MIG Welding Copper: A Complete Guide to Techniques, Settings & Challenges

A fabricator gets a rush order for a set of thick copper bus bars, fires up the MIG machine they use every day on steel, and watches the bead go cold, lumpy, and half-penetrated while the part soaks up heat like a sponge. Copper is not steel, and MIG welding it demands a completely different approach to preheat, gas, current, and prep. This guide walks through exactly what changes, why, and how to get clean, sound copper welds the first time.

Why MIG Welding Copper Is Different From Welding Steel

That bus bar is still sitting on the bench, and it is still not welded. The bead came out cold, lumpy and half-merged, which usually points to one of two things: the joint never got hot enough to fuse properly, or it did get hot and the heat drained away before it could do any work. With copper, both failure modes are common. So here is the answer before we go further.

Yes, MIG (GMAW) can weld copper, and for heavier sections it is often the preferred process over TIG because it deposits faster and handles thick sections in fewer passes. But it is not a case of loading the same consumables and dialing in the same amperage. Copper demands high heat input, deliberate preheat, clean or deoxidized base metal, and filler metal designed specifically for copper. Get those four things right and MIG copper welds are routine. Get them wrong and you get the cold, lumpy, porous bead every time.

The Physics Behind the Difference

Copper is not just a different color of steel. Its core physical properties change almost every decision you make at the torch.

Property	Pure Copper	Carbon Steel	Why It Changes Your Technique
Thermal conductivity	About 401 W/m·K (0 to 100 °C), roughly ten times that of carbon steel	Around 45 to 60 W/m·K	Heat races away from the arc instead of soaking into the joint. You must supply more heat and preheat the part to hold it.
Melting point	Approximately 1083 °C (1981 °F)	Around 1450 °C (2642 °F)	You can push a lot more current into copper before you cook the filler, which is why spray transfer works so well here.
Thermal expansion	Coefficient of roughly 9.4 µm/m·K	Roughly 12 µm/m·K	Combined with the conductivity, the workpiece heats and moves over a wide area, so gaps close and distortion develops quickly.
Metallurgical response to heat	No phase change, so it cannot be hardened by heat treatment	Hardenable by quenching and tempering	Heat only ever softens cold-worked copper. There is no heat-treat fix for a bad copper weld.

TWI’s guide to welding copper and its alloys puts it plainly: copper conducts heat away from the weld far faster than steel, so the fusion zone cools before it can properly wet out. That single fact explains the majority of copper MIG failures, including poor penetration and the bright, brittle-looking bead that forms when the wire simply sits on the surface.

There is another copper-specific wrinkle: copper readily absorbs hydrogen and oxygen. When ordinary oxygen-bearing copper (often called tough pitch copper) is heated, its copper oxide content comes out as porosity in the weld. That is why deoxidized and oxygen-free grades dominate welding work, and why phosphorus-bearing filler metals exist.

The Four Things That Actually Change

Once you accept the physics, the practical differences fall into a short list:

1. Heat management. Heat input, preheat temperature and interpass control become the primary variables. A technique that works on 6 mm steel will starve on 6 mm copper.
2. Base metal condition. Oxygen-bearing copper welds poorly. TWI’s weldability assessment of copper alloys details how alloy content changes cracking tendency and preheat needs.
3. Filler metal. Standard ER70S-6 steel wire will not do. Copper filler is governed by AWS A5.7/A5.7M, the American National Standard specification for copper and copper-alloy bare welding rods and electrodes, which covers ERCu, ERCuSi, ERCuNi and ERCuAl classifications.
4. Shielding gas. Pure argon works for thin material, but heavier copper usually needs an argon-helium blend to keep the arc hot and stable. Gas also has to be clean and dry.

Copper is also a different story from aluminum in this respect. If you have welded aluminum with MIG before, the parallel is instructive: different filler, different gas choices, much stricter cleaning discipline. Our aluminum MIG welding guide covers that metal in detail, and the copper process shares the same “prepare the surface obsessively” mindset.

What This Guide Covers

Here is the roadmap. We start with how to identify the copper grade in front of you, since that single decision predicts everything else. Then we cover filler metal selection, the preheat and shielding gas settings that make the process work, separate techniques for thin sheet versus thick plate and bus bar, a full defect and troubleshooting breakdown, copper fume safety and exposure limits, real applications across electrical, marine and HVAC industries, and finally a step-by-step workflow you can put on the shop floor tomorrow. The short version: copper welding is a heat management problem first and a technique problem second.

Understanding the Copper Grades You Will Actually Weld

Before you strike an arc for MIG welding copper, alloy identification needs to be your first step. A quick check of the mill cert, a stamped grade, or the copper.org designation only takes a minute, but it will dictate nearly every parameter for the job. If the material is unidentifiable, treat it as tough pitch copper and plan for a deoxidized filler with preheat.

The root of most issues comes down to oxygen. While pure copper itself is predictable, the way oxygen is trapped within each grade is what determines whether you’ll get a clean bead or a porosity nest.

The Pure Copper Family

ETP copper (C11000), more commonly known as tough pitch copper, is the electrical grade you’ll see in bus bars, bushings, and coil work. It can carry up to roughly 0.1% oxygen, locked in as stringers of copper oxide (Cu2O). This oxide has little impact on the wrought mechanical properties, however, when melted during welding, those stringers migrate to the fusion boundary and weld surface to promote porosity and erratic arc behavior. This is exactly why TWI’s job knowledge on welding copper and its alloys consistently recommends deoxidized filler metal and preheat for heavier sections. Despite these welding hurdles, ETP remains prized for its conductivity, rated anywhere from 97 to 101.5% IACS.

Phosphorus deoxidized copper (C12000 through C12300) solves this issue at the source. With only 0.01 to 0.05% phosphorus added during production, the oxygen is tied up during solidification, leaving no Cu2O stringers to gas off into the weld pool. For most fabricators, this is the easiest pure copper grade to MIG weld, with a significantly lower risk of porosity.

At the top of the cleanliness scale sits oxygen-free copper (C10100, C10200, and C10500). With effectively no oxide phase present, it welds beautifully with the correct filler and produces a very clean bead. The trade off is cost, as it’s almost exclusively reserved for vacuum service, electronics, heat sinks, and high conductivity buswork. For precise values by spec, the Copper Development Association A1050 datasheet is the place to reference.

Alloys That Make Copper More Weldable

When strength, corrosion resistance, or service in harsh environments are required, pure copper is often swapped for an alloy, and in many cases, this makes MIG welding far easier.

Cupro-nickel (90/10 C70600 and 70/30 C71500) is the go to for marine, desalination, and heat exchanger work. The addition of 10 to 30% nickel drastically lowers its thermal conductivity to a level similar to low carbon steel. According to TWI’s weldability assessment of copper and copper alloys, this stability allows cupro-nickel to be fusion welded with little to no preheat, turning what would be a heat hungry job into a routine one.

For added strength, silicon bronze (C65100) and aluminium bronze (C61400, C95200, C95400) are strong contenders. Silicon bronze is especially useful for joining copper to steel, pairing well with ERCuSi-A filler. Aluminium bronze provides a considerable jump in strength over pure copper with good corrosion resistance, however, it is more susceptible to hot cracking and demands a low contamination, freshly stored filler wire.

Brasses fall into a much tighter window of weldability. Those with zinc content under around 15% can be welded with a zinc-free filler and proper heat control. Once zinc climbs higher, the risk of porosity and hot cracking rises drastically.

Grades to Avoid Altogether

Not every copper based material is a candidate for fusion welding. TWI explicitly states that welding of nickel silvers (around 45% Zn/10% Ni), leaded gunmetal, and high-zinc brasses (around 40% Zn) is not recommended. For leaded alloys specifically, this isn’t just a weld quality concern. Lead fume is a serious health hazard, one that OSHA identifies alongside other welding fume risks.

Base metal	Common grade	MIG weldability	Preheat need
Tough pitch copper (ETP)	C11000	Weldable with deoxidized filler, porosity risk	Essential above ~5 mm, up to 350 to 450 °C on heavy sections
Phosphorus deoxidized copper	C12000 to C12300	Easiest pure copper to weld	Recommended above ~2 to 3 mm
Oxygen-free copper	C10100 / C10200 / C10500	Very weldable, clean bead	Still needed for thick sections
Cupro-nickel 90/10 and 70/30	C70600 / C71500	Excellent, seawater service	Usually none
Silicon bronze	C65100	Good, preferred for copper to steel	Rarely
Aluminium bronze	C61400 / C95400	Good, high strength, watch hot cracking	Low to moderate on thick sections
Low-zinc brass	Under ~15% Zn	Weldable with zinc-free filler	Rarely
High-zinc brass	~40% Zn	Not recommended	Not applicable
Nickel silver and leaded gunmetal	C74600 and similar	Not recommended	Not applicable

Take the time at the bench to write down the UNS number on your job traveler and match it to its filler before power is applied. That five minute habit will solve the hardest half of MIG welding copper before the arc is ever struck.

Filler Metal Selection: ERCu, ERCuSi-A, ERCuNi and More

Filler metal is the cheapest insurance in copper work. Choose badly and the arc punishes you immediately: spatter, cracks, a bead that crumbles under a chip hammer. Choose well and the wire behaves as though it was made for the job in front of you. The copper filler family is small and standardized, so the choices are learnable in about an hour.

Every copper and copper-alloy MIG wire here is classified under AWS A5.7/A5.7M, the specification covering copper and copper-alloy bare welding electrodes and rods. That is why a supply house can hand you ERCuSi-A without a long conversation. Copper fillers are entirely separate from the aluminum wire families you know from MIG welding aluminum, which fall under a different AWS specification. Our all-position welding rod guide is a good primer on classifications.

ERCu: deoxidized copper, the copper-to-copper default

ERCu is the workhorse: 98% or more copper plus manganese, aluminum, silicon and iron deoxidizers. Molten copper absorbs oxygen and hydrogen aggressively, and as the weld metal cools that gas leaves voids behind that no grain structure can close. A deoxidizer takes the oxygen before it reaches the pool, so the metal stays clean.

The most common commercial form is Crown Deoxidized Copper wire: 98% minimum copper with phosphorus, silicon, tin and manganese, roughly 40% IACS conductivity and about 29,000 psi tensile. The conductivity figure comes from the Copper Development Association. IACS is the International Annealed Copper Standard, where pure annealed copper equals 100% IACS, so a welded joint conducts roughly two-fifths as well as the solid bus bar between them. Grounding and structural work can live with that, but high-current bus bar design is usually a bolted joint. ERCu also gives the best color match of any copper filler, which matters on decorative copper panels.

ERCuSi-A: the versatile silicon bronze

ERCuSi-A is a copper-silicon wire with roughly 6% silicon and a small tin addition, and its value is dissimilar metal work: copper to steel, copper to brass, and aluminum with the right procedure. It is also the standard answer for brass, being zinc-free. Tensile strength is about double that of ERCu, so it earns its keep on load-bearing joints, and it distorts far less than pure copper filler. The trade-off is conductivity: silicon bronze lands in the low-to-mid 20s percent IACS, so it does not belong in a current-carrying path that ERCu would serve. It is also a different color than copper, a visual tell.

ERCuNi: built for salt water

ERCuNi comes in two grades. ERCuNi-A is 90% copper and 10% nickel for 90/10 cupro-nickel base metal; ERCuNi-B is 70/30 for 70/30 base metal, for surfacing, and as a brazing filler. The reason to reach for these is corrosion: nickel markedly improves resistance to seawater attack and dezincification, which is why cupro-nickel piping, heat exchangers and desalination equipment are welded with copper-nickel filler rather than plain copper. Match the nickel content of the filler to the base metal, and use ERCuNi-B on 90/10 base only when extra strength justifies the mismatch. The Copper Development Association’s guide to welding copper and its alloys explains that matching logic.

ERCuAl: aluminum bronzes for strength and hard service

ERCuAl-A1, A2 and A3 add roughly 6%, 10% and 12% aluminum to copper, making them the strongest and hardest wires in the family. They offer good corrosion resistance in seawater and acidic process environments, plus excellent wear resistance for overlay work. Typical uses are copper-to-steel joints where strength matters more than conductivity, bearing and bushing surfacing, and heavy copper-to-copper structural work. The costs are conductivity, which drops into the single digits, and increased crack sensitivity, so keep heat input and bead profile under control.

AWS class	Best base metals	Notes
ERCu	Deoxidized and oxygen-free copper	About 40% IACS, ~29,000 psi tensile, best color match
ERCuSi-A	Copper to steel, brass, copper alloys	Zinc-free, ~50,000 psi, low conductivity
ERCuNi-A / -B	Cupro-nickel 90/10 and 70/30	Seawater and heat exchanger service, also a brazing filler
ERCuAl-A1 / A2 / A3	Copper alloys, copper to steel, overlay	Strongest and hardest, wear and corrosion resistant

Brass: keep zinc out of the wire

Brass is copper with zinc, and zinc boils uncomfortably close to the melting point of the brass itself. In the arc, zinc flashes to vapor, it oxidizes, and the gas gets trapped in a rapidly solidifying deposit, leaving a porous, ugly, weak joint. The cure is a zinc-free filler: silicon bronze (Cu 6560) or aluminum bronze (Cu 6180). Some copper-silicon alloys are also acceptable, so the rule is simple: if the wire specification lists zinc, pick a different wire. Leaded brasses are effectively not weldable: lead segregates along grain boundaries and cracks the joint.

How to match like with like

• Strength: filler should match or exceed the base metal. ERCu at roughly 29,000 psi handles copper-to-copper; load-bearing or copper-to-steel joints need silicon or aluminum bronze.
• Conductivity: for current-carrying joints, ERCu at about 40% IACS is the best copper-family choice. Every alloy addition costs conductivity.
• Environment: match corrosion service first. Fresh water, HVAC and electrical work suit ERCu; seawater and marine service suit ERCuNi; aggressive service suits aluminum bronze.
• Cleanliness: copper filler is soft and easily contaminated. Keep it sealed until use, wear gloves feeding it, and keep dirty spools off the drive rollers.

Preheat, Shielding Gas, and the Settings That Make It Work

Copper MIG is a heat management problem before it is a technique problem. Copper drains the weld pool far faster than steel, so the arc must deliver enough energy to keep the puddle fluid and the base metal warm enough to accept it. Get both right and the bead lays down clean; get them wrong and you get a cold, lumpy, half-penetrated lump.

The Default Preheat Rule

Most shops settle on one rule: preheat anything thicker than about 2 mm. Under 2 mm you can usually skip it, provided the joint is clean and the heat keeps moving. From 2 to 5 mm a light preheat in the 10–100 °C range helps. Beyond that it is essential.

The TWI guidance on welding copper and its alloys shows the jump. Under 5 mm: preheat 10–100 °C, current 175–240 A. At 10–12.5 mm: preheat 200–300 °C, current 300–400 A. Above 16 mm: preheat 350–450 °C, current 350–600 A.

Those numbers exist because copper conducts heat roughly 20 times better than mild steel along the joint. A bead on 20 mm copper must reheat a large mass of metal pulling heat away from the fusion line. Preheat narrows that differential, slows cooling, and lets the puddle wet the joint and push ahead of solidification instead of freezing behind the arc.

Shielding Gas: Argon, Argon-Helium, and What Not to Use

Shielding gas keeps oxygen and moisture off the molten copper and sets the arc characteristics that decide how much heat you can push into the joint.

• Pure argon for thin to moderate sections, up to about 5 mm. Clean, stable arc, easy to start, and the go-to for sheet.
• Argon-helium mixes for thicker material. Helium raises the arc voltage and broadens the heat pattern, adding penetration and stabilising the puddle. A common blend is 75% helium / 25% argon, and TWI recommends argon-helium above about 5 mm.
• Nitrogen mixes are largely obsolete for copper MIG. Once used to add heat and improve wetting on heavy sections, they are avoided now because nitrogen is reactive with copper and promotes porosity.

Do not confuse copper with aluminum MIG welding, where argon plus a trace of oxygen is deliberately used to break down aluminum oxides. Copper is the opposite: you want a clean, inert, moisture-free shield, and higher purity is genuinely better. Unsure what is flowing through the regulator? The gas supply basics are worth a read before you light the torch.

MIG Parameters for Copper

These are starting points, not commandments. Grade, joint design, wire diameter and machine all shift the numbers. Treat the table as the middle of the range TWI identifies as workable for copper, plus the higher-current GMAW settings used on heavy bus bar and plate.

Thickness	Wire Dia.	Current (DCEP)	Preheat	Arc Voltage	Wire Feed Speed	Gas / Flow
Under 5 mm	0.030–0.035 in	175–240 A	10–100 °C (50–210 °F)	17–20 V	200–300 ipm	100% argon, 20–30 cfh
6–10 mm	0.035–0.045 in	250–350 A	150–250 °C (300–480 °F)	22–26 V	250–350 ipm	75% He / 25% Ar, 30–40 cfh
10–12.5 mm	1/16 in	300–400 A	200–300 °C (400–575 °F)	24–28 V	300–400 ipm	75% He / 25% Ar, 40 cfh
Over 16 mm	1/16 in	350–600 A	350–450 °C (660–845 °F)	26–32 V	145–260 ipm	75% He / 25% Ar, 40 cfh

Note the trend: current, preheat and voltage climb together with thickness, while wire feed speed on the heaviest row drops. At 350–600 A you are laying a large, flat, deeply penetrating spray bead, and more wire only widens the puddle past the point where it stays fluid.

Polarity, Spray Transfer, and the Rest of the Setup

Run copper MIG on DCEP, electrode positive, also called reverse polarity. That is MIG standard anyway, but on copper it matters more because the heat distribution has to support the heavier amperages above. Reversed, the arc looks cold and unstable and the bead will not flow.

On thicker sections, set the machine for spray transfer rather than globular or short-circuit. Spray gives a stable, high-heat arc that melts the base metal and lays down good penetration. The MIG machine setup basics show how to find that window, since the transition point moves with wire diameter and gas mix.

Hold the torch nearly vertical with a slight push, 5 to 15 degrees off vertical, and keep the arc length short and consistent. A wandering torch angle changes the effective heat input and leaves a lopsided bead.

Travel speed matters as much as amperage. Too fast and the pool freezes ahead of the arc, giving lack of fusion. Too slow and you get burn-through on thin material plus a wide, overheated, distorted bead. The practical range for most copper GMAW work is roughly 8–14 in/min. If beads come out cold, convex and poorly wetted, slow down before adding amperage.

Joint Design: Wider Grooves, Tighter Gaps

Copper needs different joint geometry than steel, and it runs counter to muscle memory. Crown, a major copper manufacturer, advises preparing joint angles 10 to 20 degrees wider than you would for steel, with a typical overall V-groove of about 70 degrees.

The reason is access. A wider groove lets the torch reach the root without the copper walls shadowing the arc. On thick copper those walls act as heat sinks and the arc struggles to reach the bottom. It also lets you lay down more filler without excessive amperage.

Root gaps go the other way. Copper’s high thermal expansion closes the joint fast as the part heats, so tighter gaps are needed, which makes fit-up fiddly. Clamp carefully, check fit often, and expect the gap to shift as you work. Do not force a mismatched gap closed and leave it, because the part will move as you weld.

On bus bar work, a square butt on a backing bar or copper plate is often cleaner than a V-groove and saves time without sacrificing strength. Match filler to base metal as covered in the filler and process guide: ERCu for copper to copper, ERCuSi-A for copper to steel.

A Quick Pre-Weld Sanity Check

• Base metal identified and filler matched to it
• Joint cleaned to bright metal, oxide and grease removed
• Preheat measured at the joint, not guessed at the torch
• Correct gas connected, correct flow set, cylinder warm enough
• DCEP confirmed on the machine
• Wire clean and dry, gun liner in good shape
• Lagging or chill bars planned for heavy work

Get these seven right and the welding becomes far less of a fight. Copper will still test you, but it turns into a repeatable process instead of a guessing game.

Techniques for Thin Copper Sheet vs Thick Plate and Bus Bar

Copper is not one welding job, it is two. A 1.5 mm lid and a 25 mm vessel wall share a metal but nothing about arc behavior. On thin material, copper chills the puddle so fast the pool shrinks before you can control it, and the flash of melting punches through the sheet. On thick material the pool wants to sit and grow, and you must pour in energy to keep the arc stable. MIG is good at the second job and only adequate at the first.

Thin Copper Sheet: Under 3 to 5 mm

Below roughly 3 mm, most of the arc energy leaves through the base metal. Copper conducts heat at about 401 W/m·K, so the puddle is chilled from every side. Expect a narrow, restless arc and edges that lift or roll when you stop.

The most common failure in thin copper is not a weak weld. It is a hole: heat in a small puddle on thin, stiff sheet has nowhere to go but through.

• Back it with copper. A backing or chill bar under the joint absorbs heat and stops burn-through, the single most effective trick for thin sheet. Two bars cover both sides of a lap or flange joint.
• Lower amperage, faster travel, longer stickout. Use the smallest wire that feeds reliably, keep the arc in short-circuit mode, and drag a controlled speed so the puddle never pools.
• Consider TIG or pulsed MIG. The heat control of TIG welding separates good parts from scrapped ones, because you can fade the foot pedal where the sheet thins out. Pulsed MIG gets close: peak current melts the wire and puddle, background current holds the arc, and average heat input drops without the arc going cold.

Thick Plate, Bus Bar, and Tank Work

This is where MIG earns its keep. Filler manufacturers point to GMAW for heavy sections of copper, because spray transfer keeps the arc stable and the deposition rate high enough to hold the puddle against the base metal’s cooling effect. DC reverse polarity (DCEP) is standard, with copper alloy wire feeding fast enough to sustain a spray pattern. Practically: 1.2 mm wire on lighter sections, 1.6 mm on heavy ones, argon for thinner material, and an argon-helium mix for more arc voltage and deeper penetration.

Two representative jobs:

Job	Joint and filler	Starting settings	Passes
6 mm copper bus bar splice	70° V-groove, ~1 mm root gap, ERCu or ERCuSi-A	DCEP 260-300 A, 26-28 V, 1.2 mm wire, argon 12-15 l/min, preheat 100-150 °C	Root, fill, then cap
12 mm tank nozzle flange	70° V-groove, ~1.5 mm root gap, ERCuSi-A	DCEP 330-380 A, 30-32 V, 1.6 mm wire, argon or 75/25 Ar-He at 18-22 l/min, preheat 250-350 °C	Multi-pass fill and cap

Treat those numbers as a starting point and prove them on a coupon. Copper joints move as they cool, so generous root gaps are usually right. TWI’s guidance on welding copper and its alloys agrees: heat input and joint design must be reworked for copper.

Controlling Distortion

Copper’s high thermal expansion means the finished part moves more than you expect. Four habits help:

• Tack in a planned sequence. Space tacks evenly along the joint and, on a closed section, work around it in opposing pairs.
• Weld balanced and backstep. Alternate welds on opposite sides and step back over finished weld to heat the preceding bead evenly.
• Clamp with copper, not steel. Copper chill blocks draw heat away and stop the part moving, without cold spots.
• Watch the ends. Long straight seams distort most at the terminations, so close the joint near where you started.

Copper to Copper vs Copper to Steel

Copper-to-copper is a metallurgical match, and a deoxidized filler such as ERCu or ERCuSi-A does the work cleanly. Copper-to-steel is different: the metals expand at different rates, copper wets steel poorly, and a straight copper filler on steel will run and pore. Silicon bronze, ERCuSi-A, is the standard answer because it bridges the gap and tolerates the mismatch, classified under AWS A5.7 for copper consumables. For many transition joints, brazing or a bolted interface is more reliable.

Choosing the Right Process

Process	Best suited to	Main limitation
MIG / GMAW	Thick plate, bus bar, tanks, production pipe	Poor heat control on thin sheet
TIG	Thin sheet, complex and high-integrity joints	Slower, lower deposition
FSW	Solid bus bar, electrical and battery parts	Needs rigid fixturing, limited access
Laser-MIG hybrid	Thick copper where preheat is impractical	Cost, setup time, joint properties

Hybrid laser-MIG welding is the most active research front. In a study on laser-MIG hybrid welding of 8 mm pure T2 copper, single-pass full penetration was achieved without preheating for the first time, with joint tensile strength at 82.1% and elongation at 96.8% of the base metal. The catch is microstructure: hybrid-welded copper showed minimum electrical conductivity and average thermal diffusivity reduced by 62.0% and 34.7% against base metal, largely from microvoids and impurities. Fine for structural work, not for bus bars where conductivity counts.

Laser welding alone is limited by copper’s optical properties. Copper absorbs only about 5% of infrared laser light at 1000 nm, so most energy reflects off the surface, which is why blue laser research is advancing so fast. For background on the options, laser welding systems explained is worth a read, but budget for the optics problem, not the machine price.

Common Defects, Their Causes, and How to Fix Them

Copper welding problems are not mysterious. They are the familiar steel defects, only faster and far less forgiving. Copper pulls heat from the pool so quickly that a weld can freeze before it wets the base metal, it expands as it heats, and it picks up hydrogen and oxygen from everything it touches. Find the symptom first, then the root cause, then the fix.

Defect	What it looks like	Likely root cause	Fix
Porosity	Gas pockets, holes, or a spongy bead that weeps when cut	Moisture or contamination on the base metal, dirty or corroded filler, poor or wrong shielding gas, hydrogen in the arc	Clean joint and wire, verify flow
Lack of fusion / incomplete penetration	Bead rides on the base metal with no sidewall fusion, root gap visible from the back	Low heat input, no preheat, travel speed too high, wrong shielding gas, excessive wire feed speed	More preheat, slower travel
Cracking	Longitudinal cracks along the bead or fusion zone, sometimes as the arc stops	Thermal expansion under restraint, or crack-sensitive alloys such as nickel-aluminum bronze	Relieve restraint
Excessive distortion	Pulled angles, bowed flanges, a joint that will not close	High thermal expansion plus heat sinking away from the joint	Backstep, chill bars
Spatter	Orange or bronze globules thrown from the pool	Insufficient shielding gas, low flow, long stick-out, wire feed too high for the amperage	Fix flow and stick-out
Burn-through	Holes in thin sheet, or a collapsed melted-through bead	Too much heat on low mass, no backing support	Lower amps, add backing
Poor bead appearance or colour mismatch	Rough, oxidised, dull bead that does not match the base metal shade	Air exposure, contaminated gas, dirty wire, mismatched filler colour	Purge, brush or pickle

Porosity: The Number One Copper Complaint

Porosity is the defect most likely to cause a failure in service. A copper bus bar with gas pockets may carry current, then heat up at the joint and fail in switchgear nobody wants to fail.

The TWI guidance on copper weldability is blunt: hydrogen control is necessary, meaning clean, grease-free wires and rods plus high-purity shield gases for TIG or MIG.

Work the causes in order of likelihood:

• Contamination. Fingerprints, cutting oil, an oxidised edge or a dusty, corroded wire all make pores that no amount of extra heat will close. Clean mechanically right before the arc.
• Oxygen-bearing base metal. Tough pitch copper holds oxide inclusions that release gas as the pool solidifies. Deoxidized or oxygen-free metal is safer.
• Gas problems. Under-flow lets the arc suck in air, over-flow makes turbulence that drags air in at the nozzle. Check the cylinder: nitrogen mixes are largely obsolete for copper.
• Hydrogen in the arc. It makes cracks, not just pores, and comes from moisture and hydrocarbons. Dry wire, dry gas, clean steel.

Porosity also does hidden damage. Research on copper welds has found microvoids of 1 to 2 micrometres in the fusion zone, plus carbon, silicon, manganese and tin impurities that measurably cut conductivity and corrosion resistance. You will not see the microvoids, but you will see their consequences: a bus bar running hot, a fitting corroding at the weld.

Porosity in Brass and Other Copper Alloys

Brass is where the zinc does the damage. As the pool solidifies, zinc comes out of solution and releases gas, leaving a joint that looks fine on the surface and is weak underneath. Use a zinc-free filler such as silicon bronze (Cu 6560) or aluminium bronze (Cu 6180). High welding speeds also cut pore size. Keep gas flow high enough to displace the zinc vapour, add enough preheat to keep the pool fluid, and expect more porosity in high-zinc brasses. The TWI job knowledge on welding copper and its alloys covers the wider family.

Lack of Fusion: When the Weld Will Not Stick

Lack of fusion on copper is a heat problem, not a filler problem. The base metal soaks heat away faster than the arc delivers it, the leading edge of the puddle solidifies against cold copper, and the bead sits there like a rolled coin on a plate. Three levers fix it: more preheat, slower travel, or more heat in the arc. On thicker sections an argon-helium blend raises arc voltage and heat input without overloading the torch. Check the torch angle too, since poor wetting shows up first at the sidewall. The aluminum MIG welding guide covers the same parameter logic for a metal with very different thermal behaviour, a useful contrast.

Cracking and Distortion: The Expansion Problem

Cracking is a thermal expansion problem. Copper expands as it heats and wants to shrink back as it cools, so if the joint is rigid enough to prevent that, the metal tears, usually along the centreline of the bead. Relieve restraint before changing filler: widen the root gap, use a less rigid tacking and welding sequence, and control interpass temperature. Nickel-aluminum bronze is crack-sensitive, so heavy-section procedures recommend a nickel-free filler.

Distortion follows the same physics, and root gaps close fast during welding: a 3 mm gap can be welded shut by the time you reach the root. Set the joint for the heat you will add, not the drawing dimension. Weld balanced and backstep, tack lightly, and use copper chill bars. Pre-set the parts on large assemblies rather than fighting distortion afterwards.

Spatter, Burn-Through and Bad-Looking Beads

Spatter on copper is usually a gas and wire feed mismatch: check flow, cup size and stick-out, and match wire feed to the amperage window for the wire diameter. Burn-through on thin material is a heat problem, answering to lower amperage, quicker travel and a backing bar. A dull, rough bead in a different colour than the base metal means air reached the puddle, so check gas flow, nozzle condition and joint cleanliness, then brush or pickle the bead to restore the colour match.

None of these defects is unique to copper. What makes them distinct is how fast they appear and how hard they are to back out of once they are in the part. Clean base metal, correct gas, right preheat and matched filler will do most of the work.

Safety: Copper Fume, Metal Fume Fever, and Exposure Limits

When MIG welding copper, the most immediate threat is often the one you can’t see. The arc reaches several thousand degrees, vaporizing copper near 1,083 °C (1,981 °F). That vapor oxidizes almost instantly into fine copper oxide particles, small enough to reach the gas-exchange region of the lungs. Add the deoxidizers found in ERCu filler, along with mill scale, cutting oils, or brazing residue, and the particulate load only increases. For that reason, joint prep is just as much a safety step as it is a quality one.

What Copper Fume Actually Is

These particles are primarily copper(I) and copper(II) oxides. Unlike bulk copper, fume is respirable, meaning it’s designed by its size to bypass many of the body’s natural defenses. This size difference is exactly why the exposure limits for fume are far stricter than they are for copper dusts and mists.

OSHA and NIOSH Exposure Limits

There are two categories of copper particulate, and the gap between their limits is telling.

Contaminant	OSHA PEL (8-hour TWA)	NIOSH REL (8-hour TWA)	NIOSH IDLH
Copper fume	0.1 mg/m³	0.1 mg/m³	100 mg Cu/m³
Copper dusts and mists	1 mg/m³ (as Cu)	1 mg/m³ (as Cu)	100 mg Cu/m³

The copper fume limit is backed by the NIOSH Pocket Guide entry for copper fume, which lists its REL and IDLH. The revised IDLH was based off of its analogy to copper dusts and mists, detailed in the dusts and mists entry. OSHA’s standard interpretation on the PEL documentation cites the driving health effects for this number, including metal fume fever, upper respiratory tract irritation, nausea, and a metallic or sweet taste in the mouth. At 0.1 mg/m³ over an eight-hour TWA, this limit is often exceeded in unventilated bays.

Metal Fume Fever

The most common acute reaction to copper fume is metal fume fever, also known as welder’s flu or Monday fever. Symptoms typically appear within a few hours of heavy exposure and include chills, muscle aches, nausea, fever, a dry throat, cough, lassitude, and a metallic or sweet taste. Skin or hair discoloration can also occur from vaporized skin oils. According to the NIOSH Pocket Guide, eye, nose, and pharynx irritation, nasal septum perforation, and dermatitis are also reported, with liver, kidney, and lung damage seen in animal studies at high exposures.

While symptoms are usually self-limiting within a day or two, that doesn’t make them harmless. Treating them as “just the flu” is a control failure. If they persist or recur, seek medical evaluation.

Controls: Ventilation First, PPE Second

Engineering controls are your primary defense. A movable exhaust hood or suction arm should be positioned within a few inches of the arc to capture the plume before it reaches your breathing zone, paired with adequate general dilution ventilation. For repetitive jobs, such as long bus bar runs, a dedicated arm is necessary. Keep in mind that fine fume is often invisible, so a lack of a visible plume is not proof of a safe breathing zone. For gas handling considerations, it’s worth reviewing welding gas supplies, cylinders, and safety basics and industrial gas supply services and safe practices.

Respiratory protection is strictly a backup. N95s may be suitable for low-intensity work with strong capture, while P100 or higher filtration is better for heavier fume generation. For enclosed spaces, unknown concentrations, or extreme conditions, supplied air is the appropriate choice. Above all, fit testing is more important than the rating on the mask. On top of respiratory protection, a welding helmet with the correct shade, flame-resistant clothing, leather gloves, and safety glasses under the helmet are needed to protect from radiation, spatter, and heat. With thick copper requiring preheat between 350 and 450 °C (662 and 842 °F), radiant heat should also be factored in with barriers, hydration, and job rotation. For equipment setup specifics, refer to the welding equipment guide.

Pre-Job Safety Checklist

• Identify the alloy. Know if you’re welding tough pitch copper, oxygen-free copper, or a brass alloy, as zinc-bearing materials create their own fume profile.
• Verify local exhaust. Have your hood running and positioned near the arc before striking it.
• Inspect gas and cylinders. Check regulators, hoses, leaks, and cylinder security.
• Degrease the joint and wire. Remove oils, grease, and any flux residue before welding.
• Don full PPE. Confirm your respiratory protection is correct for the task and properly fit tested.
• Plan for heat. Establish a preheat schedule, radiant barriers, hydration breaks, and job rotation.
• Have an emergency plan. Know your fire watch, ventilation failure response, and how to report fume symptoms.
• Track exposure. Keep exposure records and report any symptoms of metal fume fever to your supervisor and a physician.

Copper MIG Welding Across Industries: Real Applications

Copper’s place in fabrication comes down to two traits: corrosion resistance and its ability to carry heat and electricity. As TWI notes in its copper weldability guidance, those are the exact reasons it’s found across so many industries. The challenge is that each one places a different demand on the joint, so the process must match it.

Electrical Bus Bar and Switchgear Fabrication

For current-carrying hardware, conductivity is everything. Heavy copper plate, often between 6 mm and 25 mm thick, is joined to build bus bars and switchgear paths. Pure copper is 100% IACS (International Annealed Copper Standard). A deoxidized copper ERCu weld will only reach around 40% IACS, as deoxidizers and alloying elements disrupt electron flow.

Because of this, many shops don’t rely on the weld to carry full current. It’s common to weld for mechanical support while keeping a bolted joint as the primary conductor. If a design does require full conductivity through the weld, the filler and heat input must be verified against the Copper Development Association material data, as results can vary. For high deposit rates, MIG with spray transfer using ERCuSi-A or ERCu is efficient, while TIG is better for smaller or more complex joints.

HVAC, Refrigeration, and Seawater Systems

Brazing still dominates most HVAC and refrigeration copper tube, and for good reason. However, larger industrial chillers or high-purity systems may call for fusion welds. For tube work, ERCuSi-A is a popular MIG filler, as it resists porosity, especially through copper-to-brass transitions.

For marine and desalination work, copper-nickel (90/10 and 70/30) takes over. Its nickel content lowers thermal conductivity compared to pure copper, often eliminating the need for preheat. ERCuNi with argon shielding is the standard, with MIG used for heavier pipe and TIG for thin-wall, critical seawater penetrations. With tight acceptance criteria for potable systems, NDE is often required on these seams.

Brewing, Distilling, and Food Equipment

Copper is prized in stills and mash kettles for both heat transfer and its effect on flavor. In product-contact areas, full fusion welds are rare. Most sanitary joints are brazed with food-grade silver alloys, finished smooth to prevent any crevice for bacteria. Where MIG or TIG fusion is used, it’s more often for non-product-contact parts like frames and guards before polishing.

EVs, Renewables, and Emerging Applications

Electric vehicles are pushing copper welding to its limits. Thin busbars, hairpin windings, and motor terminations leave very little room for heat. Excess input can distort parts or weaken the HAZ. For this reason, many manufacturers have shifted to laser welding systems, with hybrid laser-MIG only used on thicker structural copper.

In renewables, copper shows up in solar thermal absorbers, wind generators, and grid hardware. Brazing handles most thin absorber tubing, while MIG can cover bus hardware. For any multi-material assembly combining copper and aluminum, the two processes need to be planned separately, as a direct copper-to-aluminum fusion joint is its own challenge. At research scale, pure copper is even used in experimental plasma confinement, though those are bespoke, highly qualified builds.

Where MIG Is the Wrong Answer

Knowing when to not reach for a MIG gun is just as important as knowing how to use one. As TWI guidance on welding copper and its alloys explains, laser welding is favored for smaller and thinner components where arc processes simply deposit too much heat. Keep the following in mind:

• Thin electronic copper (0.2 mm to 1 mm): Use laser or resistance welding over MIG.
• Tube under 6 mm: Brazing is faster, cheaper, and often more reliable.
• Sealed or ultra-pure joints: TIG with back purge offers far more control.
• Critical high-conductivity paths: Bolted, crimped, or silver-brazed connections can outperform a fusion weld for electrical performance.

Process Choices by Application

Application	Preferred Process	Typical Filler	Key Nuance
Bus bar and switchgear	MIG (spray) or TIG	ERCuSi-A / ERCu	May not need to carry full current, verify conductivity if required
Refrigeration and HVAC tube	Brazing, MIG on larger tube	Silver brazing alloy / ERCuSi-A	Local annealing; brazing preferred for most tube sizes
Seawater piping and heat exchangers	TIG for thin wall, MIG for heavy	ERCuNi	Low preheat, watch restraint and hot cracking
Brewing and distillery	Brazing for product contact, TIG for fittings	Food-grade silver brazing / ERCuSi-A	Must be smooth and crevice-free for sanitation
EV battery and motor components	Laser, resistance, hybrid laser-MIG	Copper-specific alloys	Minimal HAZ required due to thin, precise parts
Solar thermal and renewables	Brazing on absorbers, MIG on bus hardware	Silver brazing / ERCuSi-A	Balance cost with weather exposure and serviceability
Thin electronic copper	Laser or resistance welding	None (autogenous)	Arc processes deposit too much heat at this scale

Across every one of these jobs, the common thread is fit for purpose. Copper earns its place where heat and current justify its cost, but it’s only worth the effort when the process is chosen for the joint. For welders looking to expand into these precision applications, following a structured welding training path will go a lot further than applying steel practices to copper.

Step-by-Step Practical Workflow and Pro Tips

Knowing why copper behaves the way it does gets you halfway. The rest is a repeatable procedure any shop can follow.

Step 1: Identify the Alloy First

Everything downstream depends on this. Read the marking or mill cert, or run a conductivity test if the material is unknown. ETP copper (C11000), deoxidized grades, copper-nickel and the bronzes all behave differently under the arc. The TWI guide on copper weldability puts it plainly: oxygen content and the type of deoxidizer decide whether you are fighting porosity.

Step 2: Joint Preparation and Cleaning

Oxides, greases and drawing lubricants turn into porosity, so the joint area must be cleaned thoroughly. The crown procedure for copper makes the same demand: clean the joint area thoroughly, and weld with higher preheat temperatures and faster welding rates than you would use on steel.

• Mechanical cleaning: Bronze or stainless bristle brush, or grind to bright metal. Never use a wheel that has touched steel.
• Degreasing: Solvent wipe, then dry fully, the same logic as prepping aluminum for welding.
• Oxide removal: Grind the root land and groove faces back to bright copper.

Geometry matters. Because of the high thermal conductivity, groove angles should be 10 to 20 degrees wider than those used for steels, with a typical overall V-groove near 70 degrees. Keep root gaps tight: copper expands more than steel, so a wide gap closes fast in the puddle.

Step 3: Preheat Planning and Measurement

Preheat is a range tied to thickness, alloy and joint design. Light material under 5 mm may need very little; heavy bus bar and plate want the low hundreds of degrees Celsius at the joint before you strike. Measure at the joint rather than the plate edge, using temperature-indicating crayons for shop work or a pyrometer on large sections.

Two cautions from the crown procedure: never use a hydrogen atmosphere, because the oxygen in the copper creates an explosion risk, and do not push preheat until the plate glows and loses strength. Annealing ETP copper C11000 is done at 475 to 750 C for 15 to 60 minutes with no hydrogen, so at those temperatures you are heat treating rather than preheating.

Step 4: Machine Setup

• Polarity and transfer: DCEP (electrode negative) for spray transfer on heavier sections, the same convention used for aluminum MIG welding. Short-circuiting transfer on thin sheet burns through and spits.
• Gas: Pure argon for light to medium thickness; argon-helium blends for heavy copper, where you need higher arc voltage and a wider heat pattern.
• Wire: Copper or copper-alloy wire sized to the plate, with a matching liner and contact tip.
• Clamp: Close to the joint, on the part you are welding, since copper pulls heat away from anywhere else.

Step 5: Welding Sequence

Weld in short segments and keep the heat moving. Backstepping with 25 to 50 mm overlaps keeps the puddle small and lets heat soak into the base metal instead of cooking a hole. A slightly wider weave than on steel fills the groove better, but a steady travel speed is what stops heat pooling.

Plan tacks before the first arc: small, spaced so the joint can still move as it heats, and skipped on simple butt joints where a short bead sets position. Let the joint cool toward the preheat range between passes. Stacking heat without a chance to recover is how thick copper cracks or distorts.

Step 6: Post-Weld Work

Cool naturally, then clean the bead and heat-affected area: remove the black or grey oxide film with a wire brush and knock off spatter before it turns into a corrosion site. Inspect for undercut, incomplete penetration, porosity and toe cracks. Stress relief is a decision, not a ritual: Crown recommends 200 to 250 C for ETP copper where dimensional stability matters.

Pro Tips From the Shop Floor

• Run practice beads first. A 100 mm coupon in the same alloy beats any parameter chart.
• Use copper chill bars to pull heat from the joint and keep preheat lower on delicate assemblies.
• Keep the gas flowing. Pre-flow before and bleed slowly after the arc.
• Watch for hydrogen. It comes from dirty wire, contaminated gas or moisture on the base metal.

Printable Pre-Job Checklist

• [ ] Alloy and grade confirmed, filler matched
• [ ] Joint cleaned to bright metal and degreased
• [ ] Groove about 70 degrees, root gap tight
• [ ] Preheat range set, crayons or pyrometer on hand
• [ ] DCEP, spray transfer, feed speed and gas set
• [ ] Tip, liner, clamp, practice bead, chill bars ready

Follow this order on the first job and the second one gets noticeably easier.

Conclusion: Turning a Difficult Metal Into a Repeatable Process

Strip away the arc noise and the parameter tables and copper welding comes down to one idea: it is a heat management problem first and a technique problem second. The fabricator who opened this guide with a cold, lumpy bead on a copper bus bar was not fighting the machine. They were fighting a metal that pulls heat out of the puddle faster than the puddle can wet the sidewall, then expands hard enough to close the root gap and twist the part as it cools. Get the heat right and most of the technique follows.

Treated as a controlled heat input problem with four levers, copper stops being the metal that ruins your afternoon and becomes the metal that needs more planning than mild steel.

The four levers you actually control

1. Base metal type. Oxygen-bearing tough pitch copper (ETP) with its Cu₂O stringers fights the weld; phosphorus-deoxidized and oxygen-free copper do not. Copper-nickel, silicon bronze, aluminum bronze and low-zinc brass behave differently again. Identifying the alloy before you strike an arc is the single biggest determinant of success, and TWI’s copper weldability guidance helps when the mark is unreadable.
2. Filler selection. ERCu carries phosphorus and silicon deoxidizers that prevent porosity; ERCuSi-A, ERCuNi and ERCuAl cover copper-to-steel, seawater service and higher-strength needs. The AWS A5.7/A5.7M standard governs classification and matching.
3. Preheat and heat input. Above roughly 2 mm, preheat stops being optional. Thick sections want 350°C to 450°C, wider 70° V-grooves, backstepping and interpass control.
4. Cleanliness and gas quality. Grease, oxide, moisture and dirty wire all feed porosity. If base metal, wire and gas are clean, dry and correctly flowing, most copper defects never start.

Argon is the default shielding gas, but helium and nitrogen-based mixes run higher arc voltages, and that higher voltage puts more energy into a puddle fighting thermal conductivity near 401 W/m·K. Levers three and four are linked: as thickness climbs, high heat input shielding gases separate a sound weld from a cold, porous one.

Choosing the right process, and knowing when to walk away

TIG and MIG are the preferred welding processes for copper, with oxyacetylene and MMA also usable for tough pitch copper repairs. ETP copper datasheets rate MIG, TIG, WIG and other arc processes suitable for the alloy, while resistance welding is not recommended. Where MIG struggles: pulsed MIG for thin sheet, TIG for heat-sensitive work, friction stir welding for flat bus bars, and laser-MIG hybrids for high-volume thin material. If your instincts came from steel, note that aluminum MIG welding has its own parallel polarity, gas and cleanliness rules.

Safety is the one non-negotiable

Copper fume causes upper respiratory irritation, nausea and metal fume fever. The OSHA permissible exposure limit for copper fume is 0.1 mg/m³ as an 8-hour TWA, with 1 mg/m³ applying to copper dusts and mists and a NIOSH IDLH of 100 mg Cu/m³. Per the NIOSH Pocket Guide to Chemical Hazards, metal fume fever symptoms include chills, muscle ache, nausea, fever, dry throat, cough, lassitude and a metallic or sweet taste. Local exhaust at the arc comes first, respirators second, and preheat at 400°C plus radiant heat makes PPE a heat-stress decision too. OSHA’s welding safety fact sheet covers respiratory protection detail.

Building a copper-capable setup

You likely already own a machine that can do the job. You need genuine duty cycle, DCEP spray transfer at high wire feed speed, 100% argon or an argon-helium mix, and a clean spool of .035 or .045 ERCu. Our welding equipment guide covers machine and cylinder setup, and the gas supply guide walks through regulators and flow meters. Handle wire properly with the welding prep and cleaner guide.

Finally, practice on scrap of the same alloy and thickness before you touch the customer’s part. Copper rewards a procedure, and a procedure gets rehearsed.

Key takeaways

• Copper welding is heat management first, technique second.
• Identify the base metal alloy before anything else.
• Preheat becomes necessary above about 2 mm.
• Move to argon-helium or nitrogen mixes as thickness climbs.
• Match filler to base metal under AWS A5.7/A5.7M; keep wire and gas clean.
• Default to TIG or MIG; MMA and oxyacetylene suit tough pitch copper repairs.
• Treat the 0.1 mg/m³ copper fume limit as a hard constraint; local exhaust first.
• Consider friction stir welding or laser-MIG hybrids when volume justifies them.

Copper rewards the welder who asks four questions before the arc is struck: what is this alloy, what filler goes with it, how much heat does it need, and is everything clean and dry? Answer those, and a difficult metal becomes a repeatable process.

Frequently Asked Questions

Can you MIG weld copper?

Yes. Gas metal arc welding (MIG) is one of the preferred processes for copper and is often the best choice for heavier sections such as bus bar, plate, and tanks because it delivers high deposition rates and good heat input. It does require correct filler metal, preheat on thicker material, and usually argon or argon-helium shielding gas.

Is MIG or TIG better for welding copper?

It depends on thickness and control needs. MIG is generally faster and preferred for heavy sections, while many welders choose TIG for thinner copper because it offers better heat control and precision. For very thin or delicate copper, processes like laser or resistance welding may be better than either.

What filler wire do you use for MIG welding copper?

The common choices are ERCu deoxidized copper for copper-to-copper joints where color match and conductivity matter, ERCuSi-A silicon bronze for joining copper to steel, ERCuNi for copper-nickel base metal and marine service, and ERCuAl aluminum bronze grades for higher strength. Match the filler to the base metal and application.

What shielding gas is used for MIG welding copper?

Pure argon works well up to about 5 mm thickness. For thicker copper, an argon-helium mixture (commonly around 75% helium) provides higher arc voltage and more heat input, which helps overcome copper’s rapid heat loss through conduction.

Do you need to preheat copper before MIG welding?

Preheat is generally necessary above about 2 mm thickness and becomes essential on thick sections. Typical guidance ranges from roughly 10-100 C for thin material up to 350-450 C for thicknesses over about 16 mm, depending on section size and alloy.

Why does my copper weld keep getting porosity?

Porosity usually comes from oxygen in the base metal, moisture or contamination on the filler, poor gas coverage, or hydrogen in the arc. Use deoxidized or oxygen-free base metal where possible, clean grease-free filler and high-purity shielding gas, and choose the right gas mix and flow rate.

Is copper welding fume dangerous?

Yes. Copper fume can cause upper respiratory irritation, a metallic taste, nausea, and metal fume fever, with symptoms like chills, muscle aches, and fever. Both OSHA and NIOSH set an exposure limit of 0.1 mg/m3 as an 8-hour time-weighted average for copper fume, so local exhaust ventilation and appropriate respiratory protection are important.

Can MIG weld copper to steel?

Yes, with the right filler. Silicon bronze (ERCuSi-A) or deoxidized copper filler is commonly used to join copper to mild steel, but note that deoxidized copper is typically used where strength is not critical. Match filler and technique to the service requirements of the joint.

Does welding copper reduce its electrical conductivity?

It can. Research on welded pure copper showed the minimum electrical conductivity and average thermal diffusivity of joints dropped by 62.0% and 34.7% versus base metal due to microvoids and impurities in the fusion zone. Filler selection and cleanliness strongly influence how much conductivity is retained.