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Metalworking & Soldering

4 min read

A solar dish can solder at 230-350°C, braze at 600-900°C and melt aluminium at 660°C, because heating 1 kg of aluminium from 20°C to a melt takes about 971 kJ, which a dish netting 0.75 kW supplies in about 22 minutes. Copper at 1,085°C is marginal for a 1.5 kW dish, since a 5 cm spot at that temperature radiates about 379 W away, so keep copper work to small pieces or use a larger, more accurate dish.

Part of High-Temperature Applications

Solar metalworking uses the focus of a dish as a clean, smokeless torch for soldering, brazing and small melts.

What The Dish Can Supply

Take the parent page’s 1.8 m dish delivering about 1.5 kW at the focus. Of that, a crucible or work-piece absorbs part and loses part by radiation and conduction. Assume it nets 0.75 kW, half.

Melting aluminium. Inputs (standard handbook values, recalled): specific heat 0.897 kJ/kg·K; latent heat of fusion 397 kJ/kg; melting point 660°C; start at 20°C, so a rise of 640 K.

  1. Heat to the melting point = 1 kg × 0.897 × 640 = 574 kJ.
  2. Heat to melt = 397 kJ.
  3. Total = 971 kJ per kilogram.
  4. Time at 0.75 kW = 971,000 ÷ 750 = 1,295 s, about 22 minutes. For a 0.25 kg charge: 324 s, about 5.4 minutes.

The crucible itself also takes heat, so allow longer. The parent page says a 1.5 m dish easily melts aluminium; the arithmetic agrees for small charges.

Copper. A surface at temperature T radiates σT⁴ per square metre (σ = 5.67 × 10⁻⁸ W/m²·K⁴). At 1,085°C (1,358 K), σT⁴ = 193,000 W/m². A 5 cm spot (area π × 0.025² = 0.00196 m²) loses 379 W by radiation alone. At 660°C (933 K), σT⁴ = 43,000 W/m² and the same spot loses 84 W. So copper at 1,085°C eats about a quarter of a 1.5 kW dish’s power before any heat reaches the metal, and the loss grows with the surface the crucible shows. The parent page’s note that copper needs a 2 m or more dish or a very accurate smaller one is sound.

Tasks In Order Of Difficulty

  • Soldering (230-350°C): easy. Position the joint at the focus edge, apply flux, feed solder (Soldering Basics; the 60/40 solder there contains lead, so keep it away from food and drinking water).
  • Brazing (600-900°C): needs the whole joint at temperature; use a larger dish or trough.
  • Aluminium casting: small crucible, preheated mold, dry everything (Casting).
  • Copper: marginal.
  • Steel heat treatment (800-1,100°C): possible but hard to control (parent page).

What You Need

  • A tested dish and a tracker (Accuracy & Focus Testing)
  • A fireproof work platform at the focus, adjustable in and out
  • A small graphite or clay crucible and long tongs
  • Flux and dry molds
  • Dark eye protection, leather gloves, sleeves and shoes

Method: A Small Aluminium Melt

Step 1: Dry the crucible, the aluminium scrap and the mold completely. Any moisture trapped in a melt or mold flashes to steam and throws metal.

Step 2: Aim, track, and set the crucible at the focus.

Step 3: Melt in small charges. Skim the dross when the metal is liquid.

Step 4: Cover the dish before pouring, then pour into a dry, preheated mold.

Common Mistakes

  • Damp scrap or mold. Water in molten metal causes an explosion.
  • Galvanised or painted scrap. It gives off toxic fume.
  • Pouring with the dish uncovered. Your attention is off the focus.

Safety

Molten metal, fume and concentrated sunlight at once

Molten aluminium burns through skin and clothes, and water meeting it flashes to steam and throws metal. Metal fumes from coated scrap are poisonous; work outdoors, upwind. Concentrated sunlight ignites materials at once and blinds without pain; use dark eye protection that blocks infrared and keep the dish covered when not working.

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