, with the assistance of a galvanometer, having but few coils, and therefore offering only a small resistance.
“The intensity of the current ought to be such as to admit only of a feeble evolution of gas-bubbles at the cathode, but it would become prejudicial to the beauty of the deposit if gas-bubbles were allowed to adhere to its surface.
“The same molds, as employed for depositing copper, may also be used for depositing iron, only it is advisable, in employing molds made of lead or gutta-percha, to cover them previously with quite a thin film of galvanic copper, formed, in a few minutes, in the usual way, and then oring [sic] them, after having washed the molds with water, immediately in the iron-bath.
“The film of copper may be removed from the deposit either by mechanical means, or by immersion into strong nitric acid.
“The deposited iron is very hard, and rather brittle, so that some precaution must be taken in separating it from the mold. By annealing, it acquires the malleability and softness of tempered steel.
† This is a different Cassius Clay than Muhammad Ali.
https://www.pfonline.com/articles/iron-plating(2)
The iron plating bath is particularly useful for when large build-ups (50–100 thousand[th]s of an inch) are required. There are a number of different baths available: ferrous chloride, ferrous fluorborate, ferrous sulfamate and ferrous sulfate are common examples. Of these baths, the most common is the ferrous chloride bath…
Then Kushner gives the recipe: 40–60 oz./gal. of ferrous chloride dihydrate, 20–35 oz./gal. of calcium chloride, 185–210°F, 20–80 amps per square foot without agitation or up to 200 with agitation (which must not be air to prevent oxidizing the ferrous ions), high-quality iron anodes, pH 0.2–1.8 using HCl. There is some confusion in the recipe.
https://patents.google.com/patent/US2745800A/en
By John Poor, 1953.